EPA-600/1-79-006
January 1979
HUMAN HEALTH EFFECTS OF
MOLYBDENUM IN DRINKING WATER
by
Willard R. Chappell
Robert R. Meglen
Rafael Moure-Eraso
Clive C. Solomons
Theodora A. Tsongas
Philip A. Walravens
Paul W. Winston
edited by
Willard R. Chappell
Robert R. Meglen
Environmental Trace Substances Research Program
University of Colorado
Boulder, Colorado 80309
Grant No. R-803645
Project Officer
Paul Heffernan
Laboratory Studies Division
Health Effects Research Laboratory
Cincinnati, Ohio 45268
HEALTH EFFECTS RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Health Effects Research Laboratory,
U.S. Environmental Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the views and policies
of the U.S. Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendation for use.
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FOREWORD
The U.S. Environmental Protection Agency was created because of increas-
ing public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled land
are tragic testimony to the deterioration of our national environment. The
complexity of that environment and the interplay between its components re-
quire a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution
and it involves defining the problem, measuring its impact, and searching for
solutions. The primary mission of the Health Effects Research Laboratory in
Cincinnati (HERL) is to provide a sound health effects data base in support of
the regulatory activities of the EPA. To this end, HERL conducts a research
program to identify, characterize, and quantitate harmful effects of pollutants
that may result from exposure to chemical, physical, or biological agents found
in the environment. In addition to the valuable health information generated
by these activities, new research techniques and methods are being developed
that contribute to a better understanding of human biochemical and physiologi-
cal functions, and how these functions are altered by low-level insults.
This report provides an assessment of the present environmental exposure
to molybdenum and the health effects of such exposure. A guideline for maxi-
mum concentration in drinking water is(^pivs>posed.
R. J. Garner
Director
Health Effects Research Laboratory
111
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ABSTRACT
Molybdenum is a trace element which occurs widely in nature and plays an
important role in industrial society. The primary industrial use of molybde-
num is as an alloy in steels. Major releases to the environment have been
associated with several industries including molybdenum mining and milling,
uranium mining and milling, and oil refining.
Molybdenum also plays an important biological role as a micronutrient for
plants and animals. At high levels it can be toxic to animals. While concen-
trations in surface waters are generally less than 5 ygMo/L, concentrations
as high as 500 ygMo/L have been reported in some drinking waters. Concentra-
tions in water greater than 20 ygMo/L are almost certainly anthropogenic.
Conventional wastewater and water treatment technologies are ineffective in
the removal of molybdenum.
The average human intake via food for the United States is 170 ygMo/day
while the average intake via drinking water is less than 5 ygMo/day. While
no adverse health effects have been reported in the United States, there are
reports in the Russian and Indian literature of both biochemical and clinical
effects in humans at intakes ranging from 1 to 10 mgMo/day. Rapid urinary
excretion appears to provide considerable protection at intakes less than
1 mgMo/day. This report reviews the data on molybdenum as it relates to the
effects of its occurrence in drinking water.
The report also reviews the results of an interdisciplinary study carried
out by the authors. The authors recommend a guideline of 50 ygMo/L for the
maximum concentration in drinking water.
This report was submitted in fulfillment of Grant No. R-803645 by th>
Environmental Trace Substances Research Program, University of Colorado un5
the sponsorship of the U.S. Environmental Protection Agency. This report
covers the period from April 7, 1975 to September 30, 1978, and work was
completed as of September 30, 1978.
Correspondence should be addressed to:
Willard R. Chappell
Director, Environmental Trace Substances Research Program
Campus Box 215
University of Colorado
Boulder, Colorado 80309
IV
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CONTENTS
Foreword iii
Abstract iv
Figures vii
Tables ix
Acknowledgments xi
1. Introduction 1
Comment on Concentration Units 2
2. Conclusions and Recommendations 3
Conclusions 3
Recommendations 4
3 . Chemical Properties 6
General Chemistry 6
Bioinorganic Chemistry 7
4. Measurement Techniques 10
Sampling „ 10
Analyses 12
Colorimetric Methods 12
Atomic Absorption Spectrophotometry 13
Other Methods 13
5. Production and Use
Production 15
Industrial Use of Molybdenum and Its Compounds 17
6. Environmental Fate 19
Rocks and Soils 19
Air 20
Industrial Exposure - Mining and Milling 20
Industrial Exposure - Smelting 22
Water 24
Plants 27
Pood 28
Industrial Sources 30
Coal Combustion 30
Molybdenum Mining and Milling 30
Molybdenum Smelting 31
Uranium Mining and Milling 31
Steel and Copper Milling, Oil Refining, and
Claypit Mining 32
Removal Technology 32
7. Biochemistry arid Metabolism 34
Biochemical Function 34
Work of the Colorado Molybdenum Project 35
Discussion 37
v
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Conclusions 38
Metabolism 39
Tissue Distribution in Aninuils 44
8. Biological Effects 47
Toxicity - Animals 47
Introduction .47
Livestock 47
Laboratory Animals - Acute Toxicity 49
Laboratory Animals - Chronic Toxicity 52
Summary 60
Human Dietary Intakes and Nutritional Requirements 61
Biological Effects of Molybdenum in Humans 65
Deficiency 65
Toxicity 65
Human Health Effects and Present Study 67
References 78
Appendices
A. Supplementary Calculation of Guideline 92
B. Analysis for Molybdenum, Spectrophotometric Method 95
C. Analysis for Molybdenum, Atomic Absorption Spectrophotometry . 93
VI
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FIGURES
Number Page
1 Eh-pH diagram of areas of predominance of aqueous species of
molybdenum 8
2 Atmospheric release in metric tons/year molybdenum 16
3 R-value versus serum uric acid concentration ( r = -0.84) 33
4 R-value versus log (serum molybdenum) (r = -0.84) 39
5 Percent of molybdenum dose remaining in the gastro-intestinal
tract versus time, and percent of dose in accumulated urine
versus time after dosing -40
6 Percent of molybdenum-99 dose accumulated per gram of tissue
(wet weight) for kidney and blood versus time after dosing 42
7 Percent of molybdenum-99 dose accumulated per gram of tissue
(wet weight) for liver and adrenals versus time after dosing 43
8 Weighted average molybdenum concentration of ten tissues (dry
weight basis) from male rats receiving indicated molybdenum
concentrations in their drinking water 44
9 Hypothetical dose-response curve for molybdenum . 48
10 Oxygen consumption in sleeping rats versus molybdenum concentration
in drinking water 54
11 Unstressed animal activity as measured by number of squares entered
per minute in an open field arena 56
12 Effect of cold stress on serum ceruloplasmin in rats given molyb-
denum and copper in a defined diet at the indicated levels 57
13 Calculated daily intake of molybdenum, according to sex and age
groups for the whole United States 62
14 Calculated daily intake of molybdenum per kilogram body mass,
according to sex and age groups for the whole United States 63
Vll
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Number Page
15 Contribution of major food categories to daily molybdenum intake
of male subjects, according to age 64
16 Plasma molybdenum concentrations of control subjects and of workers
in a molybdenum smelter in Denver 69
17 Urinary concentrations of 14 workers in a molybdenum smelter .. 70
18 Histogram of plasma molybdenum concentrations in 42 normal subjects
from the Denver area 73
19 Comparison of the daily urinary molybdenum excretion in male sub-
jects with different daily intakes from food and water 77
A-l Molybdenum concentration of water required to produce a 50%
increase in daily intake through water-based beverages 93
Vlll
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TABLES
Number Page
1 Some Physical Properties of Molybdenum .............................. 7
2 Consumption of Molybdenum by End Use (1974) ........................ 17
3 Some Use Applications for Molybdenum and Its Compounds ............. 17
4 Concentrations of Molybdenum in Various Rock Types ................. 19
5 Soils with Anomalous Molybdenum Concentrations ..................... 20
6 Comparison of Dust Survey Levels in a Colorado Molybdenum Mine ..... 21
7 Molybdenum Content of Respirable Dust in a Colorado Molybdenum
Mine [[[ 23
8 Molybdenum Levels in Respirable Dust and Total Dust ................ 24
9 Molybdenum Content of Waters and Stream Sediments Contaminated
by Various Industrial Activities ................................. 26
10 Molybdenum Content of Foodstuffs ................................... 29
11 Uric Acid Production of Lysed Human Erythrocytes Using Thin
Layer Chromatography ............................................. ^6
12 Results of C14 ATP Formation ....................................... 37
13 Molybdenum Concentrations (ppm dry weight basis) in Tissues of
Rats Given Different Levels of Molybdenum as Na2MoO4 in their
Drinking Water [[[ 45
14 Effect of Molybdenum on Litter Size in White Rats .................. 50
15 Representative Weights for Rats on Various Dietary Regimes ......... 51
16 Summary of Test Results ............................................ 59
17 Serum Ceruloplasmin and Uric Acid Concentrations in Molybdenum
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Number Page
18 Comparison of Serum Ceruloplasmin and Uric Acid Levels and of
the Urinary Excretions of Molybdenum and Copper in 1975 and
1977 in the Golden Area 73
19 Comparison of Serum Ceruloplasmin and Uric Acid Levels and of
the Urinary Excretions of Molybdenum and Copper in Two Groups
of Workers at Water Treatment Plants 74
20 Comparison of Serum Ceruloplasmin and Uric Acid Levels and of
the Urinary Molybdenum and Copper Excretions in Residents of
Summit County; Males Only 75
21 Comparison of Serum Ceruloplasmin and Uric Acid Levels and of
the Urinary Molybdenum and Copper Excretions in Residents of
Summit County; Females Only 75
22 Comparison of Biochemical Assays in Male Subjects from the
Denver, Breckenridge, and Frisco Areas 76
A-l Variability of Daily Molybdenum Intake 93
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the work of many graduate students,
undergraduate students, and technicians. We also appreciate the assistance
of several officials of local cities and municipalities. Without their co-
operation the community tap water surveys and human sampling program would
have been impossible. Special thanks are given to Ms. Terry Tedeschi for
her expert handling of the administrative details encountered during this
study. The preparation of this report was simplified and its contents were
greatly improved by the editorial assistance and many helpful suggestions of
Ms. Kathy K. Petersen.
The following is a list of the authors and their affiliations:
Willard R. Chappell, Ph.D.
Director, Environmental Trace Substances Research Program
University of Colorado, Boulder, Colorado
Professor, Department of Physics
University of Colorado, Denver, Colorado
Professor, Department of Preventive Medicine
University of Colorado Medical Center, Denver, Colorado
Robert R. Meglen, Ph.D.
Director, Analytical Laboratory
Environmental Trace Substances Research Program
University of Colorado, Boulder, Colorado
Rafael Moure-Eraso, M.Sc.
Health and Safety Office
Oil Chemical and Atomic Workers International Union
Denver, Colorado
Clive C. Solomons, Ph.D.
Professor and Director of Orthopedic Research
University of Colorado Medical Center, Denver, Colorado
Theodora A. Tsongas, Ph.D.
Clinical Instructor, Department of Preventive Medicine and
Comprehensive Health Care
University of Colorado Medical Center, Denver, Colorado
Philip A. Walravens, M.D.
Assistant Clinical Professor, Department of Pediatrics and
Department of Preventive Medicine
University of Colorado Medical Center, Denver, Colorado
XI
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Paul w. Winston, Ph.D.
Professor, Department of Environmental, Population, and
Organismic Biology
University of Colorado, Boulder, Colorado
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SECTION 1
INTRODUCTION
Two-thirds of the elements occurring in the earth's crust are more abun-
dant than molybdenum, yet it is among the fifteen trace elements that are
essential to plants and animals. Molybdenum is also economically important as
a component of metal alloys, fertilizers, catalysts, and anti-corrosive agents.
Its rapidly growing production and use represents a potential for increased
release and distribution in the environment. The purpose of this report is to
describe the human health effects that water-borne molybdenum may have.
In spite of its relatively low natural abundance the amount of soil mo-
lybdenum available for plant uptake appears to be sufficient in most areas.
Geochemical anomalies leading to molybdenum deficiencies in plants have been
described; however, nutritional deficiencies in humans have not been docu-
mented. Therefore, the human nutritional requirement is probably low, or
modern food distribution practices tend to ameliorate any deficiencies in
locally produced foods.
Molybdenum is similar to other essential trace elements in that it exhi-
bits a detrimental biochemical effect when the animal's intake exceeds the
optimum amount. The symptoms of molybdenum toxicity and the dietary concen-
tration at which symptoms occur varies with species. Much has been published
about the effects of excessive molybdenum consumption by cattle, and the
molybdenum-induced disease in ruminants, molybdenosis, has been well docu-
mented. However, little is known about the human health effects. The fol-
lowing sections review the existing literature relevant to the assessment of
the human health effects of exposure to anomalously high concentrations of
molybdenum. This report also'includes the results obtained by the authors
in a study of human exposures to molybdenum in water, food, and air.
Most surface and ground waters contain about 1 ygMo/L. Stream waters
draining undisturbed molybdenum mineral deposits are also low. Therefore,
waters containing more than 10 to 20 )jgMo/L are usually associated with human
activities such as mining, upgrading, or other industrial processing. Several
municipal water supplies in Colorado exhibit anomalously high molybdenum con-
centrations. This report describes the measurement of several biochemical
parameters on selected groups of individuals who receive their drinking water
from these sources. The study also includes an extensive food sampling pro-
gram to determine the molybdenum intake derived from food consumption.
Since relatively few humans are presently exposed to high molybdenum con-
centrations in food or water, we have also included studies of humans who
have been industrially exposed to molybdenum compounds. These data are
1
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supplemented with relevant animal studies. The conclusions; presented in
Section 2 are therefore based upon the best available data from the literature,
and the authors' own work on both humans and animals.
Brief descriptions of the chemistry of molybdenum, its production, use,
and environmental fate precede the discussion of the biochemistry and biologi-
cal effects. The reference list does not cite all of the published work on
the subject. However, it represents the bulk of the relevant work upon which
the conclusions and recommendations are based.
COMMENT ON CONCENTRATION UNITS
Concentrations of trace constituents in solid samples are reported in
parts per million (ppm). This unit is equivalent to micrograms of analyte per
gram of sample. Concentrations of an analyte in liquid samples are reported
in micrograms analyte per liter of solution. This convention is convenient
since most natural waters contain less than 20 ngMo/L. Concentrations of ten
to one thousand times the "natural" levels are only observed as a result of
anthropogenic activities. In order to emphasize the order of magnitude dif-
ferences between natural and contaminated waters large concentrations have not
been converted to the more common multiples such as mg/L.
However, the units of mg/L are used in portions of Sections 7 and 8 since
the Laboratory Studies involved concentratiDns of thousands of micrograms
molybdenum per liter of water. These concentrations are therefore converted
to milligrams per liter to facilitate the reading of the text.
It should again be emphasized that concentrations of milligrams per liter
in waters are several orders of magnitude a.oove natural levels.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS
While conclusive evidence that molybdenum is required by humans is lack-
ing, there is general agreement that it should be considered as one of the
essential trace elements. The absence of any documented deficiencies in
humans indicates that the required level is much less than the average intake
of 180 ygMo/day in the United States.
Except for industrial workers, the intake of molybdenum is almost entire-
ly via food and beverages, including water. Abnormally high food intakes, as
high as 10 to 15 mgMo/day, have been documented in India and the U.S.S.R. and
are suspected in Turkey. Because of the interregional distribution system in
the United States, it is unlikely that molybdenum toxicity will be encountered
in humans due to food intake. There could be exceptions where significant
parts of the diet come from one particular geographical area or for indivi-
duals on a limited diet.
Natural molybdenum concentrations in ground and surface waters are rarely
more than 10 to 20 ygMo/L. Concentrations significantly higher than these
levels are almost certainly due to industrial contamination. Since conven-
tional wastewater and water treatment facilities remove very little (0 to
20%) molybdenum, drinking water concentrations will be close to those of the
untreated source.
Several industries have effluents which have high concentrations of mo-
lybdenum. These include molybdenum mining and milling, molybdenum smelting,
uranium mining and milling, copper mining and milling, shale oil production,
and coal-fired power plants. The aqueous effluents from these industries have
molybdenum concentrations ranging from 100 to 800,000 ygMo/L. The most fre-
quently observed environmental impact is molybdenum toxicity in cattle.
There is great species variation in the susceptibility of animals to
molybdenum toxicity. Acute toxicity in other than the laboratory setting has
only been seen in cattle and sheep. Cattle are by far the least tolerant of
molybdenum, while sheep are somewhat more tolerant. Rats, guinea pigs, and
poultry are more tolerant than cattle and sheep, and less tolerant than pigs.
Symptoms of molybdenum toxicity also vary with species. In the case of cat-
tle, severe diarrhea is a common symptom. Other animals may suffer from loss
of weight, sterility, anemia, connective tissue lesions, and pathological
changes in the liver and kidney depending on the species and dose. The only
clinical symptom described in humans is a gout-like disease. Laboratory and
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field studies indicate that molybdenum is a biological antagonist to copper.
Thus, the usual mechanism of molybdenum toxicity is the induction of copper
deficiency. Symptoms in cattle and other animals can often be reversed by
the addition of supplemental copper to the diet.
While there are people in the United States with water supplies contain-
ing greatly elevated concentrations of molybdenum, the corresponding daily
intakes are still at least an order of magnitude less than those people in the
U.S.S.R. who were described as exhibiting a gout-like disease. Industrial
workers in the United States are exposed ~o intakes that can be as high as
50 mgMo/day without violating OSHA standards.
Molybdenum in the diet and in water is readily absorbed and rapidly ex-
creted. The rapid excretion, primarily in urine, provides an effective mech-
anism for regulating molybdenum concentrations in blood and presumably other
tissues. In our study of humans having water supplies containing as much as
200 ygMo/L, we found that while urinary concentrations were increased, serum
molybdenum concentrations remained normal. No changes in copper metabolism
were observed. However, Deosthale and Gopalan (1) have observed an increase
in daily urinary copper excretion in human subjects receiving 500 to 1,000
ygMo/day in their diets.
In summary, no biochemical or clinical effects were observed in humans
whose water supplies contained up to 50 ygMo/L. Increased urinary excretion
of molybdenum was observed in humans whose: water supplies contained 50 to 200
ygMo/L. Deosthale and Gopalan (1) observed increased copper excretion in
humans having daily intakes of 500 to 1,500 ygMo/day, but they did not ob-
serve any changes in uric acid excretion. In a study of molybdenum workers
exposed to a minimum daily intake of 10 mgMo we found greatly increased blood
and urine levels of molybdenum. We also found significant increases in uric
acid excretion, but these levels were still within a normal range for humans.
We also found a significant increase in serum ceruloplasmin compared to nor-
mals, consistent with the results of Deosthale and Gopalan (1). In addition,
we found an increased xanthine oxidase activity. However, these levels were
still within a normal range for humans. Kovalskii and others (2), studied
a human population receiving 10 to 15 mgMo/day. They found greatly increased
uric acid levels, decreased copper excretion, and a high incidence of a gout-
like disease. They postulated that the increased uric acid excretion and
gout-like disease were due to increased xanthine oxidase (a iriolybdemrn-
containing enzyme) which was in turn due to the abnormal molybdenum intake.
RECOMMENDATIONS
In view of the absence of any documented human cases of molybdenum defi-
ciency, it seems likely that the average daily intake of 180 yg Mo is well
above the minimum daily requirement. All micronutrients are characterized by
a range of deficiency, a range of sufficiency, and beyond these, toxicity.
Since the exact size of the range of sufficiency is unknown, it would seem
prudent to avoid large increases beyond the average dietary intake of 180
ygMo/day. On the other hand, clinical effects have been reported at 10,000
to 15,000 ygMo/day and biochemical effects in the range of 500 to 10,000 ygMo/
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day. The "no-effect" level cannot be pinpointed with certainty, but we feel
it is probably not less than 500 ygMo/day. A concentration of 50 ygMo/L in
drinking water would contribute about 100 ygMo/day to the total diet which
would make the average total daily intake 280 ygMo/day. Even for 15 to 17
year old males, the group having the largest average dietary intake at 250
ygMo/day, the total would be 350 ygMo/day. Since no differences were seen be-
tween humans on 1 ygMo/L and on 50 ygMo/L, it appears that 50 ygMo/L repre-
sents a "safe" level in drinking water. (A supplementary method for calculat-
ing this level is found in Appendix A.)
Thus, we believe 50 ygMo/L represents a useful guideline for drinking
water. Our data indicate that somewhat higher levels can probably be toler-
ated, at least for short periods of time. Where levels above 50 ygMo/L occur
it is almost certain to be as the result of industrial contamination. While
such concentrations are not often encountered, recent studies by ourselves and
others show that some communities in different regions of the United States
do have finished water supplies containing more than 50 ygMo/L. Other such
communities will be encountered in the future.
It is not likely that a drinking water standard for molybdenum is
required. But we believe that 50 ygMo/L represents a prudent guideline.
In order for a recommended level to be useful, the analytical techniques
used or recommended by regulatory agencies must be capable of accurate and
precise measurements at or, preferably, below that level. The present tech-
nique recommended by E.P.A. (Methods for Chemical Analysis of Water and
Wastes; EPA 625/6-74-003) and used for monitoring purposes (e.g., in N.P.D.
E.S. compliance) is not adequate. This flame atomic absorption technique does
not have a detection limit sufficiently low to monitor most drinking waters.
The method is also highly susceptible to the interferences presented by most
industrial effluent matrices. There are other methods of analysis which have
detection limits in the range of 1 to 10 ygMo/L which could be validated for
routine use (e.g., atomic absorption with electrothermal atomization, colori-
metric methods). Direct flame atomic absorption without preconcentration and/
or extraction to separate the analyte from interferences is not likely to
provide accurate analyses. In view of the guideline recommended above and
the existing recommended level for irrigation water (10 ygMo/L for continuous
irrigation), we strongly recommend that EPA adopt a different analytical
technique for general use. Two such techniques are described in Appendices
B and C.
