REPORT NO. 8
REVISED
guidance
for the
I control of
radiation hazards
in
uranium mining
SEPTEMBER 1967
Staff Report of the
FEDERAL RADIATION COUNCIL
-------
REPORT NO. 8
REVISED
GUIDANCE
FOR THE
CONTROL OF
RADIATION HAZARDS
IN
URANIUM MINING
SEPTEMBER 1967
Staff Report of the
FEDERAL RADIATION COUNCIL
-------
FEDERAL RADIATION COUNCIL
MEMBERS
SECRETARY or HEALTH, EDUCATION. AND WELFARE ICHAIRMAN)
SECRETARY OF AGRICULTURE
SECRETARY OF COMMERCE
SECRETARY Or DEFENSE
SECRETARY Or LA«OR
CHAIRMAN. ATOMIC ENERGY COMMISSION
SPECIAL. ASSISTANT TO THE PRESIDENT FOR SCIENCE AND TECHNOLOGY IAOVISER)
P. C. TOMPKJNS. EXECUTIVE DIRECTOR
C. C. PALMITER. SPECIAL ASSISTANT
WORK.NO GROUP
H. N. DOYLE
G. M. DUNNING
W. MANN
H. •. MITCHELL
J. P. O'NEILL
A, B. PAR K
R. 0. STOTT
J. G. TERRILL. JR.
E. C. VAN BLARCOI.
r. WESTERN
DEPARTMENT Or HEALTH. EDUCATION. AND
WELrARE
ATOMIC ENERGY COMMISSION
DEPARTMENT OF COMMERCE
DEPARTMENT Of DEFENSE
DEPARTMENT Or LABOR
DEPARTMENT Or AGRICULTURE
DEPARTMENT Or THE INTERIOR
DEPARTMENT Or HEALTH. EDUCATION. AND
WELrARE
ATOMIC ENERGY COMMISSION
ATOMIC ENERGY COMMISSION
NATIONAL ACADEMY OF SCIENCES - NATIONAL RESEARCH COUNCIL
ADVISORY COMMITTEE TO THE FEDERAL RADIATION COUNCIL
C. L. COMAR (CHAIRMAN)
S. ABRAHAMSON
H. L. ANDREWS
V. P. BOND
G. W. CASARETT
L. H. HEMPELMANN
S. P. HICKS
B. MAC MAHON
J. E. RALL
W. L. RUSSELL
E. L. SAENGER
S. WARREN
CORNELL UNIVERSITY
UNIVERSITY OF WISCONSIN
PUERTO RICO NUCLEAR CENTER
• ROOKHAVEN NATIONAL LABORATORY
UNIVERSITY OF ROCHESTER
UNIVERSITY OF ROCHESTER
UNIVERSITY OF MICHIGAN
HARVARD SCHOOL OF PUBLIC HEALTH
NATIONAL INSTITUTES OF HEALTH
OAK RIDGE NATIONAL LABORATORY
UNIVERSITY OF CINCINNATI
NEW ENGLAND DEACONESS HOSPITAL
AD HOC PANEL ON MINING PRACTICE AND ECONOMIC FACTORS
E. C. VAN BLARCOM (CHAIRMAN)
H. N. DOYLE
R. D. EVANS
FRANZ. JR.
MOUADAV
JENSEN
'NEILL
O. A.
O.
C. R.
J. P.
R. G. STOTT
J. WESTFIELO
ATOMIC ENERGY COMMISSION
U. ». PUBLIC HEALTH SERVICE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
COLORADO BUREAU OF MINES
U. S. PUBLIC HEALTH SERVICE
NEW MEXICO DEPARTMENT OF PUBLIC HEALTH
DEPARTMENT OF LABOR
ATOMIC INDUSTRIAL FORUM INC.