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SECTION 3
CHEMICAL PROPERTIES
GENERAL CHEMISTRY
The chemistry of molybdenum is so extensive that it is necessary to limit
the discussion to chemical and physical properties that are important under
conditions which occur naturally in the environment or in biological systems.
Molybdenum, atomic number 42, is a transition element with the outermost
electronic structure 4d55s!. It exhibits oxidation states from 2- to 6+.
However, the oxidation states below 2+ are generally found only in organo-
metallic compounds. Of the remaining oxidation states, only 3+, 4+, 5+, and
6+ are important in aqueous solution. Oxides and sulfides of these oxidation
states are the principal solid inorganic species which occur naturally. Crys-
talline oxides and sulfides are well characterized and are widely used indus-
trially. Molybdenum also forms the tetrahedral poly atomic anion molybdate,
9 — Pi —
MoO^ , and isopolyanions such as MO7O24 . These anions form salts with a
variety of cations and these compounds are used in industrial applications.
Some of the important naturally occurring compounds are the minerals: molyb-
denite, powellite, wulfenite, ferrimolybdite, ilsemannite. Table 1 shows the
chemical formulas for these compounds. Most of these minerals are only
slightly soluble at natural pHs and oxidation-reduction conditions.
The relative stabilities for naturally occurring minerals in equilibrium
with water and dissolved species can be illustrated by using theoretical Eh-
pH diagrams. The species oxidation potential, relative to the hydrogen elec-
trode, is plotted as a function of pH. Such a diagram shows the regions? of
stability for the various species under consideration. Figure 1 shows an Eh-
pH diagram for the predominant aqueous molybdenum species (3). The shaded
area represents the range of natural Eh-pH characteristics which have been
measured for shallow ground waters and fresh waters (4). From this diagram
it can be seen that most natural waters exhibit conditions under which
? —
MoO^ would be the principal stable species. At pHs below 5 and under more
oxidizing conditions the hydrogen molybdate aniori (HMoO^~) is expected to be
the predominant species. Sucii Eh-pH conditions are likely to be found in
some acid mine drainages or industrial effluents. Similarly, at very low
pHs and under slightly less oxidizing conditions, the cationic MoO2+ is the
principal species present.
When there are significant concentrations of other ionic species such as
iron, calcium, sulfate, etc., these species can also be included in the Eh-pH
diagram. The presence of some of these species can significantly alter the
aqueous molybdenum chemistry. However, the following generalization remains:
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TABLE 1. SOME PHYSICAL PROPERTIES OF MOLYBDENUM
Color metallic silver
Atomic number 42
Atomic mass 95.94
Atomic radius 1.40A
Density 10.28 g/cm3 (20°C)
Melting point 2620 + 10°C
Boiling point 4825°C
Formulas of common minerals:
Molybdenite MoS2
Powellite
Wulfenite
Ferrimolybdite Fe2O3-3.52MoO3-10.4H2O
Ilsemannite M°3°8
the principal dissolved molybdenum species in the natural environment is
molybdate. For a more detailed description of molybdenum mineral solubility
and aqueous geochemistry the reader is directed to the work of D. Kaback (3).
BIOINORGANIC CHEMISTRY
In order to understand the biochemistry of the molybdenum-bearing enzymes
it is helpful to examine the chemical interactions between molybdenum and bio-
logically important molecules. While the exact nature of the molybdenum-
protein interactions is incompletely understood, several generalizations re-
garding these interactions can be made. The 3+, 4+, 5+, and 6+ oxidation
states of molybdenum are the most important in biological systems (5). A pro-
tein can bind to a transition element through oxygen, sulfur, or nitrogen. In
general, higher oxidation states lead to oxygen binding while lower oxidation
states favor sulfur or nitrogen binding. Oxygen and sulfur are favored over
nitrogen as ligands. This is partly due to the ability of oxygen and sulfur
to partially neutralize the positive charge of the metal atom (6). The
neutral nitrogen in most ligands does not afford this added stabilization.
The coordination number of molybdenum with most organic ligands is five or
six. This is in contrast to other biologically important transition elements
such as iron, copper, and zinc which are four coordinate (6).
There is considerable evidence that the molybdenum in xanthine oxidase
and other enzymes is bound through the mercapto group of cysteine residue (7).
Both Mo (V) and Mo (VI) are known to form complexes with histidine. The mo-
lybdenum enzymes are catalysts in oxidation-reduction reactions. Molybdenum
7
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-0
12 13 14
Figure 1. Eh-pH diagram of areas of predominance of aqueous species
of molybdenum [after Kaback (3)]. Shaded areas shows Eh-
pH characteristics of fresh waters [after Baas-Becking et.
al. (4)1.
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frequently occurs in enzymes where there are other electron carriers such as
iron/sulfur, iron/heme, and iron/flavin. Molybdenum participates in electron
transfer, and may be preferred over other metals in this role since it has
stable oxidation states from 3+ to 6+ and can undergo multiple electron
transfers (6). The biochemistry and metabolism of molybdenum will be dis-
cussed in Section 7.
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SECTION 4
MEASUREMENT TECHNIQUES
SAMPLING
Waters, geological solids, plants, animal tissues, and biological fluids
all have complex chemical characteristics which must be considered before an
adequate sampling protocol can be devised. The choice of a sampling procedure
depends upon field sampling conditions, tie chemical characteristics of the
material being sampled, and the purpose to which the analyses will be put.
Since the purpose of this report is to present data on the human health ef-
fects of molybdenum in drinking water, we shall limit our discussion to sam-
pling methods which have a direct bearing on this study. Therefore, w^^.ter
sampling and analysis will be discussed in detail.
No single method of collection and treatment is suitable for all types of
water samples. Each chemical constituent of water may affect the stability of
other constituents. Preservation methods which ensure quantitative recovery
of molybdenum may not effectively preserve other trace elements. Therefore,
it is essential that the adequacy of the sample treatment be confirmed for
each element of interest before extensive sampling is undertaken. The fol-
lowing section describes the results of a study of sampling factors which
affect the validity of molybdenum analyses.
Adsorption of trace elements on the surface of sample walls is poten-
tially the most serious source of error ir water sampling (8). The extent of
molybdenum adsorption, at equilibrium, depends upon the container material,
ionic strength, pH, and molybdenum concentration (9). The maximum adsorption
of molybdenum on polyethylene occurs at about pH 4. At this pH about 25% is
adsorbed from an aqueous solution of 5 ygMo/L. About 3% adsorption occurs at
100 ygMo/L. Adsorption is variable and greater when iron is present in solu-
tion at 2 mg/L. Coprecipitation or coadsorption with hydrated ferric oxides
can severely reduce the recovery of molybdenum from improperly treated sam-
ples. Glass, linear, arid cross-linked polyethylene containers are suitable
for collection and storage of water samples when the samples are acidified to
less than pH 2. Samples stored at pH 2 in these bottles for three years
showed quantitative recoveries. The light weight and unbreakable nature of
polyethylene containers makes them convenient for field collection and ship-
ping. Bottles may be reused if they are thoroughly cleaned with a detergent
wash followed by a dilute nitric and/or hydrochloric acid wash. While no mo-
lybdenum contamination was found in unwashed new bottles, several other trace
elements present serious contamination problems. Acid washing of new bottles
is recommended when other trace elements are to be determined (10).
10
-------
The need for sample filtration depends upon how the analyses will be
used. When human or animal intake is to be determined, filtration may not be
appropriate since molybdenum present in suspended colloidal material is con-
sumed along with dissolved molybdenum. The tradition of filtering samples
through 0.45 micron membrane filters was based upon the desire to remove bio-
logical materials; however, this practice complicates the interpretation of
the existing literature. This poorly justified filtration practice has become
the arbitrary criterion for distinguishing between "dissolved" and "suspended"
species. Experienced workers are aware of the tendency of membrane filters
to retain species smaller than the nominal filter pore size. The effective
pore size of these filters is reduced as the membrane becomes clogged. In
addition, membranes heavily loaded with colloidal iron hydroxides can strongly
adsorb some dissolved species. These limitations should be considered when
one interprets data obtained on filtered samples.
Analyses obtained on unfiltered natural water samples can also complicate
interpretation. This is particularly true of samples which contain sediment
and/or large amounts of suspended material. These materials can serve as
sinks for adsorption and/or sources of elemental contamination through desorp-
tion or dissolution. Samples obtained from rapidly flowing streams often
carry large amounts of material which settle out under less turbulent flow
conditions. Sampling these waters requires great care. Adequate molybdenum
analyses may be obtained by allowing these waters to settle in the sample
bottle. Aliquots of the supernatant may be used for analysis. Such samples
should not be acidified since the added acid may dissolve some of the solid
material. Samples should be analyzed as soon as possible after collection.
Most drinking and natural waters can be analyzed without filtration since
most of the molybdenum present remains in solution as the molybdate or hydro-
gen molybdate species. Extremes of pH or high concentrations of other species
such as iron are not likely to be found in most drinking waters. Figure 1,
the Eh-pH diagram shown in Section 3 of this report, illustrates the stability
fields of the principal aqueous species. Furthermore, most of the naturally
occurring molybdenum-bearing minerals which may occur in sediments are not
highly soluble under the chemical conditions of natural waters.
Biological growth in stored samples can also change the trace element
content of stored samples. Since most household tap waters are chlorinated
no pretreatment is necessary. However, untreated waters may be treated with
bacteriostatic agents to retard biological activity. One mL of chloroform per
liter of sample is sufficient to prevent growth and does not interfere with
the analysis.
Freezing of samples is frequently recommended for preservation of trace
elements in waters. Winter collection of natural waters often precludes pro-
per pretreatment of samples in the field before they freeze. Since freezing
can irreversibly alter the stability of some trace elements (11), a study to
determine the effects of freezing of molybdenum in waters was performed.
This study showed that freezing does not affect the analytical concentration
of molybdenum. Treatment in the field is always the preferred method of sam-
ple preservation; however, samples which remain frozen until analysis can
later be treated with acid in the laboratory to reversibly recover molybdenum.
11
-------
When iron is present at concentrations greater than 2 mgFe/L poor iron recov-
ery is obtained which in turn affects the molybdenum recovery from uriacidified
iron rich water samples. A comprehensive survey of the current literature on
sample handling is available in an NBS Technical Note (12).
ANALYSES
Quantitative analysis of molybdenum as a trace constituent has been
attempted by a wide variety of techniques. While emission spectroscopy, x-ray
fluorescence, and neutron activation have been successfully used in the analy-
sis of aqueous samples, the cost and availability of the instrumentation re-
quired by these methods precludes their use by the majority of analytical lab-
oratories responsible for water quality monitoring. Therefore, colorimetric
and atomic absorption spectrophotometry are the most widely used analytical
detection techniques today.
Colorimetric Methods
Molybdenum forms several stable colored complexes which can be used for
detection and quantitative analysis of aqueous molybdenum. However, these
complexes have low molar absorptivities and the presence of other trace ele-
ments can interfere with the analysis. Improved sensitivity and specificity
can be obtained by using a solvent extraction technique to concentrate the
colored analyte and to separate it from potential interfererits.
The widely used thiocyariate colorimetric method makes use of the amber-
colored molybdenum-thiocyanate complex. In this method molybdenum is reduced
in acid medium by tin(II) ion. Addition of thiocyanate ion yields the amber
molybdenum-thiocyanate complex which is readily extracted into a polar organic
solvent. Ethers, alcohols, and ketones, which are immiscible with water, are
used for the extraction. The intensity of the amber colored complex in stan-
dards and unknowns may then be determined using a spectrophotometer. The de-
tection limit and sensitivity of this method depends upon the relative vol-
umes of sample and organic extractant; however, the method is capable of
detecting molybdenum in water at concentrations as low as 1 ygMo/L. Several
modifications of this technique have been published (13-16).
Another widely used colorimetric method is the dithiol method. Toluene-3,
4-dithiol reacts with hexavalent molybdenum in hydrochloric: acid solution to
form a green complex which is extractable with chloroform, carbon tetrachlor-
ide, or other organic solvents. This method is also capable of detecting mo-
lybdenum concentrations in the low ygMo/L range. Separation using ion-
Detection limit = the concentration of an element which will shift the
absorbance (or emission) signal an amount equal to the
peak-to-peak noise of the baseline (or background signal)
Sensitivity = the concentration of an element which produces an absorbance
of 0.0044 (1% absorption).
12
-------
exchange resins have also been used to preconcentrate the sample and to remove
interferents (17,18). Modifications and various improvements are described
in several references (14, 19-23),
Atomic Absorption Spectrophotometry
For most metal elements atomic absorption spectrophotometry (A.A.S.) is
the preferred method of analysis because of its speed and simplicity. How-
ever, accurate molybdenum analyses by A.A.S. can be difficult and time consum-
ing. Molybdenum is one of several metals which forms refractory oxides in
flames. This tendency can be minimized by the use of a nitrous oxide-acetylene
reducing flame. However, even in reducing flames, the propensity to remain in
an oxidized form is enhanced by the presence of other interfering elements.
One study of molybdenum absorption in flames has shown that 46 added ionic
species affect the signal intensity (24) . The magnitude of the interference
depends upon the concentration of the interferent. Therefore, complete separ-
ation of molybdenum from potential interferents is necessary to insure accur-
ate analyses. Adequate separations can be accomplished by ion exchange separ-
ation or by complexation-extraction techniques (14,16,25,26). The
complexation-extraction procedures used in the colorimetric methods have been
used for analysis of molybdenum by flame A.A.S. In these procedures the or-
ganic phase bearing the molybdenum complex is aspirated into the flame for
detection and quantification (25). These techniques tend to be more time
consuming than the colorimetric procedures.
Another difficulty encountered with the A.A.S. determination of molybde-
num is its low practical sensitivity of about 500 ygMo/L. Even with large
preconcentration factors the detection limit of about 10 ygMo/L is insuffi-
cient for many waters. Direct analysis of natural waters without preconcen-
tration is not possible since the detection limit of the method is between
50 and 100 yg/L. Most natural and drinking waters must be concentrated by a
factor of at least ten before adequate precision can be achieved.
Flamesless A.A. techniques have recently become more widely used. In
this technique atomization is accomplished by introducing a small aliquot
(usually 10-50 yL) of the aqueous sample into a graphite furnace or cup. The
graphite furnace or cup is resistance heated to about 3,000°C by passing an
electric current through it. The electrothermally induced atomization process
replaces the flame induced atomization of conventional A.A.S. This method is
much more sensitive than flame A.A. techniques. Detection limits of 1 yg/L
may be achieved without preconcentrating the sample. However, this method is
also susceptible to interferences. Several ions severely suppress the absorp-
tion signal and high dissolved salt concentrations make the detection limit
poorer. However, since most tap waters are low in dissolved solids, they may
be analyzed directly by this method. With some modifications the method is
applicable to the determination of molybdenum in biological materials when
low concentrations and only small samples are available (27).
Other Methods
Emission spectrography (E.S.) has been used for the analysis of molybde-
num in solid samples. However, water analysis by E.S. requires precipitation
13
-------
of molybdenum as a complex which can be dry mounted in the instrument elec-
trodes. This method is tedious and time consuming (13). Inductively coupled
plasma spectrometry (I.C.P.S.) is now being applied to a variety of trace ele-
ment analyses. This newly developed technique is sufficiently sensitive to
permit the direct analysis of waters without preconcentration. The ionic
emission spectra generated in the high temperature argon plasma substantially
decrease the interelement interferences observed in the lower temperature
flames of A.A.S. Detection limits for molybdenum in the low )_ig/L region are
being reported by several instrument manufacturers. This instrumentation is
very expensive; however, the advantages of high sensitivity, low detection
limits, wide dynamic range, and multi-element capability will make this tech-
nique very valuable for trace element studies. In Appendices B cind C we de-
scribe the methods we have used for colorimetric and AAS analyses.
14
-------
SECTION 5
PRODUCTION AND USE
PRODUCTION
The prinicpal mineral from which molybdenum is obtained is molybdenite
(MoS2) (28). The product obtained from the milling of crude ore containing
molybdenum, molybdenite concentrate, generally contains 90% or more MoS2-
Almost all molybdenite concentrate is then converted to technical-grade molyb-
dic oxide (MoO-^) which is the base material for production of various chemical
compounds, ferromolybdenum, and purified molybdenum. Some lubricant-grade
MoS2 is prepared directly from the concentrate by additional grinding and
flotation.
Technical grade molybdic oxide is usually produced by roasting in a mul-
tiple hearth furnace at temperatures up to 600°C (28). Pure molybdic oxide
is obtained by sublimation or selective recrystallization of technical-grade
molybdic oxide (90-95% MoO3) at about 1,000°C to 1,100°C. It is used as a
base material for metallic molybdenum and for sodium and ammonium molybdates.
Technical grade molybdic oxide is added as a charge material or directly to
cast iron and to a large proportion of steels and other alloys.
In 1976 production by the United States was 50 million kg (29). The
total world production was 76 million kg. Approximately 35% is produced as a
byproduct from ores of copper, tungsten, and uranium; the remaining 65% is
recovered from ores processed for molybdenum. Approximately 60% of the U.S.
production is presently obtained from the Climax and Henderson mines in Colo-
rado which are operated by the Climax Molybdenum Corporation, a division of
AMAX, Inc. Other producers include the Molybdenum Corporation of America,
Duval Sierrita, and Kennecott Copper Corporation. Two primary and four by-
product producers account for over 95% of the domestic output (30).
Because molybdenum occurs at concentrations of 0.5% or less in ores, sub-
stantial amounts of solid waste must be disposed of in tailing piles. In the
case of the world's largest mine at Climax, approximately 36,000 metric tons
of tailings are generated per day. This operation has released as much as
100,000 kg per year of molybdenum as aqueous effluent.
Figure 2 gives estimated atmospheric release rates for various parts of
the production sequence.
15
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INDUSTRIAL USE OF MOLYBDENUM AND ITS COMPOUNDS
The consumption of molybdenum in 1974 is summarized in Table 2.
3 lists some common molybdenum compounds and their use.
Table
TABLE 2. CONSUMPTION OF MOLYBDENUM BY END USE (1974) (29)
Alloy steel 44%
Stainless steel 21%
Tool steel 11%
Chemicals and lubricants 8%
Cast iron and steel-mill 6%
rolls
Special and super alloys 5%
Molybdenum metal 4%
Miscellaneous 1%
TABLE 3. SOME USE APPLICATIONS FOR MOLYBDENUM AND ITS COMPOUNDS
Molybdenum
Molybdic oxide
Cobalt molybdate
Molybdenum disulfide
Molybdenum pentachloride
Molybdenum hexacarbonyl
Molybdenum acetylacetonate
Molybdenum oxalate
Molybdenum dithiocarbamate
Ammonium molybdate
Molybdenum tannate
Molybdates (e.g., zinc
molybdate)
iron-base alloys; cracking catalysts;
in fertilizers
production of ammonia from hydrogen
and nitrogen
desulfurization of gasolines
lubricant
Friedel Crafts chlorination of aromatics
and alkylations; vapor phase deposition
of molybdenum coatings
vapor phase deposition of molybdenum
coatings
catalyst for polymerization of poly-
urethane foam
in photochemical systems
lubricant additive
laboratory reagent for determination of
phosphorus, bromates, cholesterol;
arsenic in feeds
coloring of leather
pigments for printing inks, lacquers,
paints; vitreous enamels
17
-------
One particular use which may increase dramatically over the next few
decades is in the catalytic hydro-cracking of coal to liquids. If this proves
to be feasible on a commercial scale, then to supply 9,490 quadrillion joules
(9 quadrillion BTUs) of energy by coal conversion in 1985 we will need to con-
vert almost 363 million metric tons of coal (32). Conversion will require
almost 1.8 million kg of molybdenum. While; this is a small percentage of the
current U.S. production of over 45 million kg/year, one estimate of coal con-
version requirements in the year 2000 would imply the need for 11 million kg
of molybdenum for this purpose (32). Other energy related uses include the
use of molybdenum in elevated temperature steels which are widely used in
steam and gas turbines, and in the steam generating sections of fossil or
nuclear-fueled power plants. These and other applications are discussed in
reference 29.
18
-------
SECTION 6
ENVIRONMENTAL FATE
ROCKS AND SOILS
The average abundance of molybdenum in the earth's crust is 10 "*% or one
part per million (ppm) (33) . Table 4 summarizes information on the molybde-
num contents of some rock types.
TABLE 4. CONCENTRATIONS OF MOLYBDENUM IN VARIOUS ROCK TYPES (33)
Rock type
Basaltic (igneous)
Granitic (igneous)
Shale
Black shale
Limestone
Phosphorite
Range
_ p
0.9-7
1-6
1-3
1-300
5-100
Aver
1.
1.
2
10
0.
30
age
5
6
4
Economic molybdenum deposits contain 200 ppm Mo or more, with the lower
concentrations generally mined as a byproduct of copper mining. Oil shale
from the Green River formation in Colorado contains approximately 30 ppm Mo
(34).
The total molybdenum concentration in soils can vary quite widely, but
most soils contain between 0.6 and 3.5 ppm Mo (35). The average content of
most soils has been reported to be 1 to 2 ppm Mo (36).
Kubota (37) has written perhaps the most recent comprehensive review on
molybdenum in soils. His assessment of the data for the United States is
that the median for U.S. soils is 1.2 to 1.3 ppm Mo with a range from 0.1 to
40 ppm.
In areas where soil has been developed from a molybdenum-rich formation
or where industrial contamination has occurred, the concentrations can be
19
-------
quite high. Table 5 summarizes data on some soils containing naturally high
levels of molybdenum and others which are high because of contamination.
TABLE 5. SOILS WITH ANOMALOUS MOLYBDENUM CONCENTRATIONS
Source Range Mean
ppm
Natural sources:
Soils covering a mineralized area (38) 27-190 76
Soil derived from a marine black
shale (39) 2~85 12
Alluvial soils - eastern footslopes
of Sierra Nevada (37)
Soils formed from volcanic ash - q
Kauai, Hawaii (37)
Industrial sources:
Soils downstream from a molybdenum mine .
and mill - Colorado (38) 4,..bU
Soil irrigated with water contaminated
by a uranium mill (40)
Two miles from molybdenum smelter -
Pennsylvania (41)
Clearly, both natural and industrial sources can give rise to extremely
elevated molybdenum concentrations in soils. Where concentrations exceed 3
to 5 ppm, a geological anomaly or industrial contamination is the likely ex-
planation. Soils adjacent to mineralized areas generally show elevated con-
centrations of molybdenum and offer a reliable guide to mineralization.
AIR
Concentrations of molybdenum in ambient air are quite low ranging from
below detection limits to 0.03 mgMo/m3 (42). Thus, under ordinary conditions
there is a negligible contribution from air to human and animal daily intake
of molybdenum.
Industrial Exposure - Mining and Milling
In the industrial setting, there can be a considerable exposure to mo-
lybdenum. Various environmental levels have been reported in mining opera-
tions. In Table 6 levels of dust are reported that were measured during two
extensive surveys of a molybdenum mining, crushing, and milling operation.
20
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The milling operation is remarkably less dusty than the others due to the high
percentage of water used in the process. However, the processes of drying,
packing and loading are potential areas of exposure to dust containing high
percentages of molybdenite (90%
The data of the 1959 Public Health Service are difficult to use to esti-
mate exposure to molybdenite because the sampling techniques used counted the
particles without determining their mass. Therefore, it is not possible to
determined the gravimetric concentration of molybdenite in the airborne dust.
In our study stationary 10L samples were collected over 10 minute periods.
The Oil Chemical and Atomic Workers (OCAW) 1975 study determined the respir-
able mass of dust at the breathing zone of the worker.* Each sample was col-
lected for 480 minutes (a full shift) and represents about 820 I, of sample
air. The concentration of molybdenum in the respirable dust collected in the
OCAW 1975 study are shown in Table 7. It is possible to estimate the exposure
to molybdenum in the mining and processing of molybdenite by assuming that a
"standard man" inhales 10 m /day during an eight hour workday. This estima-
tion is also found in Table 7 .
Industrial Exposure - Smelting
In 1975, the environmental levels of molybdenum were measured in a
smelting (roasting) operation (45) . Settled dust, impinger samples and high
volume, respirable size, filter samples were collected. Concentrations of
molybdenum, as MoO^, in settled dust samples ranged from 57% to 61%. The
quantitative analytical method was colorimetry. The molybdenum species were
confirmed by x-ray diffraction. Three impinger samples encompassing the
roasting operation gave concentrations of MoO-> , which ranged from 3 mg/m to
-j
33 mg/m . About 180 L of air were collected and passed through the same sam-
pling trains consisting of two impingers in series. Based on a time-motion
study of the exposed workers, the time-weighted average was found to be 9.5
mgMo/m3 .
In order to estimate respirable fraction, three high volume samples were
also collected in the same areas where impingers were placed. X-ray diffrac-
tion was again used to determine the molybdenum species. MoO3 was the iden-
tified compound and its concentration in the collected dust varied from 52.4%
to 77.9% MoO3. The concentrations of molybdic oxide were then determined by
spectrophotometry. Values ranged from 4.6 to l.L mgMo/m3 . For an estimation
of body burden, see Table 8.
No reports of levels of exposures in industrial operations using molybde-
num in the United States were found in the literature.
A few U.S.S.R. studies (46,47) report environmental concentrations of
MoO->. Forty samples were taken above a crucible and in the breathing zone of
*
As defined in Interim Guide for Respirable Mass Sampling AIHA Journal, Vol.
31, p. 133, 1970.
-------
TABLE 7. MOLYBDENUM CONTENT OF RESPIRABLE DUST IN A COLORADO MOLYBDENUM MINE
(Estimation of net daily body burden - DBB)
Portable Pump Model G (MSA) with two stage size selective samples
Gelman DM. (MSA PVC 5 ym filters)
Sample
number*
MINE
18
38
55
CRUSHER
62
66
102
106
MILLING
69
68
71
98
OPEN PIT
74
76
93
Location and Volume of
classification air samples
of worker (liters)
Underground loader
Underground miner
Underground loader
Crusher welder
Crusher mechanic
Crusher welder
Crusher gyro operator
Milling mechanic
Milling oiler
Milling mechanic
Milling mechanic
Bui 1 dozer operator
Driller
Loader
795
869
833
784
681
686
678
613
787
757
704
821
784
819
Respirable mass
concentration Net DBB
ym/m3 of Mo yg/dayt
0.1
0.3
3.0
49.0
27.9
8.7
0.1
3.9
0.1
1.5
12.8
1.5
11.7
0.2
2.2
22.5
36
209
6.5
36.7
0.8
11.2
96.0
11.2
87.7
1.5
,
One blank filter was analyzer for Mo for each operation (Mine, Crusher,
Milling, Open pit). Concentrations of Mo6+ in the blanks ranged from 0.2 to
1.0 ymg. The molybdenum present in the air samples was corrected for the
amounts of molybdenum found in the blanks.
^Assuming 10 m of air inhaled during an 8-hour day and 75% absorbed from
total respirable dust inhaled.
23
-------
TABLE 8. MOLYBDENUM LEVELS IN RESPI3ABLE DUST AND TOTAL DUST (45)
Base of rocister First tier Second tier
Respirable dust:
Dust concentration (mg/m3) 1.31 3.01 6.25
Percent molybdenum in dust 77.8 52.4 71.0
Molybdenum content of dust (mg/m3) 1.02 1.58 4.49
Total dust:
mgMo/m3 of air 3.04 9.11 33.28
Hours-exposure/worker* 4 1.5 1.5
8-hour TWA = (3.04 x 4) + (9..11 x 1.5) + (33.28 x 1.5) = 9.47 mgMo/m3
*
Does not total 8 hours since there is virtually zero exposure during 0.5 hour
lunch break and two 0.25 hour breaks.
workers during a smelting operation involving MoO3- The concentrations of
MoC>3 in the breathing zone of the workers involved in the process averaged
0.22 mgMo/m3. Highest values reported were 0.4 to 0.5 mgMo/m3. The area
above the crucible was also sampled showing concentrations from 1.4 to 5.4
mgMo/m3 depending on the molybdenum content of the ore. In another factory
producing high purity molybdenum the air concentration ranged from 6.4 to
10 mgMo/m of MoO3. The analytical method was colorimet.ry but no descriptJ
of the methods of sample collection is reported (46).
Another U.S.S.R. study reported air concentration in two chemical plants
producing molybdenum salts. The range of concentrfitions found in the first
plant was 0.5 to 200 mgMo/m3 of MoO3. In the second plant the concentrations
were 0.2 to 30 mgMo/m3. No analytical methods or sample sizes are given (47).
Clearly industrial exposures can lead to a greatly increased daily intake
of molybdenum. Indeed, the present OSHA standard of 5 mg/m3 soluble molybde-
num would, over an eight-hour period, lead to an average intake of 50 mg which
is more than 200 times the normal human daily intake.
WATER
It is estimated that 3.6 x 101° grams of molybdenum are released per year
into the surface waters of the world by natural processes (48). Concentra-
tions in most waters are less than 20 ymMo/L. Average molybdenum
24
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concentrations in sea water range between 4 and 12 ygMo/L (49,50). In 1963,
Durum and Haffty (51) estimated the median molybdenum content of major North
American rivers to be 0.35 ygMo/L. In an extensive survey of Colorado surface
waters, Vogeli and King (52) found that 87% of 299 samples from 197 stations
contained less than 10 ygMo/L and concluded that concentrations of more than
5 ygMo/L in the surface waters of Colorado were probably due to molybdenum
mineralization and/or molybdenum mining and milling.
Vinogradov (53) found a normal molybdenum background concentration of
3 ygMo/L in the groundwaters of the U.S.S.R. A recent study of groundwaters
in Colorado by the U.S. Geological Survey (personal communication) found that
98 out of 156 samples contained less than 1 ygMo/L—only 10 samples contained
more than 10 ygMo/L and the highest concentration found was 28 ygMo/L.
Runnells and Kaback (38) compared waters draining highly mineralized
areas with waters draining areas containing normal concentrations of molybde-
num in rocks and soils. They found that molybdenum mineralization did not
contribute significantly to molybdenum concentrations in surface waters.
(Most ore bodies contain molybdenum as MoS2)• Water from streams draining
highly mineralized areas rarely contained more than 1 to 2 ygMo/L. With the
exception of surface waters that are very acidic and have a high content of
particulate ferric iron, molybdenum occurs principally in the form of a truly
dissolved, filterable species (54).
Normal concentrations in stream sediments, as measured in the -80 mesh
fraction, are reported to be in the range of 1 to 5 ppm Mo. Concentrations
ranging from 10 to 200 ppm Mo have been found in sediments from streams that
drain relatively undisturbed natural deposits of molybdenum in the United
States (38). Sediments derived from black marine shales in England may con-
tain as much as 300 ppm Mo (39). The concentration of molybdenum in stream
sediments has been shown to increase with decreasing grain size (38). Thus,
stream sediments, as opposed to water, do reflect mineralization.
Contamination of surface and ground waters has been documented in several
studies. Kopp and Kroner (55) conducted an extensive sampling of water from
rivers and lakes of the United States from 1962 to 1967. Water samples were
taken from 100 sampling stations in the vicinity of highly populated areas,
industrial areas, recreational use areas, state and national boundaries, and
other potential problem areas. Of the 100 stations sampled, 38 had maximum
concentrations of molybdenum in water greater than 100 ygMo/L and 26 stations
had mean molybdenum concentrations in water samples greater than 50 ygMo/L.
The detection limit for molybdenum in this study was 40 ygMo/L.
There are many industries involved in the production and use of molybde-
num compounds. These industries may be sources of molybdenum contamination
of the environment. The only ones that have been studied in detail are mo-
lybdenum mining, milling, and smelting; but significant contributions of mo-
lybdenum to the environment can result from uranium and copper mining and
milling (56), shale oil production (34), and coal-fired power plants (57).
The concentrations of molybdenum in the aqueous effluents from these sources
can be as high as 850 mgMo/L (a uranium mill) (58), and the release rates can
be as high as 100,000 kg of molybdenum per year (a molybdenum mill) (58).
25
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Table 9 shows data for some waters receiving various industrial effluents. A
more detailed discussion of industrial sources is contained in another part
of this section, Industrial Sources.
TABLE 9. MOLYBDENUM CONTENT OF WATERS AND STREAM SEDIMENTS
CONTAMINATED BY VARIOUS INDUSTRIAL ACTIVITIES
Water Stream Sediment
Mean Range Mean Range
yg/L ppm
Stream below molybdenum
mine and mill (Colo.- 100-10,000 530 50-1,800
1973)
Stream below molybdenum
tailings pile (N. Mex.)
Ground water down gradient r.. 0^_
-- 50,000
from uranium mill (Colo.)
Coal-fired power plant ash
pond effluent (N. Mex.)
Leachate from retorted oil 2,500-8,300
shale (Colo.)
A survey of the finished water supplies of the 100 largest U.S. cities
by Durfor and Becker in 1964 (59) found a median concentration of 1.4 ygMo/L,
with a maximum value of 68 ygMo/L. Hadjimarkos (60) reported that the mean
concentration of molybdenum from 161 sources of finished water in 44 states
of the United States was 8 ygMo/L.
In an epidemiologic study of the relationships between water constituents
and cardiovascular disease, municipal drinking water supplies were sampled in
35 geographic areas of the United States (61). Thirty-three percent of the
3,676 samples tested contained detectable (greater than 1 ygMo/L) amounts of
molybdenum. The highest mean concentration found for an area was 52 ygMo/L.
Only those samples with detectable amounts of molybdenum were included Ln the
computation of area means. The maximum concentration in any sample, 276 yg
Mo/L, was in a sample of tap water collected in Denver, Colorado in 1975 (62) .
Some samples collected in Ohio, West Virgiiia, and South Carolina had maximum
molybdenum concentrations greater than 90 ygMo/L.. Some Nebraska, Kansas,
Florida, Washington, Delaware, Tennessee, and California samples contained
between 20 and 90 ygMo/L maximum molybdenum concentrations.
Tsongas carried out an extensive survey of tap waters in the Denver met-
ropolitan area and in several Colorado mountain communities for our study and
for a previous investigation in 1974-1976 (63). Community tap waters which
were impacted by molybdenum mining and milling operations contained molybdenum
at concentrations between 1 and 500 ygMo/L. Golden, Colorado derives its
26
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water supply from a stream draining a molybdenum mine and mill site, and
during 1971, tap water samples contained an average of 440 ygMo/L. During
late 1974, molybdenum concentrations were still averaging 440 ygMo/L, when
the mill ceased operation. However, by January of 1975, the tap water molyb-
denum concentrations had decreased to about 150 ygMo/L. In June 1975 the con-
centration was found to be 60 ygMo/L. The decrease was probably due to the
spring runoff. The concentration of molybdenum in tap water samples from
Golden during 1977 had a mean of 30 ygMo/L.
Another suburban Denver community receives water from the same source,
but stores the water in a large reservoir prior to finishing and distribution.
The molybdenum concentration of this community's domestic water has decreased
at a slower rate than that of Golden, to approximately 80 ygMo/L by October
1977.
Frisco, Colorado, a mountain community, derives its domestic water from
Ten Mile Creek, which drains another large molybdenum mining area. Silver-
thorne, Colorado receives its water from the Blue River just below Dillon
Reservoir. Tap water molybdenum concentrations in these two communities have
varied between 100 and 400 ygMo/L from 1974 to 1977.
Denver, Colorado draws water for domestic use from Dillon Reservoir at
certain times of the year. The concentration of molybdenum in Denver's tap
water varies considerably, depending upon tap location and time of year.
Since 1975 the molybdenum concentrations in tap waters in Denver have varied
between less than 1 ygMo/L and 80 ygMo/L. Barnett and others (64) reported
molybdenum concentrations in some Denver tap waters as high as 190 ygMo/L in
1969. Greathouse and others (62) report a maximum value for tap waters sam-
pled in the United States of 276 ygMo/L from a Denver tap in 1975.
It can be concluded that concentrations of more than 20 ygMo/L in sur-
face, ground, or tap waters are very likely to be anthropogenic.
PLANTS
Molybdenum plays an important role as a micronutrient for plants. In
1942 molybdenum was discovered to be a limiting factor to clover production
in some Australian pastures (65). Microorganisms require molybdenum for ni-
trogen fixation and for the enzymes which catalyze the reduction of nitrate
to nitrite. Molybdenum is now a common component of fertilizers in many
parts of the United States. Several soil parameters can influence the avail-
ability of molybdenum to plants. These include soil pH, soil moisture, sul-
fate, and phosphate. Molybdenum availability is generally higher at high pH,
low sulfate, high moisture levels, and high phosphate (37,65).
While normal concentrations of molybdenum in plants are 1 to 2 ppm, a
range of tenths of a ppm to hundreds of ppm have been observed (65). Legumes
appear to take up more molybdenum than other plants. In 1938 Ferguson and
co-workers (66) reported that a severe disease in cattle was cause by abnor-
mally high concentrations of molybdenum in forage. When the molybdenum in for-
age exceeds 5 ppm it can adversely affect the health of grazing cattle (39,67),
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The additions of amounts of molybdenum ranging from a few hundredths of
a kilogram per acre per year to several kilograms per acre per year (depending
on various soil parameters, climate, etc.) can significantly increase the mo-
lybdenum content of plants (68). Irrigation with water containing anomalous
concentrations of molybdenum can lead to plant uptakes that could be deleter-
ious to animal or human health if these constituted an important part of the
diet (69). The Water Quality Criteria Committee (70) recommended a maximum
concentration of 10 ygMo/L in irrigation water for continuous use on all soils.
Vlek (71) and Lindsay developed a model for molybdenum uptake that pre-
dicts molybdenum concentrations in plants as a function of molybdenum in ir-
rigation water and other parameters. For a common Colorado soil they found
that the use of irrigation water containing 100 ygMo/L would lead to plant
concentrations toxic to cattle in approximately 15 years.
FOOD
The molybdenum content of food varies with the type of food and the geo-
chemical region in which it is grown. Legumes, cereal grains, leafy vegeta-
bles, liver, and kidney beans are among the foods which usually contain higher
concentrations of molybdenum than fruits, root and stem vegetables, muscle
meats, and dairy products (72).
Deosthale and Gopalan (1) found widely differing molybdenum contents
(0.21 and 1.39 ygMo/gm) in two varieties of Sorghum vulgare Pers. grown and
used as an important dietary staple in some regions of India. Indian rice
was reported to contain about half as much molybdenum as the sorghum (73).
Other investigators have found great variability in the molybdenum con-
tent of foods (74,75). The variation in content was as great in samples of
the same food from within a region as it was between samples of the same type
of food grown in different areas of the world (75).
Tsongas and co-workers (76) in our study determined the molybdenum con-
tent of foods collected in a market basket sampling program which sampled
foods from six major supermarket chain stores in the Denver, Colorado metro-
politan area. There was very little variation found in the molybdenum content
of particular food items from store to store, and no seasonal trends in the
molybdenum content were apparent in samples collected over one-and-a-half
years.
In our study (77) , 10 food items making up the greatest bulk in the
'typical American' diet were sampled most extensively. Of these, 'white1
enriched bread and eggs contained the highest concentrations of molybdenum on
a wet weight basis. Ground beef, iceburg lettuce, and apples were lowest in
molybdenum. Analysis of supplemental samples provided data on the molybdenum
concentration of foods in each of 24 U.S.D.A. food categories (78) (see Table
10).
Data on the average molybdenum concentration of milk samples (whole,
skim, and 2% butterfat) collected during this study are consistent with those
28
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TABLE 10. MOLYBDENUM CONTENT OF FOODSTUFFS
Mean Mo pg/gm wet wt.
Milk, milk drinks 0.06
Cream, ice cream 0.06
Cheese 0.11
Eggs 0.086
Beef 0.04
Pork 0.029
Meat mixtures 0.039
Poultry: turkey, chicken 0.047
Fish < 0.01
Legumes: dry beans, peas, lentils, 1.63
mixtures
Nuts, nut butter 0.02
Fats, oils < 0.01
Breads 0.21
Bakery 0.27
Cereals: cooked and ready-to-eat 0.55
Mixtures: pastas, spaghetti, macaroni 0.41
Tomatoes 0.039
Citrus fruits < 0.01
Other fruits, fruit mixtures 0.036
Dark green vegetables 0.03
Deep yellow vegetables 0.24
Potatoes 0.065
Other vegetables, mixtures 0.15
Sugar, sweets < 0.01
previously reported from Colorado (63,78), California (79), the United Kingdom
(80), Germany (81), and other areas (82) . Average values for milk sampled in
all of these areas varied between 20 and 60 pgMo/L. Several investigators
have found that changes in the dietary molybdenum intake of cows do affect
the molybdenum content of milk (82,83), and the concentrations can vary quite
considerably on a local level. However, when compared with other foodstuffs,
milk does not usually contain high concentrations of molybdenum. It is im-
portant to note, however, that the greatest portion of the dietary molybdenum
29
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intake among children in the United States is provided by milk (76). Milk is
consumed in greatly varying amounts by different segments of the population of
the United States. It is therefore important to consider the trace element
content of foods relative to their contribution to the total dietary intake
when examining the impact of trace elements in the diet. Further information
on the daily intake of molybdenum can be found in Section 8 under Biological
Effects of Molybdenum in Humans.
INDUSTRIAL SOURCES
Coal Combustion
Coal combustion is probably the largest source of molybdenum to the at-
mosphere. It has been estimated that 550 metric tons of molybdenum were emit-
ted due to coal combustion in the United States in 1970, as compared to 900
metric tons of molybdenum emitted from all air pollution sources (80). The
molybdenum concentration in coal varies widely with the concentrations in
western coal ranging from 1 to 15 ppm (57). Mass balance studies indicate a
successively increasing enrichment in the outlet, ashes. A single 1,000 mega-
watt power plant may emit 909 metric tons of molybdenum per year (57).
Kaakinen (57) investigated the molybdenum concentrations and bioavail-
ability in bottom ash and fly ash. He found concentrations ranging from 3 ppm
in the bottom ash to 37 ppm in the fly ash collected by the electrostatic pre-
cipitator. Approximately 15% of the molybdenum in the bottom and fly ash is
available for uptake by plants. Dreesen and others (84) found that approxi-
mately 60% of the molybdenum in the coal ash from one plant in New Mexico
could be extracted from the ash obtained from the ash pond at this plant.
Thus, as the use of coal increases, especially with the impetus to change
to coal from other fuels, the emission, both aqueous and atmospheric, of mo-
lybdenum from this source will become increasingly important. Since the re-
lease into the surface waters of the United States averages about 1,800 metric
tons per year (85), the atmospheric emissions alone due to coal combustion
were approximately one-third the total background value in 1970.
Other energy sources such as oil shale (34) and uranium (58) are also
sources of molybdenum contamination. Some specific instances involving ura-
nium mining and milling are discussed in the part on Uranium Mining and
Milling.
Molybdenum Mining and Milling
Three molybdenum mining and milling operations have been investigated in
detail: the Climax and Urad operations in Colorado and the Questa operation
in New Mexico (38,58). While these facilities dealt with the mining and con-
centration of molybdenum as MoS2, which is very insoluble, sufficient solubil-
ization occurred during the operations to lead to concentrations on the order
of 1,000 to 10,000 ygMo/L in the waters that are decanted from the tailings.
The quantities of water released to surface streams depended on the size of
the operations, on the amount of water which was recycled, and on the quantity
of precipitation and run-off into the tailings ponds.