U. S. BUREAU OF MINES
U. S. BUREAU OF MINES
AD HOC PANEL ON EPIDEMIOLOGY
A. W. MILBCRG (CHAIRMAN)
V. E. ARCHER
M. A. CONNELL
F. E. LUNOIN, JR.
O. A. MORKEN
H. M. PARKER
G. SACCOMANNO
R. SKLTBER
B. O. STUART
J. K. WAGONER
U. S. PUBLIC HEALTH SERVICE
U. S. PUBLIC HEALTH SERVICE
GRANTS CLINIC (GRANTS, N. MEXICO)
U. S. PUBLIC HEALTH SERVICE
UNIVERSITY OF ROCHESTER
BATTELLE-NORTMWEST, PACIFIC NORTHWEST LABORATORY
ST. MARYiS HOSPITAL (GRAND JUNCTION, COLORADO]
JOHNS HOPKINS UNIVERSITY
BATTELLE-NORTHWEST. PACIFIC NORTHWEST LABORATORY
HARVARD SCHOOL OF PUBLIC HEALTH
REVIEW OF DOSIMETRY AND BIOLOGICAL MODELS
M. PARKER, BATTELE-NORTHWCST, PACfFIC NORTHWEST LABORATORY
For sale by Ih* Superintendent of Documents, U.S. Oorernment Printing Office
Washlartoo, DC. 20402- Price 40 cents
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CONTENTS
P^e
Tables and figures iv
Section I. Introduction 1
Section II. The Radiation Environment \ssociated With Uranium Mining 9
Section III. Biological Effects \ssociated With Exposure to Radon and Radon
Daughters With Special Reference to Lung Cancer 17
Section IV. Control Capabilities in Uranium Mines 33
Section V. Summary and Recommendations 43
Appendix. Dosirnetric and Radiobiological Considerations Related to the Analysis
of Radiation Hazards in Uranium Mining 49
111
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TABLES AM) FIGURES
P.gr
Table 1. Estimates of the number of mines producing uranium ore during the
calendar year as reported by the industry to the I .S. Bureau of Mines (1954-61-)
and AE(: (1965-66) 1
Table 2. Number of men employed in uranium mines I
Table 3. The uranium series 10
Table 4. Estimated distribution of mines by Workiup Level ranges from 1956
through 1959 12
Table 5. Summary of radon daughter concentrations by \\ orking Level ranges
during the third quarter of 1965 and 1966 13
Table 6. Lung cancer mortality between July 1955 and June 1965 inclusive white
miners whr began underground uranium mining before July 1955 21
Figure 1. Observed and expected annual lung cancer mortality per 10,000 miners and
95 percent confidence limits in relation to exposure 22
Table 7. \ entilation cost estimates 11 mine study 36
Table 8. \ entilation cost estimates 3 mine study 36
APPENDIX
Table 1. Energies and ranges of alpha particles in tissue 50
Table 2. Ion density in soft tissue 50
Table 3. \verage dose rates to the epithelium of the traches and main bronchi. ... 51
Table I. Thickness of bronchial epithelium in different parts of the lungs 53
Table 5. Approximate annual dose (rads) in reference atmosphere 54
Figure 1. Ixx-ation of the biological target 55
IV
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SECTION I
INTRODUCTION
1.1 This report supersedes the preliminary KRC staff Report No. 8 which was re-
leased on May 7, 1967, for discussion in the Joint Committee on Atomic Energy hearings
on radiation exposure of uranium miners. It contains background material used in the
development of guidance for Federal agencies in regulatory programs and in programs of
cooperation with States concerning radiation protection in the mining of uranium ore, and
seeks to provide guidance for long-term radiation protection in uranium mining. Periodic
review will he necessary to incorporate new information and new surveillance or control
techniques as they an- developed. The rejM>rt also includes recommendations for additional
research and recordkeeping needed to provide a (inner basis for the evaluation of radiation
risks in this industry.
1.2 The use of uranium, as a source of nuclear energy for the electric power industry,
is developing during a period when (Government procurement for military purposes is
declining. These two needs are complementary with respect to ore production, and operate
to maintain the uranium mining industry as an activity of substantial importance to the
national economy. The uranium mining industry is located in 10 Western States, 5 of
which produce over 90 percent of the total domestic uranium ore. The value of recoverable
uranium in ore produced in four of these States as a whole is about half the combined
values of copper, lead, and zinc ores produced from the same States.
1.3 The natural radioactive decay of uranium leads to the formation of various
radioactive nuclides in ore bodies; one of which, radon, is gaseous. Radon gas formed by
the radioactive decay of radium 226 escapes from exposed rock surfaces into the air of
uranium mines, where it continues to decay, generating a series of other radioactive prod-
ucts commonly termed radon daughters. Radon gas is also present in aboveground air in
concentrations that may vary with location, time of day, and weather conditions. Some of
the radon daughters contained in the air breathed by miners are known to be deposited,
retained, and to irradiate tissues in the miner's respiratory system. Studies by the U.S.
Public Health Service in cooperation with the Atomic Energy Commission and State
agencies disclose that underground uranium miners are subject to lung cancer to a degree
substantially greater than the general population, or of that in miners in other kinds of
underground mines. The excess incidence apparently is related to the uranium miner's
occupational environment, and is believed to be induced by the radioactive decay of radon
daughters in the respiratory system.