30
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The largest releases which were seen were those from the Climax mine in
Colorado. Because of large quantities of water from snow melt, the operation
had to release 3.7 to 6.2 billion liters per year (3,000 to 5,000 acre feet
per year) of water from the tailings ponds into the receiving stream. The
total molybdenum release was about 100,000 kg in 1972 (86). The result was a
significant increase ir. trie molybdenum levels in surface waters downstream.
In 1972, Dillon Reservoir, which js about 10 miles downstream from Climax, had
an average molybdenum concentration of about 300 jgMo/L (86). Climax is now
taking steps to reduce its release to about 6,300 kg per year by diverting run-
off water around the '.ai Lings ponds and treating any water that must be re-
leased.
The Urad operation (which ceased production in 1974) and the Questa oper-
ation are much smaller than the Climax operation and have much smaller release
rates. Concentrations of molybdenum in receiving waters for these operations
vary from 560 ]jgMo/L to 1,500 ingMo/L. Since the receiving stream for the
New Mexico operation flows into the Rio Grande and is greatly diluted before
any significant human use occurs, the only concern with this effluent is that
of potential impacts or livestock through irrigation. The release from the
Urad mine did lead to very high Concentrations (400 to 500 UgMo/L) in the tap
water communities downstream. The concentration of molybdenum in these water
supplies has decreased substantially since the operation ceased.
Molybdenum Smelting
The most thoroughly documented study of environmental contamination due
to molybdenum smelting was reported by Hornick and others (41). They inves-
tigated an area surrounding a plant located in western Pennsylvania. Stack
emissions, which were reported at /5 to 100 kg per day, had led to increased
molybdenum concentrations in soil,-:, and plants in the area surrounding the
plant. Soil concentrations in nearby farms were as high as 72 ppm Mo and mo-
lybdenosis WAS reported in some cattle. In addition, one sample of water from
a nearby stream was found to contain 1,000 ygMo/L. Near a molybdenum smelting
plant in the Netherlands forage was found to,contain molybdenum concentrations
as high as 80 ppm (87).
Uranium Minir,g and Mi1ling
Molybdenum is frequently Hound in high concentrations in uranium ores.
As a result jt is frequently a major contaminant in the effluents from uranium
operations. Three cases of substantial molybdenum contamination of the envi-
ronment associated with uranium mining and milling have been reported. These
involved a uranium mill in North Dakota (88), a uranium open pit mine in Texas
(89), and a uranium mill in CoJo>ado (58).
The North Dakota operation involved a plant where a uraniferous lignite
coal was ashed to upgrade the uranium content. Fly ash released from the
plant gave rise to increased soil and forage levels of molybdenum in a nearby
farm and a severe problem of molvbdenosis in cattle.
The Texas case involved open pit mines which intercepted shallow aquifers.
Elevated molybdenum concentrations in surface waters, soils, and forage were
reported and molybdenosis in livestock was documented.
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The Colorado mill began operation in 1953. In 1965 farmers located down
hydrologic gradient from the mill reported a deterioration in the health of
their cattle. A study of the mill and the surrounding area indicated that
leakage from the tailings ponds containing 860,000 ygMo/L contaminated the
underlying aquifer. The water from the farms' wells used for irrigation and
stock water contained as much as 50,000 ygMo/L. Forage taken from the farms
was shown to contain as much as 300 ppm Mo.
Aqueous effluent from some uranium operations in New Mexico are reported
to contain as much as 1,000 ygMo/L (personal communication). Uranium mining
and milling, therefore, can contribute significant amounts of molybdenum into
the environment, particularly as an aqueous effluent which can affect drinking
water quality.
Steel and Copper Milling, Oil Refining, and Claypit Mining
While the largest use of molybdenum products is in steel, very little is
known about emissions from steel plants. One study reports highly elevated
concentrations of molybdenum in forage accompanied by molybdenosis in cattle
downwind from a steel plant (90).
In some copper milling operations, molybdenum is the major contaminant in
the aqueous effluent. Concentrations in the effluent have been measured to be
as high as 30,000 ygMo/L (91).
Since molybdenum-containing catalysts are used in the refining of oil,
oil refineries represent another possible source of molybdenum into the envi-
ronment. One such incident involved a refinery in England which was reported
to have lost 0.45 to 3.6 metric tons of molybdenum per month during 1960 and
1961 (92,93). Elevated molybdenum concentrations in forage, due to these
emissions, resulted in molybdenosis of cattle.
In Missouri a claypit was found to be the source of molybdenum which
caused severe problems in a nearby herd of cattle. Some plants in the area
were found to contain as much as 750 ppm Mo (93).
REMOVAL TECHNOLOGY
The most thorough study of the removal of molybdenum by conventional
water and wastewater treatment was reported by Zemansky (94). In this study
twelve water treatment plants, ten wastewater treatment plants, and two waste-
water tertiary treatment pilot plants were sampled at various times over a one
year period. The samples were analyzed for 19 trace metals including molybde-
num. The removal of molybdenum in conventional plants was very low, averaging
only 15%. The plants studied included the Alvarado Plant in California which
supplies part of the water for San Diego as well as treatment plants in
Boulder, Denver, Golden, and other communi.ties in Colorado. The plant which
had the highest molybdenum concentrations was the Golden, Colorado plant. At
that time the average concentration during the year was 360 ygMo/L in the in-
fluent. _The average removal for the plant was 14%.
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Zemansky concluded that the reason for the consistently low removals of
molybdenum was the fact that molybdenum is present primarily as the molybdate
anion. Since many colloids present in water are negatively charged, sorption
and removal of molybdenum does not occur, except perhaps where an excess of
alum is added (a situation which is generally avoided).
Zemansky also found that most wastewater treatment plants have a very low
percentage removal of molybdenum. For most plants only 4% to 16% removal was
observed. Carbon adsorption was observed to be moderately effective (about
50% removal) at the Lake Tahoe plant.
Runnells and others (95) showed that significant quantities of molybdenum
were removed from solution below an outfall of acid mine drainage. They con-
cluded that the presence of ferric iron and the low pH led to the insolubili-
zation of molybdenum. Zander (96) demonstrated that this technique could be
used to remove molybdenum from industrial waste streams. The process used
involved the addition of ferric iron and subsequent dissolved-air flotation.
Removal efficiencies of better than 99% were obtained. Typical molybdenum
concentration in a treated effluent which initially contained 15,000 ygMo/L
was 110 ygMo/L.
One molybdenum mining and milling operation is using ion exchange to
treat their effluent. They report removal efficiencies of approximately 98%
in treating water containing 6,000 ygMo/L (personal communication).
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SECTION 7
BIOCHEMISTRY AND METABOLISM
BIOCHEMICAL FUNCTION
The biology of molybdenum is a vast topic which is described in the sci-
entific literature in several languages. No attempt will be made here to give
an exhaustive treatment of the subject, especially as several excellent recent
reviews are available (50,82,97). Utilizing these sources a brief outline of
the biological function of molybdenum is presented with special reference to
our approach to molybdenum-related effects in bone, gut, and blood cells.
Molybdenum is rapidly absorbed from food and water by gut and placenta
(98) when present as the moblydate or tricxide, out not as the disulfide, and
it is rapidly excreted via the urine in man (99). Exogenous or endogenous
inorganic sulfate specifically increases molybdeiuir excretion and reduces its
retention in tissues (100). Molybdenum also affects the utilization of
copper—increased molybdenum intake resulting in ccpper depletion and vice
versa (101). The protective action of copper and sulfate or molybdenum toxi-
city has been utilized in the therapeutic administration of copper sulfate to
livestock suffering from molybdenosis (102). Tungstate is a molybdenum antag-
onist and can induce a functional molybdenum deficiency (103). Molybdenum may
mediate the release of iron from intestinal mucosa (104) but the importance
of this effect is being questioned. Molybdenum is widely distributed in the
tissues of the body ranging from approximately 3 pern Mo in liver to approxi-
mately 0.15 ppm Mo in lung, brain, and muscle (82). The skeleton contains
over 50% of the total body molybdenum, presumably cue to the large surface
area of the mineral phase of bone and the possibility of phosphate-molybdate
exchange reactions in hydroxyapatite crystals. Bore;, tendon, and cartilage
abnormalities, as well as osteoporosis (105), have been seen in animals with
molybdenosis. Tooth enamel has appreciable quantities of molybdenum of the
order of 5 ppm and was thought to confei a degree of caries-resistance, but
this has been disputed, and the effects seen are thought to be due to an ef-
fect of fluoride alone (106).
Molybdenum in blood is present in the plasma and is also firmly bound to
red cells (107) , values of less than 5 to 15 ppb being common in the general
population. The homeostatic control of molybdenum is not as efficient as that
for sodium and potassium since molybdenum levels several times the normal
range are observed in industrial workers exposed tc molybdenum (108).
The essential biochemical function of molybdenum is related to the activ-
ities of the enzymes xanthine oxidase (109), aldehyde oxidase, sulfite oxi-
dase, nitrogenase, and nitrate rsductase (32). Molybdenum participates in the
34
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reaction of xanthine oxidase with cytochrome C and facilitates the reduction
of cytochrome C by aldehyde oxidase (110). In milk, xanthine oxidase has two
molecules of FAD, 2g. atoms molybdenum, and 8g. atoms Fe/molecule of protein.
Three isozymes of xanthine oxidase are present in milk. One of these enzymes
(KC>2) contained only molybdenum whereas the other two (KC^a and KC>4b) contain-
ed copper (111). Adaptation to feeding molybdenum or copper occurred by
changing the metal content of these isozymes. Xanthine oxidase in the body
converts hypoxanthine to xanthine and xanthine to uric acid. High intakes of
molybdenum in man (10 to 15 mgMo/day) have been associated with an increased
incidence of uric acid gout. Lower intakes, up to 1.5 mgMo/day, had no observ-
able effect on uric acid excretion but increased copper excretion (108,111).
In order to study the basic chemistry of molybdenum, nonenzymic models
for the nitrogenase system have been proposed utilizing complexes of molybdate
thioglycerol or cystine and Na BH4 acting on substrates of nitrogenase includ-
ing nitrogen (112). The rates of substrate reduction in the model system were
lower but parallel to those of nitrogenase. Nitrogenase reductions, although
exothermic, were accelerated by ATP to a greater extent than by ADP and AMP.
To explain this effect it was suggested that nucleotides form protonated com-
plexes with Moox which catalyzes the hydrolysis of the nucleotide phosphate.
Moox is converted to Mor through a dehydroxylated derivative of Moox. Thus,
molybdenum in the complex is activated by ATP for its role in nitrogenase-
like behavior. The efficiency of this model complex could be enhanced by the
addition of iron in the form of ferredoxin model compounds.
Work of the Colorado Molybdenum Project
The investigators of the present project were faced with the difficult
problem of measuring molybdenum effects at low levels of exposure. Conse-
quently, in order to detect the biochemical effects of molybdenum, very sensi-
tive subclinical tests were devised in the area of bone metabolism, xanthine
oxidase in red blood cells, and adenosine triphosphate (ATP) metabolism in
erythrocytes and platelets.
Bone Calcium Transport--
This model system employs the excised ulna of the rat with periosteum
intact and measures the in vitro transport of calcium 45 from bone to a phys-
iological bathing solution pumped over the bone (113). The rate constants
computed for the mathematical model of this three compartmental system indi-
cated a dose response for rats supplemented with molybdenum at the levels of
10 mgMo/L and 100 mgMo/L in their drinking water (114). The most pronounced
effect on calcium transport related to molybdenum exposure was a significantly
increased loss of exchangeable calcium from the bone at a rate approximately
three times the normal rate at a concentration of 100 mgMo/L in the water.
Bone molybdenum content increased almost eight-fold from 0.71 ppm to 5.3 ppm
of ash. No significant changes were seen in bone calcium and phosphate con-
tents .
The model enables one to study the effects of molybdenum ingestion in
animals at relatively non-toxic levels in water or food but would not be
suitable for lower concentrations--lower than 1 mgMo/L. Application to humans
would also be difficult to standardize and use due to the necessity for a bone
35
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biopsy. Nevertheless, this technique is useful in defining the site of skele-
tal action of molybdenum, and the results were consistent with the osteoporosis
associated with molybdenosis in animals (82).
Human Blood Cell Biochemistry—
The screening tests for humans included: (a) xanthine oxidase activity
of erythrocytes using xanthine and hypoxanthine as substrates, (b) ATP produc-
tion by red cells and platelets from C14 adenine, and (c) total intracellular
adenine nucleotide pools. Xanthine oxidase activity, producing uric acid, ATP
production from C14 adenine, and the total cellular pools of adenine nucleo-
tides and related purines were determined by a combination of thin layer chro-
matography (TLC) and high performance liquid chromatography (HPLC) (115).
Several groups of people were studied ranging in exposure to molybdenum from
workers at a Denver molybdenum processing plant to surrounding communities on
much lower average exposure to molybdenum. These methods are being prepared
for publication.
C 4 Uric Acid Production—
The results of C-*-4 uric acid produced from either xanthine or hypoxan-
thine are reported in Table 11.
TABLE 11. URIC ACID PRODUCTION BY LYSED HUMAN
ERYTHROCYTES USING THIN LAYER CHROMATOGRAPHY
Location
Mo Denver
processing
plant
Summit
Countyt
Marsden
Denver
suburb
Golden
Uric acid
cpm/mL
range x 10" 3
1.5-5.0
2.1-4.4
0.1-2.2
0.1-2.0
1.6-2.7
1.5-8.0
0.1-4.9
4.0-6.0
3.0-5.0
5.5-8.0
Serum Serum Substrate yg Mo
uric acid Mo Ug/L ingest-
mg% range range 105cpm ed/day*
4.4-7.9 18-363 Xanthine 10,000-
Hypoxanthine 50,000
3.6-8.4 5-33 Xanthine
Hypoxanthine 400-500
3.1-9.4 5-13 Xanthine
Hypoxanthine 200-300
Xanthine
5.0-9.0 5-25 Hypoxanthine 300-400
4.1-7.1 5-43 Xanthine
Hypoxanthine 500-600
N
15
23
14
10
8
Estimated from total intake of food, water, and air.
One individual with gout who had a uric acid production of 76x103 cpm/mL RBC
hemolysate was excluded from the range. This was confirmed by HPLC detec-
tion and identification of uric acid.
There was a considerable degree of scatter and no significant correlation
between C uric acid production and serum molybdenum or uric acid concentra-
tion was found within any group.
36
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C14 ATp Metabolism—
Table 12 shows the results of C ATP formation and related substances
from cl^ adenine precursor by red cells and platelet-rich-plasma (PRP).
TABLE 12. RESULTS OF C14 ATP FORMATION
Location
Denver Mo plant*
Summit County
Marsden
Denver suburb
Golden
14 ATP+ADP
AMP
R
1.5-7.8
2-40
4-34
Not viable
Not viable
2.5-11
4.9-14
6.8-12.2
2.0-17
Cells
RBC
PRP
RBC
PRP
RBC
PRP
RBC
PRP
RBC
PRP
Serum Mo : ygMo/L
18-363
18-363
5-33
5-13
5-25
5-43
*
Only 31% of this group had R values in the normal range, whereas over 90% of
all the other groups had normal R values, i.e., 4.5 or higher. In this
group alone a negative correlation of R with serum uric acid concentration
of r = -0.84 was seen (Fig. 3). A plot of R versus log (serum Mo) is shown
in Figure 4. The correlation coefficient here is also -0.84.
When sodium molybdate was added in_ vitro to platelets or red cells at a con-
centration of 0 to 100 ppm no significant change in ATP production or pool
size was seen.
Discussion
Finding methods of suitable sensitivity and capable of adaptation to
field studies on humans was difficult. Nevertheless, by the end of the period
of support considerable skill had been developed in analyzing blood from vol-
unteers in batches of 20 per day. Xanthine oxidase activity in hemolyzed red
cells is low compared to other tissues such as liver or intestine, but is mea-
surable using the isotopic technique. One subject with clinical gout had
greatly increased levels of red cell uric acid production. However, no signi-
ficant correlation of xanthine oxidase with serum molybdenum was found.
With regard to platelet ATP metabolism, the molybdenum smelter workers
had disturbances which were significantly correlated with both serum molybde-
num concentration and uric acid concentration which was elevated in this
group. None of the other groups had significant deviations of platelet metab-
olism. Whether these abnormalities were solely due to the effect of molybde-
num or whether other factors in the working environment contributed to the
37
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8
ATP + APR
AMP
345678
SERUM URIC ACID mg %
Figure 3. R value versus serum uric acid concentration (r = -0.84) .
results could not be determined. However, if plans to reduce the exposure to
molybdenum-containing dust are carried out it should be possible to re-study
these individuals after their blood levels of molybdenum have decreased to
normal.
Conclusions
Sensitive methods for detecting potential environmental toxicity have
been devised.
Significant changes in platelet adenine metabolism were seen in indus-
trial workers processing MoC>3, but not in any of the non-industrial popula-
tions on various molybdenum intakes.
No changes in red cell xanthine oxidase activity were seen in any of the
subjects which could be related to exposure to molybdenum.
38
-------
0.8
0.6
cr
S0.4
0.2
1.0 1.5 2.0 2.5
LOG (Serum Mo >jg Mo/L)
Figure 4. R value versus Log (serum Mo) (r = -0.84)
METABOLISM
For trace elements, little is known about the bioavailability, absorption,
transport, elimination, or movement in and out of tissues (116). Data on
these aspects of the physiology of molybdenum are mostly limited to the solu-
ble inorganic forms such as sodium molybdate (Na2MoO4) (82) and molybdic tri-
oxide (MoC>3) (117,118). Balance and tracer studies have shown that these
forms are readily absorbed in the gut of non-ruminant animals such as rats
(118-122) and swine (98,101,123-125) and that most intake is eliminated in the
urine within 24 hours (Pig. 5). In ruminants, elimination of molybdate is
much slower. Six to seven days are required for elimination of most of a dose
and only 10% to 15% of the dose appears in the urine. The remaining fraction
appears in the feces (116). While most researchers have studied relatively
soluble molybdenum compounds, Fairhall and others (117) also exposed guinea
pigs to dusts of molybdenite (MoS2), a highly insoluble compound. The tissue
concentrations, with the exception of the lungs, of the exposed animals did
not differ significantly from the controls; whereas exposures to dusts con-
taining similar concentrations of molybdic trioxide and calcium molybdate led
to greatly increased molybdenum concentrations in the liver, kidney, and other
39
-------
100
10
1.0
O.I
13 6 9 12 15 18 21 24
Tlm« in hours
Figure 5. Percent of molybdenum-99 dose remaining in the
gastrointestinal tract versus time, and percent
of dose in accumulated urine versus time after
dosing.
tissues. Since the principal form of molybdenum found in natural waters is
molybdate (see Section 3), animal absorption studies using this chemical spe-
cies are useful for determining a drinking water criterion.
Pharmacokinetic studies of sodium molybdate (Na2MoO4) were performed in
this laboratory (119,126,127) using starved rats. The animals were force-fed
doses of molybdenum-99 as the molybdate salt. Eighty-three percent of the dose
was absorbed in the stomach, 13% in the upper small intestine, and 3% in the
lower small intestine. No absorption occurred in the large intestine. These
results agree with the less extensive work of Bibr and Lener (120), and Nie-
lands and coworkers (118). An absorption half-time of 0.88 hours was measured
for both the stomach and the upper portion of the small intestine. The simi-
larity of absorption rates may indicate that the absorption mechanism is the
same in both parts of the gut. Uptake through the stomach wall was also ob-
served by others (120). Our data on the stomach are consistent with passive
movement through intercellular pore spaces.
It may be, however, that absorption in the intestine can proceed by a
different mechanism. Richer!: and Westerfeld (128) suggested that xanthine
oxidase might be a carrier for the transport of molybdenum. They based their
40
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suggestion on data which showed that intestinal xanthine oxidase activity was
correlated with ingested molybdate. We have successfully repeated this work
(129). In addition, we found that xanthine oxidase activity was highest in
the duodenum, the first part of the small intestine, and lower in the ileal
segment. Cardin and Mason (130) performed in vitro studies which showed that
molybdenum absorption occurred throughout the small intestine. The highest
rate was observed in the ileum, the last portion of the organ. Since molyb-
date absorption is highest at the opposite end of the intestine from the re-
gion of highest xanthine oxidase activity, it may be that the function of the
enzyme in the intestine is not as of a carrier for molybdate-.
Sulfite oxidase activity, on the other hand, has been shown to be highest
in the ileal segment (131) but there is no reason to assume that it is acting
as a carrier. The lack of a clear interpretation of all of these studies in-
dicates that much remains to be learned about the complex nature of molybdenum
absorption.
Additional evidence for the high level of absorption of molybdate has
been provided by balance studies both here and in other laboratories (98,101,
118,120,121-125). These studies have shown that between 1% and 27% of the in-
take of molybdenum appears in the feces with the remainder in the urine. This
indicates that most of the molybdenum is absorbed. Some of the inconsistencies
between laboratories and the variability of results may be due to differences
in the amounts of solid food in the diet. Balance studies performed in this
laboratory have shown that larger percentages of molybdenum appeared in the
feces when the animals obtained molybdate mixed into their food (10% to 15%)
than when it was dissolved in their drinking water (5% to 7%). This finding
indicates the importance of performing additional studies in which different
chemical species of molybdenum are supplied to the animals in different
matrices.
Our pharmacokinetic studies have shown that after molybdenum-99, given as
sodium molybdate, is absorbed in the gut, it moves into and out of the blood
very rapidly. No more than 0.2% of the dose was measured in the blood at any
time (126). The kinetic profiles of movement into and out of most tissues
were similar to the profiles observed for blood. These profiles were charac-
terized by a maximum accumulation at three to six hours and then an initially
rapid elimination followed by a much slower loss (see Fig. 6). The liver,
thyroid, and adrenals showed different profiles. Accumulation in the liver
reached a maximum (1.5% of the dose) in the first hour (earlier than most
other tissues) (Fig. 7). After six hours it had declined to 0.7%, a level
which was maintained essentially to the end of the test. At the end of the
test the liver retained more of the dose than any other tissue. This behav-
ior is consistent with the fact that the liver has been found to have the
highest concentration of molybdenum of any tissues in both man (132) and other
non-ruminants (116).
Both the thyroid (127) and the adrenal glands (126) showed secondary up-
take of the molybdenum-99 (Fig. 7). In both glands there was the typical,
initially rapid loss followed by a slower loss up to 15 to 18 hours. At this
time the decrease in accumulated molybdenum was reversed and it then slowly
increased through the remainder of the 24-hour test period. The explanation
41
-------
i.OF
5
0.01
o
*
0.001
Kidn«y
I 3 6 9 12 15
Tim* In hours
18 21 24
Figure 6. Percent of molybdenom-99 dose accumulated
per gram of tissue (wet weight) for kid-
ney and blood versus time after dosing.
of these results is not clear but they agree with those from other laborator-
ies (132) which showed relatively high levels of molybdenum in both these
glands. Such accumulation, when dietary intake of molybdenum is elevated,
might have detrimental effects on the stress response in which the secretions
of both these glands play an important role. Results of experiments which in-
dicate such effects are discussed in Section 8.
Elimination of molybdenum in the urine was very rapid. The kinetics are
biphasic and they closely follow the changes in the blood (Fig. 5) .. Seventy
percent of a dose is eliminated in the first three to six hours. The rate of
elimination then decreases arid 87% of the dose has been eliminated after 24
hours. These percentages are consistent with the results of Bibr and Lener
(120), but they are much higher than those of other researchers (e.g., 118).
It can be seen that molybdenum is eliminated rapidly in the urine after it
has been rapidly absorbed.
To test the possibility of adaptation to high intake levels, molybdenum-
99 was also given to rats which had been on molybdenum in their drinking water
at 0, 10, 100, and 1,000 mgMo/L since birth (126,133). Those on the higher
levels (100 and 1,000 mgMo/L) eliminated the molybdenurn-99 faster than those
on 0 and 10 mgMo/L. This is of interest in view of results shown in Figure 8
42
-------
>
- O.I
»
- 0.01
Llv«r
Adrenals
13 6 9 12 15 18 21 24
Tim* In hours
Figure 7. Percent of molybdenum-99 dose accumulated
per gram of tissue (wet weight) for liver
and adrenals versus time after dosing.
where accumulation is higher at 10 mgMo/L than at 100 mgMo/L indicating that
there is an adaptation to the high intake.
Little is known of the transport of molybdenum in the blood and its stor-
age in tissues. Bibr and Lener (134) tested two of the oxidation states of
molybdenum expected to be found in biological systems. (Molybdenum-V exists
in the molybdate-cysteine complex while Mo-VI is the oxidation state found in
the molybdate ion.) They showed that, at most levels, Mo-V binds to many of
the plasma globulins while Mo-VI binds to almost none. This indicates that
there may be a difference in the way that molybdenum from food (possibly Mo-V)
and from water (Mo-VI) are carried in the blood. If this is true, then uptake,
residence time, and elimination of molybdenum by cells may be different for
the two forms. Furthermore, all of the absorption studies cited above were
performed with molybdate, the inorganic form, but nothing is known about the
bioavailability and absorption of organically bound molybdenum. It is possi-
ble that much of the organically bound molybdenum exists as the molybdate-
cysteine complex, the form in which molybdenum is found in all the known mo-
lybdenum bearing enzymes (135).
Inorganic and organically bound molybdenum may differ in their bioavail-
ability and metabolic behavior. Since these differences may lead to different
43
-------
5 io
o
u
• o
* 0 10 IOO 1000
Mo Concentration in Drlnkbtf Wcttr
mf Mo/L
Figure 8. Weighted average molybdenum concentration
of ten tissues (dry weight basis) from
male rats receiving indicated molybdenum
concentrations in their drinking water.
conclusions regarding potential detrimental effects of excess molybdenum, ad-
ditional studies of various chemical forms of molybdenum should be performed.
The only facts known about the storage of molybdenum is that apparently
large amounts of active and inactive xanthine and sulfite oxidases are stored
in the liver (131). Johnson and Rajagopolan (131) depleted rats of molybdenum
by feeding them tungstate over several wee:ks. During this time liver xanthine
oxidase and sulfite oxidase activities decreased more rapidly than did the
total molybdenum in the liver. This discovery led to the suggestion that
there is an unknown non-protein form in which molybdenum is stored.
In summary, the inorganic molybdate which exists in drinking water is
rapidly and nearly completely absorbed. It moves into and out of most tissues
very rapidly, and it is excreted in the urine at nearly the same high rate as
it is absorbed. Animals receiving elevated levels of molybdate rapidly attain
balance and there is no progressive accumulation of this substance. Much
still remains to be learned of the physiology of molybdenum in humans and
other non-ruminants.
TISSUE DISTRIBUTION IN ANIMALS
Data were gathered on the levels of molybdenum in 1C) selected tissues of
rats given the element chronically at 0, 30, and 100 mgMo/L in their drinking
water. X-ray fluorescence analysis was us.ed to determine the molybdenum
44
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concentration of tissues obtained from adult rats. Figure 8 shows the average
tissue concentration plotted against intake levels. It can be seen that ani-
mals on 0 and 100 mgMo/L in their water exhibit lower tissue concentrations
than animals on 10 and 1,000 mgMo/L. The expected dose-response effect was
therefore not observed. Adaptation to the higher levels has been suggested as
an explanation for this phenomenon (133) . It should be noted that the average
values given here do not represent whole-body burdens but are biased toward
high concentrations because we sampled all tissues which were previously found
to be high in molybdenum. Only two low-concentration tissues, muscle and
brain, were included. Whole body analyses performed in this" laboratory show
smaller differences between the dose groups. However, the same basic pattern
is found. The molybdenum concentrations' found in the tissues of the rats re-
ceiving drinking water containing 10 mgMo/L are similar to those found by
Fairhall and others (111) in guinea pigs which had been given 50 mg of Mo as
by oral administration with a syringe.
The tissue concentrations in the controls were very similar to those seen
by other workers. In general, rats on a normal dietary intake of molybdenum
have a higher molybdenum concentration in their livers than in most other tis-
sues (kidney, spleen, brain, muscle, etc.) (132,136). Similar results have
been found for human tissues. At greatly increased molybdenum intakes the
highest molybdenum concentrations are found in the kidneys of the exposed ani-
mals rather than the livers. This result has also been obtained for guinea
pigs (117) and goats (50) . In addition, Table 13 also shows that there is
considerable uptake by the testes. This result is potentially significant in
TABLE 13. MOLYBDENUM CONCENTRATIONS (PPM DRY WEIGHT BASIS) IN TISSUES OF
RATS GIVEN DIFFERENT LEVELS OF MOLYBDENUM AS
Na2MoO4 IN THEIR DRINKING WATER
Mo in water
mgMo/L
Adrenals
Brain
Muscle
Kidney
Liver
Testes
0
0.57+0.09
(13)
0.2±0.06
(13)
0.1210.6
(20)
1.7±0.3
(21)
2.2±0.12
(13)
0.510.6
(19)
10
19+2.9
(14)
2.410.31
(15)
2.410.8
(16)
5419.6
(15)
15+3.9
(15)
38+5.2
(13)
100
4.310.41
(10)
0.7910.1
(10)
1.110.3
(12)
18±4.3
(12)
4.810.52
(10)
6.911.4
(10)
1,000
4015.7
(10)
5.4+0.6
(10)
7.2+0.35
(10)
77+9.2
(10)
22±4.1
(10)
76+8.5
(10)
45
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view of the fact that increased sterility was observed in the male rats re-
ceiving 1,000 mgMo/L in their drinking water. Molybdenum-induced sterility
has been previously reported in male rats (137,138) and cattle (139). Similar
increases in molybdenum concentration in the ovaries of goats have been report-
ed. The only research which has indicated a possible effect on the reproduc-
tive function in female animals was reported by Schroeder who found signifi-
cantly increased rates of young deaths and dead litters in rats on 10 mgMo/L
in their drinking water (140). These results will be discussed further in
Section 8, Toxicity-Animals.
46
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SFCTION 8
BIOLOGICAL EFFECTS
TOXICITY - ANIMALS
Introduction
While conclusive evidence that molybdenum is essential to animals has not
been reported, there is widespread agreement among researchers that this ele-
ment is indeed essential (67). t'oi. essential elements there are three ranges
of biological effacts- -defioie.^.y, adequacy, and toxicity. While these ranges
have not been well-defined for molybdenum or any other essential trace ele-
ments, Mertz (141) has proposed a tenrntix'e dose-response curve which is shown
in Figure 9. Because of the type of behavior exhibited in this curve, it is
very difficult to study the toxi <,ol og i oai properties of an essential trace
element such as molybJeiMm, zinc, toppej , cobalt, and nickel. Moreover, en-
vironmental exposures ro these element., are rarely at the extreme ends of the
curves, although acute toxacity may occur from occupational exposures. Be-
cause food sources contain \>an able amounts of essential trace elements, and
because the concentrations in plants are deLermined by various geochemical
parameters such as pH, it is reasonable to assume that animals can adapt to
moderate changes in daily intake.
Thus, the problem of defining a sate dietary intake for essential trace
elements such as molybdenum as very difficult. It is clear that the "no in-
take" option is not acceptable. On the positive side, it is also clear that
there is a "level" below which no toxic effects will occur whereas conclusive
evidence for such a "no effect'1 level is tacking for non-essential toxic sub-
stances. Of course, as intake i _, f.utnei i educed deficiency symptoms may
occur. The problem is to define this "no effect" level.
Fribeig and others (SO) recant ly reviewed the research on the toxicity
of molybdenum. One characterjscic of the toxicological properties of molyb-
denum is the great variability fiom species to species both in terms of the
toxic doses and the effect. Cattle aie by far the most susceptible (67),
while sheep are somewhat less tolerant; horses and pigs are the most tolerant
of farm animals. Rats, guinea pigs, and rabbits are intermediate between
sheep and horses.
Livestock
The most common clinical disordei in livestock due to high molybdenum in-
take is known as molybdenosis (or teart, or peat scours in some areas). This
disease is characterized by diarrhea (scouring), discoloration of hair, loss
47
-------
V,
°'o> **>
%
"o
"*
1
8?
r
<
CC
^"2 z
LJ
O
z
o
— O
N
jj
QJ
'0
,q
o
IH
0)
cu
w
r-1
0
a
w
cu
s-i
i
cu
M
O
H
-P
CD
-P
O
a
NOI1DNOJ H11Y3H
48
-------
of appetite (aneroxia), joint abnormalities, osteoporosis, reproductive diffi-
culties, lack of libido, testicular degeneration, and occasionally death (67).
Forage molybdenum concentrations of 20 to 100 ppm Mo can induce these symptoms
(67). The minimum level depends upon the copper status of the animal's diet
as well as several other factors. Normal plants contain 1 to 2 ppm Mo.
It has been shown in cattle and sheep, and some other species, that the
excess molybdenum leads to a copper deficiency that can often be reversed by
dietary copper supplementation (67). Moreover, the amount of molybdenum re-
quired to cause a toxic effect is strongly dependent on the amount of copper
in the diet. Sulfate has also been shown to interact with molybdenum and
copper (67).
Miltimore and Mason (142) have suggested that a copper/molybdenum ratio
of 2:1 represents a critical level and that molybdenosis can be anticipated
if the ratio falls below 2. Alloway (143) , on the other hand, believes the
critical ratio may be closer to 4:1.
At lower levels of molybdenum intake subclinical effects in cattle have
been reported by Thornton and others (144). They nave described the occur-
rence of decreased weight gains, decreased fertility, and delayed maturity in
cattle grazing on forage containing 5 ppm Mo and 10 ppm Cu. These cattle
showed no clinical manifestations of molybdenosis.
Another clinical disorder in livestock (sheep) which has been reported to
be associated with molydenum is "swayback," which is neonatal enzootic ataxia
encountered in lambs (145). This disorder involves the demyeliniation of the
cerebrum in congenital cases and in the spinal cord for postnatally acquired
cases. The mechanism involved in this pathology appears to be an insufficient
production of cytochrome oxidase (146,147). A similar myelin anomaly has been
reported in copper-deficient rats (148).
All ruminants are not as sensitive as cattle and sheep. Concentrations
as high as 1,000 ppm to 7,000 ppm Mo in the diet were required to produce tox-
ic effects in deer (growth reduction)(149). These results demonstrate the
wide variation in toxicity of molybdenum among different species.
Laboratory Animals - Acute Toxicity
Considerable research has been done on the toxicity of molybdenum to gui-
nea pigs, rats, mice, and rabbits. These animals are much less sensitive than
cattle and sheep. As with cattle and sheep, the level at which molybdenum
toxicity is observed depends on the copper status of the animal.
The symptoms vary from species to species. In young rabbits the effects
of molybdenum toxicity are loss of appetite, loss of weight, baldness, and
dermatosis (150). In rats and guinea pigs, loss of appetite, retardation of
growth, loss of body weight, arid sterility have been reported (138,151).
For cattle a lethal dose of molybdenum is on the order of 10 mgMo/kg body
weight (152), whereas for rats (153) it is about 100 to 150 mgMo/kg body weight
and for guinea pigs (117) 250 mgMo/kg body weight. Thyroid function was
49
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drastically reduced in rabbits at 5,000 ppm Mo in the food (154). This concen-
tration was lethal to rats (155). Mortality was also produced in our Labora-
tory by force-feeding rats a daily equivalent of 2,500 ppm Mo (about 50 mgMo/
day at the ingestion rate of 20 roL t^O or 20 g food). Mortality was 100% in
10 days. Acromotrichous anemia was the most obvious effect. Liver and kid-
neys, normally dark red, were almost white in all cases.
Rats exhibit relatively mild effects at lower intakes (118,133) as long
as they are well-cared for. Rats given molybdenum as molybdate at 250 or
1,000 mgMo/L over their lifetime in their drinking water did not exhibit any
severe toxicity .symptoms. Rat pups from dams that b;tcl been on 1,000 mgMo/L
most of their lives were maintained continually on the same intake. This ex-
posure produced stunting, rcugh hair, sterile males,, and some hyperactivity,
but it did not shorten the life span noticeably. P'emales produced pups which
were reduced in size (40% to 50% of normal); however, the number of pups was
only slightly less than from untreated dans (Table 14), The1 maternal perform-
ance of these animals was similar to rats on much Lower levels. The number of
abnormal births, still-born, and resorbed fetuses was within normal limits,
and there were no indications of increased incidence of tumors or teratogenic
effects. Other workers (118) have found similar results for rats on 500 arid
800 ppm Mo in their food. At intermediate dose levels (50, 100, 250 ppm Mo)
growth rates were significantly reduced but not as dramatically as at 1,000
mgMo/L (Table 15) and litters were somewhat smaller than in controls. Other-
wise there were no obvious clinical symptoms of toxLcity,
TABLE 14. EFFECT OF MOLYBDENUM ON LITTER SIZE IN WHITE RATS
Mo in water (mg/L) 0 10 100 1,000
N 17 25 16 6
Pups/litter 11.2 to.5 12.4+0.3 8.6+0.9 10.0^0.56
Range 8 - 15 6-36 2-14 9-12
The most consistent effect of nigri molybdenum intake in rats is the de-
pression of growth or weight loss. Since tnls is usually accompanied by anor-
exia, it has been suggested that the weight loss is due to reduced food intake
rather than some other metabolic process. Some others suggest that the rats
develop an ability to recognize the prese/ice of molybdenum in the diet and
voluntarily reject the food (156). This lypothesis nas boen disputed by
Arrington and others who reported pair-feeding experiments which demonstrated
that reduced food intake was not the primary cause of stunting (155).,
It was mentioned previously that mala rats whose mothers were on 100 to
1,000 mgMo/L in their water and who theru??lves were on the same level showed
a high incidence of sterility. The effect of excess molybdenum on male fer-
tility has been reported earlier in both calves ard rats. In a study reported
by Jeter and Davis (138; Long-Evans rats received varying amounts of sodium
molybdate in their diets. Seventy-five percent of the males which were fed
50
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TABLE 15. REPRESENTATIVE WEIGHTS FOR RATS ON VARIOUS DIETARY REGIMES
~ JT • i ^ • j_
Defined diet
Fully Exposed From weaning
61 days
-
Mo ppm 56 days
0.5 326±12.9 (8) 245±14* (8)
10 267+15.1 (8) 261+13* (8)
Mo in water
mgMo/L
0 251±9.6 (15)
10 202+12* (30)
100 210±14.1* (21)
1,000 108±7.9* (15)
Mo in water
mgMo/L
0
5
10
60 days
250+15.4 (8)
245+10.4 (8)
242±14.5 (8)
76 days
329+6.5* (8)
316±5.9* (8)
312±5.4* (8)
7C
Different from 0 mgMo/L group at p = <.05. Values without * are not statis-
tically different from controls.
from weaning on a diet containing 80 ppm Mo and 140 ppm Mo were found to be
sterile. The limited histological examinations which were performed showed
varying degrees of seminiferous tubule degeneration in the rats receiving high
doses of molybdenum. Thomas and Moss (139) found severe degeneration of the
seminiferous tubules in male calves which were given a diet containing 300 ppm
Mo to 400 ppm Mo (4 to 8 mgMo/kg body weight) over 130 days. The calves were
also reported to show a marked decrease in libido. In a part of Section 7
(Tissue Distribution in Animals) data were presented showing that molybdenum
is readily accumulated in the testes of rats.
Some of the effects of high molybdenum intakes (equivalent to about 100
ppm Mo in the diet or more) that have been reported are summarized below:
Rats Rabbits
Loss of appetite Loss of appetite
Loss of weight Loss of weight
Rough coat Alopecia
Growth retardation Dermatosis
Anemia Anemia
Mendibular exostoses Skeletal deformities
51
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Rats Rabbits
Bone deformities Joint deformities
Histopathology of liver and Decreased thyroxin
kidney disease
Increased liver copper
Increased xanthine oxidase
Increased uric acid
Impaired alkaline phosphatase
and sulfide oxidase activity
Male sterility
Laboratory Animals - Chronic Toxicity
While considerable research has been done on the effects of very high
levels (>100 ppm) of molybdenum in the diet, considerably less work has been
performed on long-term, low level exposure. As noted in Section 5, the con-
centration of molybdenum in water rarely exceeds 1 ugMo/L and the highest lev-
el observed outside of an industrial water was 50 mgMo/L. Thus, it is impor-
tant to study the effects of levels of molybdenum corresponding to perhaps
5 to 10 ppm Mo in the diet.
Schroeder (140,157) has studied reproduction effects in mice receiving
10 mgMo/L in their water and 0.25 ppm Mo in their food. He found significant-
ly increased rates of young deaths and dead litters in this group after three
generations. The controls received the same food with 1 mgMo/L in the water.
His findings suggested the following order of toxicity with respect to repro-
duction :
Hg > Cd > Pb > Se > Me > Ti > Ni > As.
Suttle (158) fed animals a copper deficient diet, thereby depleting their
copper stores. He then supplemented the diet with copper and various levels
of molybdenum and measured the rate of copper repletion. In guinea pigs he
found a significant reduction in response' in the group fed 4.5 ppm Mo in their
diet as opposed to the controls which received 0.5 ppm Mo in their diet.
Moreover, he found that the influence of molybdenum on copper utilization was
not linear. In fact, the first increment, of 4 ppm Mo gave as great a reduc-
tion in copper utilization as the change from 25 to 100 ppm. Thus, small in-
crements seem to have disproportionately large effects. Somewhat similar re-
sults have been found in our own work which is reported below. Suttle sug-
gested that the copper-molybdenum interaction may have a greater significance
for non-ruminants, including man, than has previously been suggested.
In our own work molybdenum at 5 and 10 ppm produced deleterious effects
on some animal functions. No additional animal treatments were required to
produce these effects. However, effects on other animal functions were only
observed when the animals were subjected to stress. The application of stress
to experimental animals was chosen to amplify the effects of subclinical toxic
doses. The working hypothesis is that the potential toxicant will alter the
animals' ability to adapt to an externally applied stress, or that stress will
intensify the otherwise subclinical effect of the toxic substance and clinical
52
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symptoms will be expressed at normally subclinical doses. In addition, stress
produces a measurable response in several physiological activities. Objective
physical measurements simplify the quantification of the response to experi-
mental variables. Examination of the stress response is also useful in re-
lating the laboratory animal experiments to humans since humans are subjected
to a wide range of stresses.
An experimental protocol was developed to test the rats. Several differ-
ent stresses were applied and their effects on various physiological and be-
havioral activities were measured (122). Male, Sprague-Dawley rats were
either (a) raised from dams maintained on identical test diets (fully exposed)
or (b) bought as weanlings (3 weeks old) and started immediately on the test
intake of molybdenum. Physiological and behavioral tests were begun at 7
weeks and continued until sacrifice at 17 weeks. Oxygen consumption was mea-
sured on sleeping rats once each day for four days. After this, behavior and
activity were tested in an Open Field arena (159) once a day for four more
days. Each animal was then subjected to a non-destructive psychological
stress (fear) and the immediate response was measured. The entire sequence
was repeated separately for oxygen consumption measurements and for Open Field
testing. The determination of oxygen consumption proved to be a useful mea-
sure of general body metabolism and health. The behavioral tests were ex-
pected to be sensitive indicators of impaired neural function.
At 15 weeks the rats were subjected to 4 days at 4°C to 5°C, and at 17
weeks they were kept at 37°C for 16 hours and then sacrificed. Tests carried
out and data gathered will be discussed below. By using several different
approaches it was possible to identify some of the subtle effects of excess
molybdenum.