1.1 Emphasis is focused on the radiation hazards associated with underground
uranium mining, since control is more difficult than that which can be achieved in open
pit mining or in subsequent milling operations. Open pit mining, being an aboveground
operation, presents no special problems in radiation protection. Primary attention is given
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in this report to the evaluation of radiation hazards resulting from the inhalation of radon
and radon daughters in the confines of underground mines and methods by which these
hazards can be controlled. In making this review, it is recognized that the benefit derived
bv the application of progressively more stringent control requirements must be evaluated
in light of the resulting reduction in radiation risk and the total impact of the more strin-
gent controls.
1.5 In addition to the recognized authority of the States to establish health and
safety standards for mining operations conducted within their respective jurisdictions,
responsibilities have been designated to certain Federal agencies. They include the De-
partment of the Interior in the administration of the Federal Metal and Nonmetallic
Mine Safety Act; the Department of I-alx>r in the administration of the Walsh-Healey
Act; the Department of Health, Education, and Welfare in providing technical advice
in the matter of health standards and control of health hazards; and the Atomic Energy
Commission in the regulation of source material (i.e.. uranium and thorium), after re-
moval from the place of hit in nature. States, Federal agencies, and the mining in-
dustry all have a direct interest in the development of uniform standards applicable to
the practical problems of uranium mining, including a standard for radiation protection.
1.6 In the preparation of this staff report technical experts from various Federal
and State agencies, industry, and individual nongovernment scientists assisted in de-
veloping information concerning mining practices, economic aspects of uranium mining,
epidemiological evidence for associating adverse health effects with radon daughter con-
centrations in mine atmospheres, and considerations involving basic radiobiological
mechanisms and the absorbed dose to tissue resulting from breathing mine atmospheres.
Coordination of information between the staff of the Federal Radiation Council, the
National Council on Radiation Protection and Measurements, the USA Standards In-
stitute (formerly called the American Standards Association), and the Atomic Industrial
Forum was achieved by including individuals associated with these organizations in the
FRC task groups. In addition, the staff has had the benefit of consulting with repre-
sentatives of organized labor, members of the International Commission on Radiological
Protection, and with individuals in other countries where uranium is mined.
1.7 The Federal Radiation Council Working (Group, which consists of senior tech-
nical personnel from the agencies comprising the Council, has supplied advice and informa-
tion from their respective agencies for inclusion in the rejxirt. Personnel from the U.S.
Bureau of Mines, Department of Interior, also have actively participated in the preparation
of this report.
Scope of Uranium Mining Industry
Production
1.8 The domestic uranium mines in 1966 produced approximately SO percent as
much uranium ore as was produced in 1961, the year when the industry was at its peak.
This curtailed rate is due to a stretch-out of (Government procurement contracts designed
to balance procurement with requirements and to tide the industry over an interim period
between diminishing (Government needs and the developing needs for the electric power
industry. Procurement under the remaining (Government contracts during the period
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July 1967 through 1970 is estimated at 28,300 tons of USO8. Procurement for electric power
plants in this period will be of the order of 18,000 tons of UaOg.
1.9 The U.S. Atomic Energy Commission estimates that "by 1980 the United States
will have between 120- and 170-million kilowatts of electricity generated by nuclear power,
with a midrange of this projection about ISO-million kilowatts—the best single esti-
mate." ' This estimate takes into account the growing demand for electric power, engi-
neering and economic factors, and the current rapid acceptance of nuclear power. An
increase of this magnitude in nuclear power plant facilities will probably require on the
order of 250,000 tons of I'sOs over the 15-year period from 1966 through 1980 inclusive,2
taking into account the pertinent factors of probable reactor types, sizes, and characteris-
tics, as well as enrichment factors and fuel economy. About 100-million tons of mined ore
would be needed to provide this quantity of uranium, assuming no major change in the
UsOs content of ore processed (currently about 0.23 percent UaOg). Although this is in
excess of presently known low cost domestic ore reserves (economic at a price of $10 per
pound of UaOs), it is considered reasonable to expect that additional ore will be discovered
to keep pace with demand. The 1966 mining rate was 4.2-million tons. To meet the esti-
mated total 15-year requirement, this rate is expected to rise to 16- to 18-million tons annu-
ally by 1980. The accuracy of these future estimates, however, is subject to the possible
effect of foreign trade in uranium.