Growth—
The effects of possible toxicants on growth is a common measure of toxi-
city (160). A significant reduction in growth was observed for rats receiving
5 mgMo/L to 10 mgMo/L (in the form of sodium molybdate) in their drinking
water (Table 15). However, these concentrations of molybdate in the animals'
food produced no effect. This difference can be explained by the difference
in total molybdenum ingested. (A rat receiving 10 ppm Mo in food ingests
about 250 jigMo/day while a rat receiving 10 mgMo/L in water ingested 350 to
400 ygMo/day from water and 25 ygMo/day from its food.) Weight losses dimin-
ished as the rats' weights approached 500 to 600 grams. For these larger rats
doses of 1,000 mgMo/L were required for significant weight differences.
Tests on Unstressed Rats--
Oxygen Consumption (V-O^)—The sleeping and awake unstressed metabolic
rates (measured by oxygen consumption) of animals receiving 10 mgMo/L in
water were significantly higher than control animals (no added molybdenum—
see Fig. 10). Oxygen consumption was also elevated in animals receiving 50
mgMo/L and 100 mgMo/L. However, no clear dose-response effect was observed
since the oxygen consumptions were only slightly higher than that for the
10 mgMo/L animals. This result is similar to Suttle's results in this phenome-
non. However, we believe that the toxicity of molybdenum is not simply a
function of the copper to molybdenum ratio as proposed by Miltimore and Mason
53
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6.0
0 0.5
8
10
MOLYBDENUM (mg/L) IN DRINKING WATER
Figure 10. Oxygen consumption in sleeping rats versus molybdenum concentra-
tion in drinking water. The numbers in parentheses are the num-
ber of rats in each test group.
54
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(142), but that it also depends on the dietary copper intake in the sense that
there is a threshold or "adequate" value. When copper intake falls below that
value the animal is much more susceptible to molybdenum toxicity than when the
copper intake is more than adequate. Since the rats which received the higher
levels of molybdenum (50 to 100 mgMo/L) all received adequate copper (7 to 10
ppm Cu), the effect of the additional molybdenum exposure was reduced.
The largest elevation above the controls was obtained with a group of
rats on food low in copper and elevated in molybdenum (2.5 and 10 ppm, respec-
tively) . In these animals, the sleeping V~02 was nearly double that of the
control group receiving 10 ppm Cu and 0.5 ppm Mo (161). In an attempt to ex-
plain this result we examined the fine structure of the adrenal cortex of ani-
mals receiving 0, 10, and 100 mgMo/L in their drinking water. The animals re-
ceiving the highest level showed statistically higher numbers of mitochondria
in their cells when compared to controls. Animals receiving 10 mgMo/L also
had higher mitochondria counts but the numbers were not statistically signifi-
cant (162). Thus, the larger number of mitochondria could result in higher
oxygen consumption.
Open Field—Activity levels (number of squares entered) in the Open Field
were higher for unstressed rats receiving 10 mgMo/L than control animals (see
Fig. 11). No difference was observed for animals receiving 5 mgMo/L. Rats on
10 mgMo/L increased their activity considerably during the last three days of
the five day trials. This was not true for the control rats (163). In Open
Field testing it is commonly observed that rats will settle down to consistent
activity in trials two through five, but molybdenum appears to have modified
this result. This behavior may be the result of the generally higher metabol-
ic rate.
Plasma Glucose—Glucose is mobilized under stress by epinephrine and cor-
ticosteroids. Copper-containing enzymes are important in the synthetic path-
ways of both of these hormones (164). Therefore plasma glucose was measured
as a possible indicator of molybdenum toxicity. The plasma glucose levels for
unstressed experimental animals and controls Were similar (average 155 mg%).
However, stressed animals receiving elevated molybdenum showed significantly
lower plasma glucose levels compared to stressed controls on normal levels of
molybdenum. The largest reductions were observed with animals receiving the
lowest copper levels (2.5 ppm).
Ceruloplasmin--Ceruloplasmin is a copper-containing enzyme which is syn-
thesized in the liver. The enzyme is found in plasma, and under stress it
can be mobilized from liver stores. Because of the known interactions between
molybdenum and copper (165), the activity of this enzyme was measured as a
potential indicator of toxicity.
Molybdenum at 5, 10, and 50 mgMo/L in water had no effect on the cerulo-
plasmin activity in rats eating Lab Chow. This food contains 7 to 10 ppm Cu
and this concentration appears to be adequate (164). Animals receiving a de-
fined diet low in copper (2.5 ppm) and excess molybdenum showed significantly
lower ceruloplasmin activity (p<0.001) (Fig. 12). Animals receiving 10 ppm
Cu and excess molybdenum also showed slight reductions in ceruloplasmin
55
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50
UJ
K
2
Z
tr
40
30
Q
UJ
o:
UJ
UJ
CO
ui
a:
O
CO
20
10
(lOmgMo/L)
I
4
DAY
Figure 11. Unstressed animal activity as measured by the number of squares
entered per minute in an open field arena. The rats received
0, 5, or 10 mgMo/L in drinking water. Eight animals in each
group were tested on five successive days. By convention, the
results of the first two days; were not recorded.
56
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75
£50
en
Q.
O
£ 25
O
0.5 Mo- 10 Cu (mg/Kg)
10- 2.5
r
CONTROL
EFFECT OF 24-HOUR
COLD
COLD STRESS
Figure 12.
Effect of cold stress on serum ceruloplasmin in rats
given molybdenum and copper in a defined diet at the
indicated levels. Vertical bars show one standard
error. There were eight animals in each test group.
57
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activities (p=0.05). These results suggest that copper must be below "ade-
quate" in order for increased molybdenum to measurably reduce ceruloplasmin
activity.
Hematology—The appearance of achromotrichous anemia in rats receiving
elevated molybdenum levels suggested that some hematological factors might be
affected. However, 5, 10, 50, 100, or 1,000 pprn Mo in food or water produced
no significant effects on hematocrit, hemoglobin, or blood cell numbe:rs.
While one study (165) did report anemia in rats receiving 400 ppm Mo in their
diet (as sodium molybdate) other studies (118) have failed to reproduce this
result.
On the other hand, there is a rather consistent picture of hematological
complications in rabbits on high doses of molybdenum. Several authors have
reported decreased hemoglobin and hematocrit in rabbits on rations containing
1,000 to 4,000 ppm Mo (150,165,166).
Tests on Stressed Rats—
Drop Stress--The drop stress is a short-term non-destructive fear stress
which was expected to amplify the effects of molybdenum on sympathetic nervous
system activity via altered secretion of catecholamines. After repeated
free-fall in a cage, the metabolic response consists of a rapid decline in
oxygen consumption followed by a rapid rise to more than double the rate ob-
served for the sleeping animal. After the peak consumption rate is attained
the rate decreases for 15 to 20 minutes until the animal reaches its awake
resting rate. Rats (bred from dams on the same level) exposed to 10 mgMo/L
from conception showed shorter total response times than controls (p<0.05).
No other measured parameters showed significant effects (161).
Open Field Behavior—Rats were subjected to a similar drop stress and
immediately placed in an open field area for a four minute test. The open
field test was repeated (the stress was not repeated) 24 hours later and four
days later to determine any persistent effects of the original stress. The
immediate response to the drop stress for all animal groups was a large re-
duction in activity (squares entered). Tne activity in all groups decreased
to about the same level and no significant effects of molybdenum were found
on any aspects of behavior. There was no indication of memory impairment
when the tests were repeated. Other tests of memory, using a different pro-
tocol, also showed that excess molybdenum had no effect.
Summary of Results—
Some deleterious effects of molybdenum were found at 5 and 10 ppm Mo.
The animals showed reduced growth, increased metabolism as measured by oxygen
consumption, and increased activity as measured by open field tests. Some of
the deleterious effects were brought out by application of stress. In all
cases the excess molybdenum produced a reduction in the animals' stress re-
sponse. No simple relationship between dose and response was observed. Evi-
dence exists for an adaptation at higher molybdenum doses (near 100 ppm) .
Some of the deleterious effects appeared only when the animal was receiv-
ing a diet which was low in copper. Results obtained in these experiments
58
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indicate that the copper:molybdenum ratio is not sufficient for predicting the
response to excess molybdenum. There exists a minimum adequate copper concen-
tration below which relatively small increases in molybdenum intake can in-
duce adverse effects. Excess molybdenum produced no effect on memory or hema-
tological measures such as blood cell counts, hematocrit, or hemoglobin.
The results of the tests performed on the rats are shown in Table 16.
The designations low and high dietary molybdenum correspond to 1 ppm Mo or
less and 5 ppm Mo and 10 ppm Mo, respectively. A plus sign (+) indicates an
increase in the measured parameter and a minus sign (-) indicates a decrease.
A zero (0) indicates no significant difference from control. The effect of
molybdenum on ceruloplasmin activity and plasma glucose concentrations showed
a dependence on dietary copper intake. These variables are indicated where
relevant. (Low copper is 2.5 ppm; higher copper is 7 to 10 ppm.)
TABLE 16. SUMMARY OF TEST RESULTS
Open Field Test
(Activity)
Unstressed Drop stressed
High Mo +
Low Mo Control
Metabolism
(Oxygen consumption: V-02)
Unstressed Drop stressed
High Mo + ++
Low Mo Control ++
Plasma glucose
(Concentration in mg%)
Unstressed Heat stressed
Short Long
term term
„• v. „ n
High Mo 0
^ Low Cu + 0
Low Mo Control
(continued)
59
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TABLE 16. (continued)
Ceruloplasmin
(Activity of enzyme)
Unstressed Drop stressed
High Cu Low Cu High Cu Low Cu
High Mo
Low Mo Control
Summary
The toxicological properties of molybdenum are very complex. No doubt
this complexity is caused in part by the role of molybdenum as an essential
trace element, thus leading to a more complicated picture than is the case for
a substance which does not play such a role.
The most susceptible species are generally ruminants and in particular,
cattle and sheep. Since the mid-1930's when molybdenum was shown to be the
cause of a long-recognized disease in cattle known as "peat scours" or
"teart," many cases of molybdenosis in livestock have been reported. In most
cases the sources of the excess molybdenum were natural. But in some cases,
industrial activity was the cause. In most cases the symptoms--which are nor-
mally diarrhea, emaciation, and male sterLlity--can often be reversed by the
addition of copper to the animal's diet.
The interaction among molybdenum, copper, and sulfate has been studied in
several species. One hypothesis has been that a Cu-Mo-S compound is formed
which renders copper unavailable to the animal (167,168).
Rats, rabbits, and guinea pigs are much less susceptible to molybdenum
toxicity than cattle and sheep. Whereas clinical effects can be induced in
cattle grazing forage containing 10 to 20 ppm Mo (normal plants contain 1 to
2 ppm Mo), 100 to 400 ppm Mo in the diet Ls required to induce clinical mani-
festations in rats. The symptoms vary from species to species and include:
weight loss, growth reduction, skeletal deformities, and sterility in males.
Subclinical effects have been reported in rats, mice, and guinea pigs
receiving dietary levels that are 5 to 10 times normal (the equivalent of
5 to 10 ppm Mo in food). These consist of reproductive effects, effects on
copper metabolism, growth retardation, and a generally lowered response to
stress.
These results indicate that for several species the upper end of the
range of sufficiency or the safe range is about 5 to 10 times the "normal"
level. Since no diets have yet been devised which are so low in molybdenum
as to induce a deficiency, the lower end of the safe range (that is, the mini-
mum daily requirement) is as yet unknown.
60
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HUMAN DIETARY INTAKES AND NUTRITIONAL REQUIREMENTS
It is estimated on the basis of balance studies (169-171) that 30% to 80%
of the molybdenum consumed is absorbed. Based upon urinary excretion of sub-
jects in our studies, we estimate the minimum absorption of molybdenum from
food and water to be between 25% and 30% of the intake. It is not known whe-
ther molybdenum in drinking water is absorbed more rapidly than that contained
in food. Little information is available on the relative amounts of molybde-
num absorbed by humans under varying conditions or the influence upon molybde-
num absorption of other components of the diet, particularly protein and other
trace elements. The discussion on Tissue Distribution in Animals in Section 7
presents information on the absorption of molybdenum which has been derived
from rat and other animal studies.
Water is usually considered to be a minor factor in the intake of molyb-
denum and many other trace elements (60,72,82). If we assume a molybdenum in-
take from food at about 180 ygMo/day (from our work), persons consuming one
to two liters of water per day which contains less than 10 ygMo/L will not de-
rive a significant portion of their daily molybdenum intake from their drink-
ing water. If a person consumes water containing 40 to 50 ygMo/L, such as in
the Denver metropolitan area, then the water contributes one-fourth to one-half
of the daily molybdenum intake. If we assume a food intake of 180 ugMo/L, and
a water supply containing 180 ygMo/L, as in some mountain areas of Colorado,
then the water may contribute one-half to two-thirds of the daily molybdenum
intake. Further information on the amounts of molybdenum present in drinking
water is presented in Section 6, Water.
Based upon the work of a number of investigators, Friberg and others (50)
state that the usual intake of molybdenum via the diet can vary between 100
and 500 ygMo/day.
Investigators in the U.S.S.R. have estimated the daily intake of molybde-
num from the diet at 329 to 376 ygMo/day for adults (172), 156 to 161 ygMo/day
for children (173), and 200 to 500 ygMo/day for children and adults (174).
On the basis of total diet samples from different areas of the United
Kingdom, Hamilton and Minski (175) reported a mean daily intake of 128 ygMo/
day.
Schroeder and others (132) estimated that the average diet in the United
States contained 335 ygMo/day, with a range between 210 and 460 ygMo/day,
based on analyses of samples of complete hospital diets.
Wester (176) examined duplicate samples of the hospital diets of two
patients, during six periods of five days each. Average daily intakes for
each period varied between 250 and 1,000 ygMo/day.
Tipton and Stewart (177) found daily intakes averaging 110, 120, and 460
yg Mo for three subjects in a balance study. Wester (170) studied four
healthy subjects in a 10 day metabolic study. The daily intake of molybdenum
varied between 115 and 245 yg Mo. Robinson and others (171) found a daily
61
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intake in New Zealand of 46 to 96 yg Mo in a metabolic study with a diet based
on meatloaf.
Our study (76) has found the average daily intake of molybdenum in the
United States to vary between 120 and 240 JgMo/day, depending upon age, sex,
and family income. We estimated the average daily intake of molybdenum via
the diet in the United States to be 180 ygMo/day, based on a market basket
sampling program, and a method of estimation using U.S.D.A. published esti-
mates of food consumption (77). Figures 13 and 14 show our calculated daily
molybdenum intake for each age group and the daily molybdenum intake for each
age group per kg body mass. The intake by males was generally greater than
that by females. Persons with a lower annual family income generally had a
higher intake of molybdenum per day, which was assumed to be due to the
greater proportion of legumes and cereals making up their diets.
300-
0
CM
l
I I
ro (O
i i i
CJ IT) 00
Age Group
CM
Figure 13.
Calculated daily intake of molybdenum according to sex
and age groups for the whole United States. Open blocks:
males; shaded blocks: females; cross-hatched areas: male
and female children less than nine years old.
62
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Age Group
Figure 14. Calculated daily intake of molybdenum per
kilogram of body mass, according to sex
and age groups tor the whole United States.
Block legends as in Figure 13.
The proportional contribution of molybderium to the total dietary intake
varies with age because of the differences in the make-up of the diet for
these age groups (see "Food" in Section 6 on the molybdenum content of foods.)
For example, milk makes up the largest contribution of molybdenum to the diets
of children in the United States (approximately 30%), whereas grains and le-
gumes provide most of the molybderium in adult diets in the United States.
Figure 15 illustrates how the contribution of molybdenum to the total dietary
intake from foods in 24 categories changes with age.
Molybdenum's essentiality is related to its role in the molybdenum con-
taining metalloenzymes xanthine oxidase, sulfite oxidase, and aldehyde oxidase.
The tissue concentration of aldehyde oxidase has been shown to be related to
the molybdenum intake in animals, and indirect evidence points to this rela-
tionship with the other enzymes (bO).
Minimum dietary requirements for molybdenum compatible with satisfactory
growth and health cannot be given for any animal species including the human
species. Molybdenum deficiency has not been observed under natural conditions
with any animal species (82). Although a deficiency of molybdenum in plants
63
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AGE-
mifk
cheese
ice cream
beef
pork
poultry
fish
meat mixtures
eggs
beans
peanuts, nuts
citrus fruit
tomatoes
potatoes
cereals
pastas
breads
bakery
fats
sugar
vegetables-dk. green
vegetables- yellow
vegetable mixtures
frmts
Figure 15. Contribution of major food categories to daily molybdenum
intake of male subjects, according to age. The vertical
width of each bar is proportional to the fractional portion
which the food type contributes tc the daily total intake.
Examples: Milk contributes 29% of the daily intake of
young children and only 10% among 75 year old adults.
The contribution from breads increases from 5% to 13% as
the individual's age increases. The age scale shown is
nonlinear but follows the groups shown in the previous
figures. There is a slight decline in bean consumption
among 18 to 19 year old males.
has been implicated in the etiology of human illness in South Africa (178),
human requirements have not been determined. The information available from
balance studies is difficult to assess due to varying amounts of molybdenum
in the diets, and inconsistent results with respect to positive and negative
balance in subjects with differing dietary intakes of molybdenum (169-171) .
Results of some epidemiological and animal studies have indicated that
molybdenum exerts some effect in reducing the incidence of dental caries (179-
183). In some of the epidemiologic studies, the association with a high mo-
lybdenum water supply was not clear,- food sources may have been more important
contributors of molybdenum. Two separate epidemiologic studies in the United
States in California (79) and in Colorado (63) were not able to find evidence
64
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supporting the hypothesis that molybdenum exerts an anti-caries effect. In
the California study, dental caries rates were compared in children living in
areas with and without molybdenosis problems in cattle. In the Colorado study,
dietary intakes of molybdenum among children examined were considered to be
comparable due to similar sources of food supply. No consistent association
could be found between the use of a high molybdenum content water supply and
a reduced incidence of dental caries.
BIOLOGICAL EFFECTS OF MOLYBDENUM IN HUMANS
Deficiency
While molybdenum is considered an essential nutrient, the experimental
induction of deficiency states in animals has been difficult and generally re-
quired the addition of tungsten to the diets, Only recently have molybdenum
responsive syndromes been described in chicks (184) and in goats (185). In
humans, nutritional deficiency of molybdenum, per se, has not been described.
There has been, however, a report of deficient function of sulfite oxidase, a
molybdenum-metallo enzyme, in an infant with neurological abnormalities who
died at nine months of age in a decoricate state (186). There were excessive
amounts of S-sulfo-L-cysteine, sulfite, and thiosulfate in the patient's urine,
concomitant with decreased amounts of inorganic sulfate. Such a pattern could
be explained by a block in the conversion of sulfite to sulfate. Since three
of the patient's siblings had died in infancy with neurological disorders, the
authors postulated that this disorder was an inherited metabolic disease that
they called "sulfite oxidase deficiency,"
Toxicity
Because of the known toxic effects of molybdenum in animals, there has
been concern over possible deleterious health effects of molybdenum exposure
or ingestion in humans. Many of the studies on industrial molybdenum exposure
were performed in the U.S.S.R., the results of which have been summarized in
an excellent critical review by Friberg and others (50).
Early signs of pneumoconiosis were found by Mogilevskaja (cited in ref.
50) in the chest x-rays of a 44 year old woman exposed for five years to mo-
lybdenum and MoO3 in dusts at concentrations ranging from 1 to 3 mgMo/m , and
in a 44 year old man exposed for fo\ir years to concentrations varying between
6 to 19 mgMo/m*. Fully developed radiological findings of pneumoconiosis were
noted in a 34 year old man after seven years of exposure to the latter concen-
tration, where most of the dust particles were below five microns in diameter.
The three subjects, who were among 19 workers in a molybdenum reducing shop,
had variable respiratory complaints.
Molybdenum-induced hyperuricemia was reported by Kovalskii and others in
1961 among inhabitants of a molybdenum-rich province in Armenia (2). In this
large and complex study, 27% of the adult population of two settlements com-
plained of joint pains, particularly of the knees, the interphalangeal joints,
and the. metatarso-phalangeal joints of the feet. Articular deformities,
erythema, and edema of the joint areas were noted; hepatomegaly was present
65
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and renal and gastro-intestinal disorders were also reported but no specific
details given. Hyperuricemiai and hyperuricosuria were demonstrated in 17 sub-
jects, in some "healthy" subjects from the same region, and in five controls
from a molybdenum-poor province. Elevations of blood molybdenum levels were
found in the sick subjects, accompanied b^ decreases in blood copper concen-
trations. Urinary excretion of molybdenum did not differ among the "sick" and
"healthy" subjects from the Armenian province, but excessive urinary excretion
of copper was noted in the sick subjects. The reported control l«;vels of
blood and urinary copper raise some doubt as to the accuracy of the measure-
ments in this study, and the control blood molybdenum concentrations a]so seem
high. Serum xanthine oxidase activity was approximately doubled in the sick
persons, and was noted to be proportional to increments in blood molybdenum
levels. Analysis of the dietary intakes of the inhabitants of the molybdenum-
rich province showed daily consumptions of molybdenum to average 10 to 15 mg
Mo and that of copper to approximate 5 to 10 mg Cu. Outside the molybdenum-
rich province, daily intakes of 1 to 2 mg Mo and 10 to 1'5 mg Cu were found.
While this study lacks adequate control groups and has trace element Vcilues
which are difficult to interpret, the data presented seems to show increased
uric acid levels in blood and urine of the sxibjects living in the molybdenum-
rich province, which may have caused a gouty-type syndrome.
Hyperuricemia is also mentioned in two othejr studies in the U.S.S.R. ac-
companied by arthrolgias, and elevations of serum bilirubin, cholestrol and
globulin levels among workers in copper-molybdenum plants. No precise labor-
atory values were presented, hence the validity of these reports is difficult
to assess (50). Ecolajan, in 1965, examined 500 workers from a molybdenum and
copper mine and compared these workers to a control group of equal size from
the general population in the area. The dust levels in the mines were report-
ed to exceed the maximum permissible concentration (in U.S.S.R., MFC = 6 mgMo/
m ) some 10 to 100 fold. Many workers complained of non-specific symptoms
such as weakness, fatigue, headaches, irritability, poor appetite, stomach
pains, weight loss, irritated skin, dizziness, anci tremor (187). The author
concluded that molybdenum exposure was accompanied by impairment of central
nervous system functions. While the studies in the U.S.S.R. may be difficult
to interpret in a scientific manner, they did stimulate further research on
the effects of molybdenum on human healtl", with specific emphasis on uric acid
and copper metabolism.
Molybdenum enrichment of sorghum hac been rioted in India, arid Deosthale
and Gopalan performed balance studies on four volunteers who were consuming
sorghum-based diets that differed only in their molybdenum content (1). The
molybdenum concentrations in the two diets provided daily intakes of ]60 and
540 yg Mo, respectively, and the authors noted an increase in the mean daily
urinary copper excretions from 24 yg to 42 ygCu/24 hours on the higher' molyb-
denum intake.
They then supplemented the sorghum with 1,000 yg Mo as ammonium molybdate
to provide a daily intake of 1,540 ygMo/day,, whici resulted in a further in-
crease in mean urinary copper excretion '10 77 ;igCa/24 hours. Urinary uric
acid excretion remained unchanged at the three levels of molybdenum intake,
whereas the mean + SD plasma copper levels increased from 80.5 + 11 ygCu/dl on
the low molybdenum diet to 113 ± 3.5 ygCu/dl at the high intake. During the
-------
balance studies, daily copper intakes remained constant at 2.4 mgCu/day, as
did the fecal copper elimination, which averaged 1.83 mgCu/day. The authors
concluded that increases in molybdenum ingestion did not impede copper absorp-
tion, but that molybdenum either prevented cellular copper uptake, or mobi-
lized tissue copper stores, or a combination of both mechanisms. The result
would be an increase in plasma levels of circulating copper with subsequent
increments in urinary copper excretion. They also suggested that changes in
uric acid metabolism would occur only at higher intakes of molybdenum.
Human Health Effects and Present Study
Molybdenum contamination of drinking water supplies occurs in some por-
tions of Colorado as a result of seepage from tailing ponds, at molybdenum
mines, into downstream creeks and rivers (58). In our study levels of molyb-
denum as high as 400 ygMo/L were found in the water supply of the city of
Golden water source which received effluent from the tailings ponds of the
Urad mine. Such high levels were a cause of concern and the U.S. Environmen-
tal Protection Agency sponsored an interdisciplinary study by this group of
the biological effects of molybdenum in humans, with the particular aim of
recommending an acceptable level of molybdenum for drinking water in the
United States. To accomplish this purpose, it was necessary to first deter-
mine normal concentrations of molybdenum in plasma and urine, and biological
samples were collected from subjects in the Denver area, where the water con-
tent of molybdenum does not generally exceed 50 pgMo/L. Since molybdenum was
known to affect copper and uric acid metabolism, the blood samples were also
assayed for ceruloplasmin (a cuproprotein that normally contains 90% of cir-
culating plasma copper) and/or copper content, and serum uric acid levels
were determined. The urinary levels of copper and creatinine were also mea-
sured, the latter being useful to judge the completeness of a timed urinary
collection, and for analysis of uric acid to creatinine and molybdenum to
creatinine ratios. The levels of serum glumatic-oxalecetic transaminase
(SCOT) were measured as an indicator of hepatic function and serum creatinine
and/or blood urea nitrogen (BUN) were also assayed as indicators of renal func-
tion. Medical histories were obtained at the time of sample collections, pri-
marily on male subjects. The known effect of female estrogenic hormones on
plasma copper and ceruloplasmin levels could mask any molybdenum-induced
changes. The results of the biochemical assays performed on these subjects
were then compared to those of subjects with high molybdenum exposure. Such
exposure arose from the presence of the metal in drinking water or in indus-
trial settings
One study of industrial exposure was performed at a roasting plant in
Denver where molybdenum sulfide is converted to molybdenum oxides (45).
Twenty-five workers at the plant responded to a general medical questionnaire
and provided a blood sample. Among these workers seven had no complaints; six
had incurred an upper respiratory infection in the preceding two weeks; six
complained of both joint pains and backaches; another four complained of joint
pains; and another four of backaches alone. Diarrhea was mentioned by five
workers, headaches by four, and non-specific hair or skin changes by eight of
the workers. In general, the complaints were not very specific and there were
no complaints of gout or renal stones among the workers. Pulmonary function
tests (forced vital capacity and forced expiratory volume/second) were normal
67
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in 22 of the 25 workers, but evidence for mild obstructive lung disease was
present in three subjects aged 32, 52, and 59 years.
Total dust samples were collected in the roaster area and an eight-hour
time weighted average (TWA) of exposure to molybdenum was calculated to be
9.47 mgMo/m3. X-ray diffraction studies of the dust showed it to be mainly
composed of molybdic oxide (MoC>3) and other soluble oxides of molybdenum.
Respirable dust samples were collected with a system that limited entry to
particles equal or smaller than 10 microns in diameter. The molybdenum con-
centration of respirable dust at the base of the roaster was 1.02 mgMo/m3 of
air. Since a worker breathes an average 10 m3 of air per eight-hour shift and
the particle size was sufficiently small to allow penetration into at least
the upper airways, the minimum daily body burden of molybdenum was calculated
to be 10.2 mg MO.
The biochemical assays performed on the workers were compared to levels
obtained from a control group, which at that time, consisted of research per-
sonnel at the University of Colorado Medical Center. The complete blood
counts were normal but the industrial workers demonstrated significant in-
creases in serum ceruloplasmin and milder increases in serum uric acid levels
(Table 17) .
TABLE 17. SERUM CERULOPLASMIN AND URIC ACID CONCENTRATIONS IN MOLYBDENUM
FACTORY WORKERS AND IN CONTROL MALES (MEAN ± SE)
Workers
Controls
p- value
Number
(25)
(24)
Ceruloplasmin
(mg/dl)
50.47 ± 1.38
30.50 + 1.33
<0.005
Uric acid
(mg/dl)
5.90 ± 0.24
5.01 ± 0.25
<0.025
Fourteen of the 24 control subjects had plasma molybdenum concentrations less
than 5 ygMo/L which was the lower limit of detection. In the 10 remaining
control subjects the plasma molybdenum levels ranged from 5 to 34 ygMo/L. In
contrast, among the factory workers, plasma molybdenum concentrations ranged
from 9 to 365 ygMo/L, and there were no samples below 5 ygMo/L. Only 10 of
the 25 samples were within the range of the control; levels of plasma molybde-
num from 35 to 100 ygMo/L were present in seven subjects, from 101 to 299 yg
Mo/L in five and greater than 300 ygMo/L in three. These differences were
highly significant (p<0.005) by the Wilcoxon Rank Sum Test (see Fig. 16).
Fourteen of the industrial workers provided urine collections which un-
fortunately were incomplete, as determined by calculation of the daily urinary
creatinine excretions. Urinary molybdenum content was hence expressed in yg/L
and for control subjects the range was from 20 to 230 ygMo/L. In the samples
from the workers only two were within the normal range. Levels from 450 to
1,000 ygMo/L were present in seven subjects, from 1,000 to 3,000 ygMo/L in
68
-------
16
14
CO
0 12
2" 10
CO
^ 8
O
i- 6
CD
.a
E 4
Z
2
™
-
"
-
-
-
-
^
1 1 Control
1-&&&I Factory Workers
n n F^1
r" ^ ?t
i i
1 1 iri 1
11-20 21-30 31-40 41-70 71-99 100-199 20Q-299 300-400
Plasma Molybdenum
<5 5-10
Figure 16. Plasma molybdenum concentrations
of control subjects and of workers
in a molybdenum smelter in Denver.
four, and 11,000 ygMo/L in one (see Fig. 17). The urinary copper excretion
was normal (less than 50 ygCu/L) in 13 of the 14 subjects, but hypercupruria
(347 ygCu/L) was present in the last sample. The urinary uric acid/creatinine
ratios were normal (i.e., less than 0.75) in the 14 workers.
This pilot investigation demonstrated absorption of molybdenum from dust
particles with subsequent excretion by the kidneys. It could not be deter-
mined whether molybdenum was absorbed through the lungs or by the gastro-
intestinal tract after swallowing of secretions. The wide range of plasma
molybdenum levels in the workers may be explained by the time of day when the
samples were taken. For some workers, the samples were taken prior to the
eight-hour shift, others in the middle of the work day, and some at the end
of the work day.
The increments in mean serum uric acid levels of the Denver factory work-
ers were not as marked as those described by Kovalskii and others (2) and hy-
peruricosuria was not found in our study. The increases in mean serum cerulo-
plasmin levels among Denver factory workers were paralleled by proportional
changes in plasma copper content, and hence are in discordance with the
Russian author's findings of decreased levels of plasma copper. Elevation of
serum ceruloplasmin could be expected, however, according to Deosthale and
Gopalan's hypothesis (188), if molybdenum exposure leads to mobilization of
tissue copper stores within the hepatocyte, with subsequent synthesis of ce-
ruloplasmin to prevent intracellular copper toxicity (164).
This pilot investigation in factory workers raised the question of whe-
ther serum ceruloplasmin levels could be used as an indicator of excessive mo-
lybdenum exposure. While the mean uric acid levels were also significantly
69
-------
11,000
10,000
/"
X.
3,000
1 2,000
^ 800 -
° 600
=3 400
200
Figure 17. Urinary concentrations of 14
workers in a molybdenum smelter.
higher than in the control group, the mean value still remained within the
normal range.
Industrial exposure to molybdenum dusts also occurs in the mining indus-
tries, and we collected samples in two groups of workers at molybdenum mines
in Colorado and New Mexico. Dust samples were taken at various levels in a
molybdenum mine by Rafael Moure who measured dust levels in various sections
of the mine. The levels ranged from 0.32 to 1.32 mgMo/m3, eight-hour time
weighted average (TWA) of respirable dust and were hence much lower than in
the factory setting. These values were compounded from 76 personal dust sam-
ples collected during the investigation. Fourteen of the samples were ana-
lyzed for MoS2 content which varied from 0.2 to 0.49 ygMo/m3. The estimated
net daily exposure ranged from 1 to 368 ]jgMo/day. Blood samples were obtained
from 16 workers at the Climax mine, most of whom were involved with the mil-
ling operation. The mean ± SE (standard error) ceruloplasmin level in these
subjects was 42.31 ± 2.54 mg/dl and the mean plasma copper levels were 129.38
± 5.47 ygCu/dl. Both mean levels were above control values for males. The
mean serum uric acid concentration was 6.5 ± 0.40 mg/dl for 15 subjects, one
having been excluded because he was taking medications known to affect serum
uric acid concentrations. Plasma molybdenum concentrations were below 5 ygMo/
L in 12 of 15 samples assayed and were 6, 9, and 28 ygMo/L, respectively, in
the remaining three samples. Urine collections were obtained in six of these
workers and the urinary molybdenum concentrations ranged from 26 to 85 ygMo/L
70
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with the urinary copper levels varying between 5 to 12 ygCu/L. Calculations
of the diurnal creatinine excretion showed low values in four of the samples.
In the remaining two, which appeared adequate, the 24-hour urinary molybdenum
excretion was 41 yg and 48 yg, respectively. Urine uric acid to creatinine
ratios were normal.
Blood samples were also obtained from 18 miners at the Questa mine in New
Mexico. The mean ± SE serum ceruloplasmin for 18 Questa miners was 40.94 ±
1.66 yg/dl and the mean serum uric acid was 6.09 ± 0.27 mg/dl for 15 subjects.
The uric acid levels of three miners were not included in the calculation be-
cause one had long-standing gout (serum uric acid: 10.1 mg/dl) and two others
were taking antihypertensive medications (serum uric acid: 8.8 and 10.2 mg/dl).
Plasma molybdenum levels were below 5 ygMo/L in 12 of the 18 samples, and
ranged from 6 to 18 ygMo/L in the remaining six samples. Because of technical
difficulties it was not possible to collect timed urine specimens in the
Questa miners, but aliquots of urine were provided by 11 miners. The uric
acid to creatinine ratios were normal. The urinary molybdenum concentrations
varied from 20 to 74 ygMo/L and the molybdenum to creatinine ratios (urinary
molybdenum in yg/L/urinary creatinine in mg/L) ranged from 0.02 to 0.05.
The results obtained in the miners differed from those of the factory
workers in various aspects. The plasma and urinary concentrations of molybde-
num were uniformly low in the miners, which might be expected, since the sub-
jects were mainly exposed to molybdenite (MoS2), a compound known to be rela-
tively insoluble. In contrast, the serum uric acid levels were higher among
the miners, but this hyperuricemia could be a consequence of polycythemia
secondary to altitude. Leadville, where the Climax miners reside, is situated
at 10,000 feet above sea level, and while the altitude of the Questa village
approximates 7,000 feet, the actual mine location is around 9,000 feet.
The mean serum ceruloplasmin levels in the miners were higher than in the
Denver control subjects, but did not attain the levels demonstrated by the
workers at the molybdenum roasting plant. Since no evidence for excessive
molybdenum exposure was present, the question of whether altitude might also
affect serum ceruloplasmin levels was entertained and a decision was made to
collect samples from subjects living at high altitudes and not involved in
molybdenum mining operations. The necessity for caution in the interpretation
of serum ceruloplasmin and uric acid levels, as indicators of molybdenum ex-
posure, became evident with these studies, while at the same time, the relia-
bility of plasma and urinary molybdenum values increased.
In addition to these studies of industrial exposure, molybdenum is pre-
sent, as previously mentioned, in some drinking water supplies of communities
in Colorado. The highest quantities of molybdenum used to be found in the
water of the city of Golden where levels of 300 to 400 ygMo/L had, on occa-
sions, been measured. However, cessation of the operations at the Urad molyb-
denum mine resulted in decreased contamination of the Clear Creek river, and
the levels of molybdenum in Golden water supplies have been decreasing since
1974. To determine whether molybdenum in water was accompanied by any health
effects or changes in biochemical indicators, we studied various populations
whose water supplies differed in molybdenum content. As controls a group of
subjects were selected from the Denver area, where the molybdenum in water
71
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fluctuates from 1 to 50 ygMo/L. Blood samples were collected on 42 male sub-
jects whose age ranged from 19 to 46 years. The mean ± SE serum ceruloplasmin
level for the group was 30.41 ± 1.03 yg/dl and the mean serum uric acid was
5.34 ± 0.20 yg/dl, both levels being in accordance with published normal val-
ues. Plasma molybdenum concentrations varied from less than 5 ygMo/L in 18
subjects to 34 ygMo/L. Fourteen subjects had levels from 5 to 9 ygMo/L; lev-
els from 10 to 19 ygMo/L were found in seven persons, from 20 to 29 ygMo/L in
one, and the two remaining subjects had plasma molybdenum concentrations of
33 and 34 ygMo/L, respectively (see Fig. 18). Red blood cell molybdenum con-
tent varied between less than 5 to 38 ygMo/L. Twenty-four hour urine collec-
tions were performed on 14 controls, for which complete results are available
on 12 subjects. The urinary uric acid to creatinine ratios were normal in
all subjects. The molybdenum concentration varied from 120 to 230 ygMo/L,
with the mean 24-hour excretion of molybdenum being 87.25 ± 18.02 yg (SE) .
Urinary copper concentrations varied from 4 to 25 ygCu/L, the mean daily cop-
per excretion being 12.70 ± 1.50 ygCu/24 hours. Urinary molybdenum to creati-
nine ratios ranged from 0.01 to 0.11.
Since the levels of molybdenum in the water of the city of Golden were
decreasing, a sample collection was organized in March 1975, at which time the
water levels of molybdenum approximated 200 ygMo/L. Blood samples were drawn
on 13 university students at the School of Mines and the mean ± SE serum
ceruloplasmin was 40.31 ± 2.55 mg/dl, a level significantly higher (p<0.005)
than for the control group. The mean serum uric acid was 4.35 ± 0.46 mg/dl,
significantly lower than for the control group. The mean daily molybdenum
excretion in four 24-hour urines was 186 + 34 yg, also significantly higher
than for the control group (p<0.01), and -he mean daily urinary excretion of
copper was 11.63 ± 4.02 ygCu; the latter value did not differ from the control
group.
Another sample collection was organized in the Golden area in September
1977, by which time the water content of molybdenum had dropped to approxi-
mately 40 ygMo/L. Blood samples were collected again in 13 students and a
comparison of the serum ceruloplasmin and uric acid levels and of the urinary
copper and molybdenum excretions is shown in Table 18.
The decrease in molybdenum water content in the Golden area was accom-
panied by normalization of the serum ceruloplasmin levels and by a decrease in
the mean daily urinary molybdenum excretion. The increase in mean serum uric
acid levels demonstrated by the students in 1977 has to be interpreted with
caution, since at the time of collection, it was evident that many of the stu-
dents had been drinking beer, and alcohol is known to increase serum uric acid
levels through inhibition of renal secretory mechanisms. The students were
asked to estimate their daily intake of all beverages and the estimated quan-
tities varied between two to three liters per day, of which approximately one-
half were water or beverages made from water. Hence, the daily contribution
of molybdenum water in 1975 would have approximated 300 yg. Calculations of
the molybdenum intake from foods show that for 20 year old male subjects
the daily ingestion approximates 210 yg, so the total molybdenum ingested in
food and beverages in 1975 would have reached 500 yg daily. At such a level
of intake, the serum uric acid levels were not affected, serum ceruloplasmin
concentrations increased, arid plasma molybdenum levels were within the normal
72
-------
20
1 8
16
(A
*- i .•
0 ' 4
M_ 10
o
« 8
.O
1 •
z
4
2
-
-
-
-
-
-
-
•••MM
•M^Ml
nl
<5 5-9 10-19 20-29 30~40
Plasma Molybdenum (pg/l)
Figure 18. Histograms of plasma molybdenum
concentrations in 42 normal sub-
jects from the Denver area.
TABLE 18. COMPARISON OF SERUM CERULOPLASMIN AND URIC ACID LEVELS
AND OF THE URINARY EXCRETIONS OF MOLYBDENUM AND COPPER
IN 1975 AND 1977 IN THE GOLDEN AREA (MEAN ± SE)
1975
(N)
1977
p-value
Serum ceruloplasmin
(mg/dl)
Uric acid (mg/dl)
Urinary molybdenum
(yg/24 hrs)
Urinary copper
(yg/24 hrs)
(13) 40.31 ± 2.55
(13) 4.35 ± 0.46
( 4) 186 ± 34
( 4) 11.63 ± 4.02
(13) 34.15 ± 1.74 <0.05
(13) 5.88 ± 0.27 <0.005
(8) 88 ± 10 <0.005
( 8) 14.20 ± 1.62
N.S.
Number of subjects
73
-------
range. Twelve of the 13 subjects in 1975 had plasma molybdenum concentrations
less than 5 jjgMo/L. The urinary copper excretion did not seem to be affected
by the elevated levels of molybdenum in the water. In contrast, the urinary
excretion of molybdenum was higher, thus indicating renal clearance of the ex-
cess molybdenum ingested.
Another occasion to compare two populations with different water concen-
trations of molybdenum occurred in the metropolitan Denver area. The molybde-
num concentrations in water of a Denver suburb fluctuated in 1977 between 80
to 100 ygMo/L while the levels in the city of Denver were generally less than
40 ygMo/L. Blood and urine samples were collected from 13 workers at the
suburban municipal water treatment plant and comparison was performed with the
results obtained from samples collected from 16 workers at a Denver area water
treatment plant (Table 19). The plasma molybdenum levels were within the nor-
mal range in the workers from both area treatment plants. Serum ceruloplasmin
levels were similar in both groups, and there were no significant differences
in the daily urinary excretion of copper, which was higher in the workers from
the suburban treatment plant.
TABLE 19. COMPARISON OF SERUM CERULOPLASMIN AND URIC ACID LEVELS
AND OF THE URINARY EXCRETIONS OF MOLYBDENUM AND COPPER
IN TWO GROUPS OF WORKERS AT WATER TREATMENT
PLANTS (MEAN ± SE)
(N) Denver (N) Suburban area p-value
Serum ceruloplasmin (16) 38.69 ± 1.59 (13) 38.46 ± 0.32 N.S.
(mg/dl)
Serum uric acid (16) 5.74 ± 0.46 (13) 6.25 ± 0.32 N.S.
(mg/dl)
Urinary molybdenum (9) 88 ± 19 (9) 97 ± 10 N.S.
(yg/24 hrs)
Urinary copper ( 8) 12.63 ± 1.65 (10) 22.9L ± 1.90 <0.005
(yg/24 hrs)
An occasion to compare another group of subjects with different concen-
trations of molybdenum in water arose from a unique situation existing in
neighboring communities in the Summit County area of the Rocky Mountains. In
the water supplies of Breckenridge and Dillon, only traces of molybdenum have
been found whereas in Frisco and Silverthorne, levels between 100 to 400 ugMo/
L have been repeatedly measured by this group. Furthermore, these communities
are situated at an altitude of 9,000 feet and hence it is possible to deter-
mine whether altitude caused the changes in serum uric acid and/or ceruloplas-
min levels found in the mines.
Blood and urine samples were collected from 28 male and female inhabit-
ants who had resided in the area for at least two /ears. Nine male and eight
74
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female subjects were from the high-molybdenum areas, and five males and six
females served as controls from the low-molybdenum areas of Breckenridge and
Dillon. The results of biochemical assays performed on the male subjects are
shown in Table 20; no significant differences were found. The plasma molybde-
num levels in subjects from both sexes and from both drinking water areas were
within the normal range. Corresponding results of biochemical tests in the
females are shown in Table 21, where, interestingly, the female residents from
the high-molybdenum area excreted significantly more molybdenum in the urine.