1.10 A substantial fraction of the known uranium deposits can be mined by the open
pit method. Currently, about one-third of the domestic ore production is derived from open
pit mines, some of which have been converted from shallow underground operations.
Current technological developments tend to increase the advantages of the open pit method
and therefore to extend the depth of economically recoverable deposits. On the other hand,
deposits discovered in the future are expected to be at progressively increasing depths, so
that the proportion of ore mined from open pits during the next 15 years may be expected
to decline somewhat. Thus, underground ore production may, by 1980, rise to somewhat
over 12-milb'on tons per year, or several times the current underground mining rate.
1.11 Some perspective on the relative significance of uranium mining to the States
involved is provided by comparison with copper, lead, and zinc mining. The latest compara-
tive statistics available appear in the 1965 issue of the U.S. Bureau of Mines Minerals
Yearbook. The total amount of copper, lead, and zinc ore produced from mines located in
the principal uranium-producing States (New Mexico, Wyoming, Colorado, and Utah)
was about 43-milIion tons, with a recoverable metal value of about $295 million. In the
same year these States produced 4 million tons of uranium ore containing about $150
million worth of uranium.
Employment in U.S. Uranium Mines
1.12 The first recorded production of uranium-vanadium carnotite-type ore in the
United States was in 1898 from the Uravan mining district, Montrose County, Colo.3
Limited production continued until about 1935 when demand for vanadium for use in
alloy steels increased the market for carnotite ores. Despite this production uptrend, there
was no sustained large-scale employment until after the Atomic Knergy Act of 1946. The
first price schedule of the Atomic Energy Commission that became effective April 9, 19-48,
-------
launched a new major mining industry. Thereafter employment in uranium mines increased
rapidly until 1961. when it declined as a consequence of curtailed Government purchases.
Open pit mining of uranium did not become significant until 1955. The number of uranium
mines providing ore during the years 1954 through 1966 and the corresponding employment
figures are shown in tables 1 and 2, respectively.
TABI.K 1.- Estimates of the number of min<>s producing uranium ore during the calendar year as reported
fry the industry to the I .S. Bureau of Mines (/9.5J-6J) and AEC (J965-66)
^ par
1954
1955
1956
1957
1958
1959
I nderp-onnd mines
450
600
700
850
850
801
1960 703
i
< )|M-n pit
mines
50
75
100
125
200
165
166
Year
1961
1962
1963
1964
1965
1966
Underground mines
497
545
573
471
562
533
Open pit
mine*
122
139
162
106
74
88
TABLE 2.—Number of men employed in uranium mines
\ ear
1954
1955
1956
1957
1958
1959
1960
Underground mines*
916
1,376
1,770
2,430
2,7%
3,9%
4,908
Open pit
mines
53
293
584
574
1,175
1,259
1,499
^ ear
1%1
1%2
1963
1964
1%5
1%6
Underground mines*
4, 182
4,174
3,510
3,249
2,900
2,545
Open pit
mines
1,047
1,074
886
726
700
359
*Kxcludes aboveground employees who may occasionally go underground.
1.13 Some further perspective is provided by considering the number of underground
miners related to the size of mine. The categories selected for this purpose are: (1) mines
employing 15 or fewer men (2) those employing from 16 to 50 and (3) those with more
than 50. The data assembled in table 1-52 of U.S. Bureau of Mines report, "Health and
Safety Study of Metal and Nonmetal Mines" 4 for 1963; are as follows:
Number of men
per mine
15 or less
16 to 50
More than 50 ...
Percent of
mine*
60
27
13
Percent of
men
16
32
52
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1.14 During 1966 a total of 621 mines produced ore, but the number in operation at
any one time probably did not average more than 100. Underground mines account for the
variability; the number of producing open pit mines was fairly constant at about 80. The
number of underground miners involved was probably less variable than the number of
operating mines because men customarily move from one mine to another when mine
operations are intermittent. Many uranium miners have worked previously in nonuranium
mines; conversely, uranium miners often quit uranium mines to work in nonuranium mines.
The turnover rate in the work force is therefore difficult to evaluate. However, in a group
of 1,888 uranium miners identified in a 1954 survey made by the U.S. Public Health Serv-
ice, only 26 percent were found still working in uranium mines 6 years later. It is also
estimated that less than 1 j>ercent work longer than 15 years in uranium mines.
Health and Safety Hazards Other Than Radiation in Uranium Mining
Injury Experience
1.15 The hazards of traumatic injury associated with mining are commonly recog-
nized. Although there are wide variations in the frequency and severity of accidents within
the industry, miners as a group are exposed to serious risks. Experience in the metal
mining industry shows that injury rates have been reduced to less than half over the last
30 years but, nevertheless, current injury rates remain about four times the average of all
manufacturing industries.4
1.16 Uranium mining became prominent at a time after the injury rate in metal
mines had been substantially reduced, but the rapid growth of uranium mining, and the
fact that much of it was concentrated in areas remote from other metal mines, necessitated
the employment of some inexperienced miners. The injury rate in uranium mines during
the late 1950's was as high as that experienced in other metal mines 15 to 20 years earlier.
Since 1960, however, experience in uranium mines has improved significantly.
Physical Hazards
1.17 U.S. Bureau of Mines records show that most disabling work injuries occurring
in underground uranium mines arise from machinery, haulage, handling of materials, falls
of persons, and falls of ground. Fatal injuries have resulted primarily from explosives,
haulage, and falls of ground. This has also been the experience in other underground metal
mines. The fatal accident rate in underground uranium mines during 1965 averaged about
1.93 per million man hours.5
Environmental Heath Hazards in I'nderground Mines
1.18 The quality of the atmosphere in underground mines is of primary importance
to the health and safety of underground workers. The mine atmosphere is subject to con-
tamination by harmful dusts and gases. Such contamination might result from drilling
(dust), blasting (gases and dust), materials handling (dust), use of diesel engines (exhaust
gases), emission of strata gases, welding and cutting, and from mine fires. In uranium mines
and some other types of mines the radiation environment must also be considered (see sec.
II). In general, appropriate measures used to control the hazards owing to common dust
and gases will serve also to reduce concentrations of radioactive materials in the mine air.
276-2910-67-2
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1.19 Most uranium ore is mined from sandstone that contains free silica, the causa-
tive agent of silicosis.6 Surveys in uranium mines having highly siliceous ores indicate that
effective dust control measures are necessary to restrict the concentration of airborne dust
to less than recommended limits. Wet drilling and wetting of blasted material before and
during loading and transportation are normally supplemented by ventilation to the extent
required for effective control of dusts and gases. Toxic gases may constitute a hazard to
the health of mine workers if ventilation is not adequate, or if men return to work areas
before the gases have been removed or sufficiently diluted by ventilation. Ventilating fans
can provide positive means for controlling the volume and flow of air underground. With a
properly arranged duct system fresh air ran be delivered directly to work areas or, alterna-
tively, contaminated air can be withdrawn from such areas and delivered above ground.
1.20 The use of diesel engines in underground mines necessitates ventilation to dilute
and remove toxic exhaust gases and to replace oxygen consumed by combustion in engines.7
Required volumes of ventilating air for individual units are stipulated on approval plates
of diesel-powered equipment approved by the U.S. Bureau of Mines for use in underground
mines.
1.21 Guides for limiting concentrations of gases, mineral dusts, and airborne con-
taminants other than radioactive substances that are likely to be encountered in under-
ground atmospheres are published annually by the American Conference of Govern-
mental Industrial llygienists. These threshold limits are widely used by engineers,
inspectors, and regulatory agencies as guides for controlling underground atmospheric
environments. These values represent conditions under which it is believed that nearly
all workers may be repeatedly exposed without adverse effect. The concept of threshold
limits is not used in radiation protection.
1.22 Control of temperature is an important factor in the operation of ventilation
systems in underground uranium mines, particularly in cold climates. Incoming fresh air
is often heated to prevent freezing of pipes and to prevent icing problems in shafts and
haulageways.
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REFERENCES
1 Seahorg. (». 'I'. Fast Breeder Power Reactors \ ^ orld ()utlk. \nnual Meeting of the Canadian
IVurlear Association: Montreal. (,)uehec. Canada, May 31. 1%7.
• Nininger. R. I). World Production and Reserves of I raniuin. Twelfth Annual Minerals Syni-
|>osiuin: \mcriran Institute of Mining. Metallurgiral and Petroleum Kngineers: Moah. Utah. June 23.
1967.
3 Coffin. H. C. Radium. I raniuin. and Vanadium De|K>sits of Southwestern (kilorado. Colo. (Jeol.
Survey Bull. 16. 1921.
* Report to Congress hy the Serretar) of the Interior. Health and Safety of Metal and N'onmetal
Mines (Suhmitted in respoiiM- to PI. 87-300. 75 Stat. 649). 1963.
1 Injury l'!x|HTience. Km ploy men t. and Worktime in the Mineral Industries 196-1-65. U.S. Bureau
of Mines Mineral Industry Survey. August 1966.