TABLE 20. COMPARISON OF SERUM CERULOPLASMIN AND URIC ACID LEVELS
AND OF THE URINARY MOLYBDENUM AND COPPER EXCRETIONS IN
RESIDENTS OF SUMMIT COUNTY (MEAN ± SE); MALES ONLY
Low molybdenum
in water
(N) (10 yg/L)
High molybdenum
in water
(N) (140 yg/L) p-value
Serum ceruloplasmin
(mg/dl)
Serum uric acid
(mg/dl)
Urinary molybdenum
(yg/24 hrs)
Urinary copper
(yg/24 hrs)
(5) 41.00 ± 0.71
(5) 6.58 ± 0.32
(4)
72 ± 14
(4) 14.95 ± 1.59
(9)
(9)
(6)
37.22 ± 2.58 N.S.
6.68 ± 0.33 N.S.
112 ± 32
N.S.
(6) 23.96 ± 6.30 N.S.
TABLE 21. COMPARISON OF SERUM CERULOPLASMIN AND URIC ACID LEVELS
AND OF THE URINARY MOLYBDENUM AND COPPER EXCRETIONS IN
RESIDENTS OF SUMMIT COUNTY (MEAN ± SE) ; FEMALES ONLY
Low molybdenum
in water
(N) (10 yg/L)
(N)
High molybdenum
in water
(140 yg/L) p-value
Serum ceruloplasmin
(mg/dl)
Serum uric acid
(mg/dl)
Urinary molybdenum
(yg/24 hrs)
Uninary copper
(yg/24 hrs)
(6) 49.33 ± 4.03
(6)
(5)
5.10 ± 0.17
57 ± 9
(5) 8.71 ± 1.28
(7)
(8)
(8)
(8)
49.57 ± 3.41
100 ± 14
N.S.
4.63 ± 0.28 N.S.
0.025
22.61 ± 7.24 N.S.
75
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An estrogen effect on serum ceruloplasmin concentrations is possible, as is
the protective effect of female hormones on uric acid levels. Also of inter-
est was the comparison between the serum levels of ceruloplasmin and uric acid
obtained in Summit County to those obtained in the Denver area control groups.
Significant elevations of serum ceruloplasmin (p<0.005) and of uric acid
(p<0.025) were noted in the male residents of the low-molybdenum Breckenridge
area when compared to the Denver subjects. Similar significant differences
were present for the high-molybdenum Frisco area residents, when compared to
Denver controls. Furthermore, the urinary excretion of copper in male Frisco
residents was significantly higher than in the Denver subjects (Table 22).
TABLE 22. COMPARISON OF BIOCHEMICAL ASSAYS IN MALE SUBJECTS FROM
THE DENVER, BRECKENRIDGE, AND FRISCO AREAS (MEAN ± SE)
Water molybdenum
content
Serum ceruloplasmin
(mg/dl)
Serum uric acid
(mg/dl)
Urinary molybdenum
(yg/24 hrs)
Urinary copper
(yg/24 hrs)
(N)
(42)
(42)
(12)
(12)
Denver
0-50 ygMo/L
30.41±1.03
5.34±0.20
87118
12.70+1.50
(N)
(3)
(5)
(4)
(4)
Breckenridge
20 ygMo/L~
41.0010.71*
6.58+0.32*
72+14
14.95+1.59
(N)
(9)
(9)
(6)
(6)
Frisco
150-200 ygMo/L
37.22+2.58*
6.68+0.33*
112+32
23.96+6.36*
Significantly higher than for the Denver control values (p<0.025 or less)
The results of these different investigations point out the difficulties
of assessing molybdenum exposure with indirect indicators, i.e. , serum cerulo-
plasmin and uric acid levels. ' While serum ceruloplasmin levels seemed to be
affected by molybdenum exposure in the molybdenum factory workers and in the
1975 Golden area students, ceruloplasmin levels were also affected by gender,
altitude, and occupation. Similarly, the potential effects of molybdenum on
serum uric acid values could be masked by age, gender, alcohol intake, and al-
titude. With so many variables, it became evident that the best indicators of
molybdenum exposure were the plasma and urinary levels of the metal. It was
encouraging to find levels of plasma molybdenum within the normal range (5 to
34 ygMo/L) among subjects who were consuming water with 200 ygMo/L. Increased
ingestion was generally accompanied by increased urinary excretion of molybde-
num (see Fig. 19), a finding noted in the Golden area students, the residents
of Summit County, and the molybdenum factory workers. In the latter, workers
where molybdenum exposure was far greater than those in any of the other situ-
ations, the plasma levels and the urinary molybdenum to creatinine ratios were
elevated. Renal clearance of excess molybdenum thus seems to function as a
protective homeostatic mechanism in humans as it does in animals, Excessive
76
-------
o
o
— 200
o>
a.
c
0)
XI
150
100
o
-§ 50
<250 200-300 300-400 400-800 >600
jjg Molybdenum Injested Daily
Figure 19. Comparison of the daily urinary molybdenum
excretion (mean ± SE) in male subjects with
different daily intakes from food and water.
Summit County I Breckenridge
Summit County II — Frisco
Golden 1975 Students in 1975
urinary copper excretion was not demonstrated in the present studies, even
when significant differences in copper excretion were shown. Nor was in-
creased urinary uric acid excretion found in the various samplings performed.
Since no biochemical changes were found in subjects consuming water containing
up to 50 ygMo/L, it can be assumed that such a level does not cause any ad-
verse biochemical or health effects. Biochemical changes were seen in some
subjects at 100 ygMo/L. Itie argument is further corroborated by the observa-
tions in the Golden area students where the changes in molybdenum water con-
centrations were accompanied by normalization of serum ceruloplasmin levels
and of urinary molybdenum excretion.
77
-------
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151. Arthur, D. Interrelationships of Molybdenum and Copper in the Diet of
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168. Mills, C.P., et al. Mechanisms of the Molybdenum Sulfur Antagonism of
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186. Irrevere, F., et al. Sulfite Oxidase Deficiency: Studies of a Patient
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APPENDIX A
SUPPLEMENTARY CALCULATION OF GUIDELINE
The guideline of 50 ygMo/L for human drinking water is based upon animal
and human studies described in the preceding sections. In order to "check"
this guideline, we have performed the following calculations based upon die-
tary intakes. While the validity of the initial assumptions necessary for the
calculations may be argued, we believe that the calculations provide a useful
order of magnitude check on the guideline.
Early humans probably did not have a "balanced" diet since availability
of game and vegetable stuffs was variable. The hunter-gatherers probably ex-
isted on a vegetarian diet for short periods of time and on a meat diet at
other times. These short-term fluctuations of food intake probably resulted
in short-term deficiencies and excesses of trace elements (see Table 10, Sec-
tion 6). Some level of adaptability to variable intakes probably still exists
in humans today. Modern food distribution practices and dietary habits tend
to minimize the fluctuations of trace element intakes. However, some differ-
ences in food consumption exist between the sexes and among the various age
groups. Socioeconomic factors and dietary preferences also lead to different
intakes. Since there is no evidence that severe deficiencies or excesses
occur in the United States' population, we may assume that the biological and
socioeconomic differences are within the adaptability of humans.
Table A-l shows the variability of dietary intake from foods for several
biological and socioeconomic groups. These results were obtained by calculat-
ing the maximum difference in daily molybdenum intake for the various groups.
(Percentages are based on the average intake of the two groups being compared.
See Figures 13 and 14, Section 8.) These calculations show that the maximum
intake difference observed between the sexes or with age is about 50%. Socio-
economic variables produce differences less than or equal to biological varia-
bles. If we assume that the observed biologically-based intake variation of
50% is the maximum perturbation that may be tolerated without observing ef-
fects , then it is possible to calculate the concentration of molybdenum in
water which would produce an equivalent increase.
Using the calculated dietary intakes (Figure 13, Section 8),, and the
availability data on consumption of water-based beverages (coffee, tea, soft
drinks from reference 77) one can calculate the molybdenum concentration in
water which would produce a 50% increase in dietary intake.
The results of these calculations for the average individual in each age
and sex category are shown in Figure A-l. The apparently high concentrations
92
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TABLE A-l. VARIABILITY OF DAILY MOLYBDENUM INTAKE
Differences
Maximum % variation
Biological
(adults only)
Socioeconomic
Other
Age (males) 47
Age (females) 47
Sex (males* vs females) 42
Age (ug/day/kg body weight) 47
Income (low* vs high) 52
Urbanization (urban vs rural*) 15
Geographic (North U.S. vs South U.S.*) 6
Meat vs vegetarian*t 58
Group with higher molybdenum intake.
Vegetarian diet estimated by substituting a proportionate increase of all
non-meat components for meats (estimate only)
CM in co =
i i i •
— IO
^ t± 92
i i
in GO
i
CO
Age Group
m to rj-
6 ih ib jn
cvi ro in «>
Figure A-l. Molybdenum concentration of water required
to produce a 50% increase in daily intake
through water-based beverages.
93
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of molybdenum in water permissible among younger individuals is mainly due to
the small amount of water-based beverages consumed by this group. However,
among adults the predicted molybdenum concentration which would produce a 50%
increase in daily intake approaches 100 ygMo/L. If we assume that water in-
take is about equal to the water-based beverage intake*, then the "permissible"
water concentration would be about 50 ygMo/L. This estimate is the same as
the guideline based upon the studies described in previous sections. The
agreement between the calculated estimate and the guideline may be fortuituous
but it serves as a useful order of magnitude check.
*
The maximum water-based beverage intake of 0.85 liters occurs among males
aged 35 to 54. Doubling this amount leads to a value close to the 2.0 liters
typically used by the U.S. Environmental Protection Agency.
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APPENDIX B
ANALYSIS FOR MOLYBDENUM, SPECTROPHOTOMETRIC METHOD
Thiocyanate has been used extensively in the colorimetric analysis of
molybdenum (13-16). The conventional analytical method has been modified
slightly to suit our needs. Water samples can be routinely analyzed with pre-
cision limits of about ±5 yg/L (without prior concentration steps). This pro-
cedure is outlined below.
ANALYSIS OF WATERS FOR MOLYBDENUM
1. Pipet 50 mL of the unknown or standards into a 250 mL separatory
funnel. (A blank should also be carried through this procedure.
The standards which we use usually range from 50 to 500 yg/L.
Typically we run four: 50, 100, 300, 500 yg/L.)
2. Add 2 mL of concentrated HC1 to the samples. The acid is required
to prepare the solution for the formation of the thiocyanate com-
plexation reaction. This reaction requires that the final solution
be about 1 M in acid.
3. Add approximately 0.2 g of sodium tartrate. This "ties up" any
tungstate which may be present. It would interfere with the
analysis for molybdenum since tungsten also forms a colored com-
plex (yellow) with thiocyanate.
4. Add 0.5 mL of 1% ferrous ammonium sulfate. Prepare this solution
by weighing out 1 g of the reagent grade Fe (N^SO^) 2- Dissolve
this salt in 100 mL of deionized water and 1 mL of concentrated
sulfuric acid.
5. Add 3.0 mL of 10% KCNS. Prepare this solution fresh daily by
weighing 5.0 g of the salt and dissolving it in 45 mL of deion-
ized water.
6. Allow the solution to stand for about 15 minutes. This step is
very important. Full color will not develop if time is not given
for the complexation reaction to be completed. The solution will
be pink, orange, or red.
7. Add 9.0 mL of 10% stannous chloride solution. Prepare this by
weighing out 100 g of the salt and dissolving it in 125 mL of con-
centrated HC1. Heat the mixture until all of the salt dissolves.
95
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The solution should be clear. Slowly add this solution to 900 mL
of deionized water. Store this solution in a bottle which contains
a few pieces of pure tin metal.
8. Allow the solution to stand for at least 15 minutes. The pink or
red color should disappear. Only a pale amber color should remain.
This is the thiocyanate-molybdenum complex.
9. Add 10.0 mL of iso-amyl alcohol to the separatory funnel.. Shake
the funnel vigorously until the color leaves the aqueous phase and
enters the organic phase. Thirty to sixty seconds should be suf-
ficient.
10. Allow the phases to separate (about 15 minutes). Drain off the
aqueous layer and discard it. Be sure to get all of the water out.
If necessary, drain off a little of the organic layer to "flush"
any water out of the stopcock bore.
11. Drain the colored organic layer into a test tube. Add a spatula
full of granular anhydrous sodium sulfate to the solution and
allow it to stand about 30 minutes. This drying step helps to
remove any turbidity which could interfere with the spectrophoto-
metric readings.
12. Read the absorbance at 465 run versus the blank. Prepare a Beer's
Law plot of absorbance versus concentration using the standards.
Use the plot to determine the concentrations of the unknowns.
In this lab we usually do a linear least squares regression analysis on
the standard points. From the equation of the line we can calculate the con-
centration of the unknowns.
The conventional thiocyanate procedure has been shortened to permit more
rapid analysis of samples. The increased speed of the analysis has been
gained at the expense of some precision. The standard error of estimate is
typically ±15 yg/L. This method is referred to as the semi-quantitative pro-
cedure. Most natural waters and aqueous samples can be analyzed by this pro-
cedure .
More Rapid Procedure
This procedure is used for the rapid processing of large numbers of sam-
ples. Up to fifty samples may be analyzed at a time. If the samples are
higher than 1 mg/L the solution should be diluted to fall in the range of the
standards. This procedure uses 30 mL test tubes. All of the steps and wait-
ing times are the same as in the method described above. A standard deviation
of about 15 yg/L can be expected when this method is used.
The following are the changes in reagent quantities to be used:
1. 10 mL samples and standards.
96
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2. 0.5 mL concentrated HC1.
3. 0.05 g tartrate (exact amount not critical).
4. 0.1 mL of 1% ferrous ammonium sulfate solution.
5. 0.5 mL of 10% potassium thiocyanate solution.
6. 2.0 mL of stannous chloride solution.
7. Extract into 4.0 mL of iso-amyl alcohol.
8. Centrifuge the test tubes and contents at 2,000 rpm for about
five minutes.
9. Draw off the colored organic phase by using an 8" Pasteur pipet.
10. Dry the extract over anhydrous sodium sulfate. (Omit if AA is
used on the organic extract!)
11. Allow the extracts to stand for about one-half hour. Read the
absorbance at 465 nm. Use 1 cm absorbtion cells.
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APPENDIX C
ANALYSIS FOR MOLYBDENUM, ATOMIC ABSORPTION SPECTROPHOTOMETRY (AAS)
Molybdenum analysis by flame AAS is susceptible to many interferences.
Direct aspiration of aqueous samples is not recommended because of these inter-
ferences. In addition, the method generally affords a detection limit (signal
to noise ratio of 2.0) of about 50 ygMo/L. Therefore the method has insuffi-
cient sensitivity and precision to adequately determine the "contamination"
of most waters.
Acceptable results can be obtained using flame AAS by preconcentrating
the samples prior to analysis. The analyte should also be separated from po-
tential interferants. Both of these requirements can be accomplished by using
a complexation-solvent extraction procedure. Several publications describing
such procedures are referenced in Section 4 of the text.
In this laboratory we have successfully used the following procedure. It
consists of the complexation and solvent extraction technique used for the
spectrophotometric determination of molybdenum described in the preceding
pages. However, in this modification molybdenum detection is accomplished by
aspirating the organic solvent containing the analyte into a nitrous oxide-
acetylene flame. Lower detection limits and higher sensitivity are achieved
by using greater preconcentration factors (sample volume to solvent volume
ratios). In addition, the sensitivity is improved when organic solvents are
used in flame analysis of refractory metals such as molybdenum.
Sample aliquots of 10 to 15 mL are taken for analysis. (This sample
volume permits a detection limit of about 5 ygMo/L.) The following method is
analogous to the solvent extraction technique described in the preceding
pages.
1. Add 0.5 g sodium tartrate and stir.
2. Add 0.1 mL 1% ferrous ammonium sulfate solution.
3. Add 0.5 mL 10% potassium thiocyanate solution; stir; wait 15
minutes.
4. Add 2.0 mL 10% stannous chloride solution; stir; wait 15 minutes.
5. Add 2.0 mL of i-amyl alcohol to the samples and 4.0 mL to the
standards; shake; allow 15 minutes for layers to separate; cen-
trifuge for about 5 minutes.
98
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6. Analyze the organic extract for molybdenum using flame atomic
absorption.
AA parameters—Perkin-Elmer 360
Lamp wavelength—313.3 nm
Lamp current—'30 ma
Flame N2O-acetylene—fuel rich, 5 cm single slot nitrous oxide head
Scale expansion x 25
Aspiration rate—the nebulizer system is slightly adjusted for
the alcohol to aspirate moderately slow
The standards are 50, 100, 200, and 300 ygMo/L. An iso-amyl alcohol
blank is used. The common practice in this laboratory is to include two to
three of each standard, depending on the size of the run, to insure that there
is surplus to monitor the calibration of the instrument.
The absorption is determined by using several six second time averages
for each sample and standard. A Beckman 10" strip chart recorder is used to
display the time integrals. After analysis the peak heights are measured and
the concentrations are calculated.
Flameless AAS or electrothermal atomization techniques which use a heated
graphite furnace or cup may also be used for molybdenum analysis. The gra-
phite furnace technique permits molybdenum determination at the ng level with
small sample volumes. This technique is, however, prone to matrix interfer-
ences when used for molybdenum analysis. The boiling point of molybdenum
metal is about 4,800°C. This is about 2,000°C above the maximum temperature
obtained in the graphite furnace. This indicates that molybdenum atomization
can only take place by a mechanism which includes the formation of a compound
which is volatile at 2,800°C. This compound then can dissociate in the vapor
phase to release molybdenum atoms. Any ions or compounds which affect the
complicated atomization mechanism will alter the sensitivity of the method.
This crucial fact is the primary reason for the extreme matrix sensitivity
of the method.
The principal interference encountered in this study results from the
simultaneous presence of two ionic species, an alkali metal ion such as potas-
sium or sodium and the sulfate ion. Neither the cation or anion produce sig-
nificant interference when they are experimentally added with other counter
ions. A sample containing 50 ygMo/L will appear to contain 30 ygMo/L when
sulfate (in the presence of sodium) is present at 1,000 mg/L. A ten percent
signal suppression results when sulfate is present at 200 mg/L.
In this study all atomic absorption measurements were made with a Perkin-
Elmer Model 360 Atomic Absorption Spectrophotometer equipped with a Perkin-
Elmer HGA 2100 Graphite Furnace. The spectrophotometer conditions were:
Silt—Alt. 0.7 nm; Mode—TCI; Exp—15X. A Perkin-Elmer molybdenum hollow
cathode lamp was operated at 30 ma. The monochromator was set to pass the
313.3 nm resonance line of molybdenum. The graphite furnace conditions were:
Dry—50 sec at 120°C; Char—40 sec at 1,800°C; Atomize—8 sec at 2,800°C. The
argon purge gas was set at continous flow (normal) at "20" units on the flow
99
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meter. Aqueous standards containing 0, 10, 20, 30, and 50 ygMo/L were pre-
pared from MoC>3. Aliquots of 25 yL were used for standards and samples. A
25 yL aliquot of 50 ygMo/L will yield an absorbance of about 0.12.
It is necessary to emphasize again the importance of testing for inter-
ferences when this method is used. Highly saline or industrially contaminated
waters represent a significant potential for inaccurate analysis when this
method is used without prior validation.
100
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO
EPA-600/1-79-006
4. TITLE AND SUBTITLE
Human Health Effects of Molybdenum in Drinking Water
5 REPORT DATE
January 1979 Issuing Date
6. PERFORMING ORGANIZATION CODE
3. RECIPIENT'S ACCESSION NO.
7 AUTHOR(S)
W. R. Chappell, R. R. Meglen, R. Moure-Eraso,
C. C. Solomons, T. A. Tsongas, P. A. Walravens, P. W. Wifiston
8. PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Trace Substances Research Program
Campus Box 215
University of Colorado
Boulder, Colorado 80309
10. PROGRAM ELEMENT NO.
1CC614
11. CONTRACT/GRANT NO.
R-803645
12. SPONSORING AGENCY NAME AND ADDRESS
Health Effects Research Laboratory -
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
Cinn, OH
13. TYPE OF REPORT AND PERIOD COVERED
Final Report
14. SPONSORING AGENCY CODE
EPA/600/10
15 SUPPLEMENTARY NOTES
Project Officer: Paul Heffernan
16. ABSTRACT
Molybdenum plays an important biological role as a micronutrient for plants and
animals. At high levels it can be toxic to animals. While concentrations in surface
waters are generally less than 5 ygMo/L, concentrations as high as 500 vgMo/L have been
reported in some drinking waters. Concentrations in water greater than 20 ygMo/L are
almost certainly anthropogenic.
The average human intake via food for the United States is 170 ugMo/day while the
average intake via drinking water is less than 5 ygMo/day. While no adverse health
effects have been reported in the United States, there are reports in the Russian and
Indian literature of both biochemical and clinical effects in humans at intakes ranging
from 1 to 10 mgMo/day. Rapid urinary excretion appears to provide considerable pro-
tection at intakes less than 1 mgMo/day. This report reviews the data on molybdenum
as it relates to the effects of its occurrence in drinking water.
The report also reviews the results of an interdisciplinary study carried out by
the authors. The authors recommend a guideline of 50 ygMo/L for the maximum concen-
tration in drinking water.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Molybdenum, Potable water, Criteria, Health
Chemical analysis, Biochemistry, Metabolism
Toxicity
b. IDENTIFIERS/OPEN ENDED TERMS C. COSATI I'lCld/GrOUp
Colorado
68G
18. DISTRIBUTION STATEMENT
Release to Public
19 SECURITY CLASS (This Report)
Unclassified
21 NO. OF PAGES
113
20 SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
PREVIOUS EDI TION IS OBSOLETE
101
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