6 I SPHS Puh. 1076. Silicosis in the Metal Mining Industry Recvaluation 1958-61 (USPHS and
Bureau Mines), 1963.
~ Holt/,. John C. Safety with Mohile Diesel-Powered Kquipment Underground. U.S. Bureau of
Mines Report of Investigations. .No. 5616, 1960.
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SECTION II
T11K RADIATION ENVIRONMENT ASSOCIATED WITH URANIUM
MINING
2.1 The atmosphere in underground uranium mines will normally contain a number
of contaminants capable of producing various deleterious effects in the respiratory system.
These entities include:
1. Airborne dust particles: Silica, alkaline earth and other metal carbonates, silicates,
vanadates and aluminates with smaller amounts of iron, molybdenum, and uranium
minerals including the radioactive uranium decay products, notably radium 226. Thorium
and its radioactive decay products might also be present in minor amounts. Atoms of the
radon decay products rapidly become attached to these particles (see par. 2.5).
2. The radioactive gas radon (and thoron if thorium is present).
3. Free ions: Single atoms of the radioactive elements resulting from the decay of
radon, <•./.'., ]x>loniiiin 218 (RaA), the first decay product of radon 222. Other free ions may
be formed by the radioactive decay of a free polonium 218 atom, or may be ejected by
recoil of a decay product from the surface of a larger particle and other mine surfaces.
1. Nuclei: Aggregations of a few molecules (c.(>., water molecules) around a polonium
218 atom or other decay product, or solid particles so small that diffusion is the dominant
transport mechanism.
2.2 The naturally occurring radionuclide uranium 238 is the parent of the radioactive
decay chain in which radon 222 is found. Table 3 presents the principal components of the
uranium series. The parallel branches in the chain from polonium 218 to astatine 218 and
from bismuth 214 to thallium 210 have been omitted; only the energies of the alpha
particles of interest are shown. .Natural thorium sometimes occurs as a constituent of
uranium ore, but in domestic ores it is generally less than 1 percent by weight of the
uranium content.
2.3 External gamma radiation intensities in domestic uranium mines seldom exceed
2.5 mR per hour.1 and the average intensities are only a fraction of this. It is accordingly
unlikely that uranium miners will be exposed to external whole-body radiation doses as
large as the Radiation Protection Guide (RPG) recommended by the FRC for occupational
radiation ex|>osure (5 rems per year). However, in mining occasional high grade ore pockets
(5 percent or greater L'jOg) external radiation levels may necessitate limitation of personnel
exposure. Beta radiation intensities near broken ores may be higher than gamma intensities
by a factor of 10,- but are of relatively minor importance as an external radiation hazard
under mining conditions.
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TABLE 3.—The uranium series
Isotope
Uranium 238
Thorium 234
Protactinium 234 . .
Uranium 234
Thorium 230
Radium 226
Svmhol
"T
"'Th
"*Pa
"T
"°Th
'"Ra
Radon 222 '"Rn
Polonium 218 "'Po
Lead 214 '"Ph
Bismuth 214
Polonium 21 4
Lead 210
Bismuth 210
Polonium 210
Lead 206
'"Bi
"4Po
"°Pb
"*Bi
"°Po
'°'Pb
Historical
name
Uranium I
Uranium X,
Uranium Xj
Uranium II
Ionium
Radium
Radon
Radium A
Radium B
Radium C
Radium C'
Radium D
Radium F
Radium F
Radium C
Half-life
4.5 X 10' yrs
24.1 days
1.18 mins
2.50 X 10s yn*
7.6 X 10« yr*
1620 yrs
3.82 day*
3.05 mint*
26.8 mint*
19.7 mins
164 X 10-* sec
22.0 yrs
5.0 days
138.4 days
Stable
Radiation
a
fry
fry
a, y
a
<*. y
a
a
fry
fry
a
fry
8
a
Alpha
energy
(MeV)
5.49
6.00
7.69
5.30
Radon and Radon Daughters
2. ( Radon 222 results from the radioactive decay of radium 226. Being an inert
gas it diffuses readily through the interstices of rock to the rock face and from there into
the air of the mine spaces. It has been observed that the rate of radon diffusion into the
mine air varies inversely with changes in the barometric pressure. As noted in table 3,
the half-lives of the first four successive daughters of radon are short. Under static condi-
tions (quiet air) radioactive equilibrium will develop in about three hours. However, an
equilibrium state is seldom found in an actively worked uranium mine area where fresh
air is being continually brought into the mine. The amount of fresh air that is brought
into a mine affects the concentration of the radon daughter products more than it affects
the concentration of radon. The radon daughter concentration in the air is reduced by
dilution and by adherence to dust particles and by preferential deposition on mine walls,
whereas additional radon is diffusing into the air from the rock surfaces and from mine
water.
2.5 Laboratory experiments have demonstrated that the unattached daughter
products of radon 222 exhibit a high rate of diffusion in air;3 that they quickly become
attached to moisture or dust particles suspended in the air (the mean lifetime existence
as free unattached ions is of the order of 10 to 50 seconds),1 and that the human respiratory
system retains a substantial portion of radon daughters attached to moisture or dust
particles and virtually all of the unattached jwrtion.4
10
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2.6 The principal dose to lungs of uranium miners is generally attributed to the alpha
particles emitted by the decay of the radon daughter products. In most United States
uranium mines the concentration of radon daughters is obtained by using a specific field
method of measurement, and the result is compared to the "Working Level" (WL), which
is defined as any combination of radon daughters in 1 liter of air that will result in the
ultimate emission of 1.3 X 104 MeV of potential alpha energy.5 The numerical value of the
WL is derived from the alpha energy released by the total decay of the short-lived radon
daughter products at radioactive equilibrium with 100 picocuries (pCi) radon 222 per liter
of air. The 100 pCi of polonium 218 give 1.3 X 104 MeV from the total decay of the polonium
218 and the same number of terminal polonium 211 atoms. The 100 pCi of lead 214 give
6.6 X 10* MeV from the decay of the resultant polonium 214. The 100 pCi of bismuth 214
give 4.8 X 104 MeV from the resultant polonium 214. The resultant total is 1.27 X 105 MeV
which is rounded to 1.3 X 10& MeV.
2.7 A significant advantage in the concept of the WL is its practical application to
field measurements of the radon daughter concentrations in mine air. ^The method of
measuring the concentration of decay products in terms of total alpha particle emission is
widely used for control and regulatory purposes. Exposure of an individual to radon
daughters in air can be estimated from the length of time the individual breathes an
atmosphere containing a stated burden of radon daughters. The Public Health Service
publications usually express exposures as "Working Level Months" (WLM), although
other time periods are sometimes used. Inhalation of air with a concentration of 1 WL of
radon daughters for 170 working hours results in an exposure of 1 WLM.
2.8 Historically, radon daughter measurements have been made in the United States
uranium mines since about 1950, and the records are preserved in large part by the U.S.
Public Health Service Occupational Health Field Station in Salt Lake City, Utah. Since
about 1960 similar records also have been kept by State regulatory agencies. The records
maintained by the USPHS Occupational Health Field Station comprise such items as
mine identity and location, identification of personnel working underground at the time
of the survey, location of sampled mine areas, and concentrations of radon daughters in
mine air expressed in terms of the WL. From time to time this agency has made sum-
maries of these records available for public purposes. The summaries generally display WL
data averaged on a calendar year basis prior to 1962. For subsequent years the reported
averages use data for the third calendar quarter. The data are further detailed according
to the State in which the mine is located; the States of main concern being New Mexico,
Wyoming, Colorado, Utah, and Arizona. Uranium mines located in these States produced
more than 90 percent of the total domestic uranium ore in 1966.
2.9 Table 4 presents a summary derived from records of radon daughter measure-
ments in underground uranium mines prior to 1960. The table shows the number of mines
measured and the percentage of mines with radon daughter concentrations falling in
various ranges of WL values. These percentages are estimated to be about the same as
the percentage of the work force whose annual average exposures fall within the WL ranges
shown in the table, and are considered to reflect the status of the whole industry during
that period of time.
11
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TABLK 4. -Estimated distribution of mines fey Working. />ere/ ranfips from 1956 throupfi 1959
Year
Number <1.0 ' 1.0- -2.9 ! 3.0-10.0
mines
measured
108
WL WL
WL
r;
19 25 33
158 20
53 28
237
18
26
28
>10.0
WL
cy
•r:
23
26
21 36 15
26 28
1
28
Total
Cfc
\
100
100
100
100
1956
1957
1958
1959
2.10 This breakdown suggests that about one-fourth of the work force was probably
exposed to atmospheres leading to annual exjwsures larger than 10 WL, and about one-fifth
was exposed at average levels lower than 1 WL. It is also noteworthy that the number ol
mines surveyed between 1956 and 1959 was but a small fraction of the total uranium
mines. This low coverage was due to the common event that many small mines were not
in operation at the lime of the survey. Many such mines were located in remote areas and
operated only a few weeks or months in each year because of such factors as available ore,
operating funds, labor supply, weather, and so forth.
2.11 In December 1960 a (Governors' conference on health hazards in uranium mines 6
was held in Denver, Oolo. This was an outgrowth of interagency studies on the occupa-
tional health problems of uranium miners carried out by the Public Health Service, the
Atomic Knergy Commission, the I'.S. Bureau of Mines, and the Department of Labor.
The objectives of the conference were to present to the Governors of States engaged in
uranium mining, data on the prevailing radon daughter levels, such as those indicated in
table 1; to present information on experience in controlling radiation hazards; and to assist
in developing cooperative programs to reduce radiation hazards in uranium mines. As a
result of this conference, many of the States placed more emphasis on their mine inspection
programs.
2.12 As a measure of the prevailing levels of radon daughters in the mines, table 5
indicates the results of samples taken in the third calendar quarter of 1965 and 1966. These
data indicate considerable success in reducing concentrations of radon daughters. The
mine operating companies are cooperating with Stale regulatory agencies to improve
control of radon daughter concentrations in working areas of the mines. The feasibility of
further reduction in the mines rej»orting average WL values between 1 and 10 involves
considerations discussed in section IV.
2.13 The significance of these recorded data is limited by several considerations:
(1) individual WL measurements usually represent a 10-minute sample at a selected loca-
tion; (2) the number of samples taken per survey is restricted by the number of survey
personnel that can be accommodated in an operating mine and by the man hours available
to collect and analyze the individual samples; and (3) the frequency of surveys in indi-
vidual mines varied widely; a single survey per year was common practice in some states,
while in others surveys were even less frequent.
2.1 I The I .S. Bureau of Mines carried out a study in 1962 on a modification of the
usual practice in selecting locations to be sampled in a mine. The report on this project 7
indicates that an estimate of the time-weighted assessment of miner occupancy in spot-
12
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TABLE 5.—.Summary of radon daughter concentrations h\ IT'nrkinft hni>' ranprs during the third quarter
of 1965 and 1966
1965
Stall-
Arizona
Colorado
New Mexico
Utah
Wyoming
Total
Number
mines
16
124
26
47
16
229
10.0
1
5
4
6
1
1966
Stati-
.Arizona
Colorado
New Mexico
Utah
Wyoming
Total
Number
mines
14
148
23
33
13
231
<^i.o
57
45
17
55
38
44
Percent of mines in each WI. range
— . - -
1.0-2.9
29
42
47
1
3.0-4.9 5.0-10.0
14
7
30
5
6
39 6
54
42
8
9
4
>10.0
1
1
sampled areas permits an approximate evaluation of the exposure of individuals for the day
of sampling. The report indicates that as few as four area samples may be sufficient to
evaluate the exposure of an individual with a probable accuracy of ±25 percent. The
examples cited in the Bureau report illustrate the method: (1) a miner working in various
areas having concentrations ranging from 3.7 to 8.0 WL had an estimated weighted exposure
of 6.1 WL, (2) another miner working in concentrations ranging from 0.3 to 6.4 WL had a
weighted exposure of 1.0 WL, and (3) a third one working in concentrations of 0.2 to 1.4
WL had a weighted exposure of 0.5 WL. The results of this study suggest that the arithmetic
average of concentrations found in the mine air does not give a reliable estimate of exposure.
Similarly, the maximum concentration found in any representative mine sampling bears
no direct relation to the exposure of individual miners.
2.15 Time-weighted assessment for evaluating exposures in uranium mines has not
been generally adopted for regulatory purposes. Rather, pertinent State regulations and the
recommendations of the USA Standards Institute stipulate a maximum concentration that,
when exceeded, is used as a basis for closing the mine area concerned. Two of the five States
(Colorado and .New Mexico), for which data are reported in tables 4 and 5, maintain
13
l 0 - «7 - 3
-------
records both of concentrations found at each inspection and of the estimated time-weighted
average exposure of individual workers who work in different areas in the mines.
2.16 Radon and its daughter products have also heen measured in other than ura-
nium mines. One study 8 showed that the air in certain nonuranium mines contained radon
daughters in concentrations of about 0.1 to 0.2 WL. JacoeJ reported radon daughter
concentrations in 24 nonuranium metal mines in Colorado ranging up to 2.0 WL, and a
similar range in five clay mines. Rabson, H a/.,10 reported on radon daughter concentrations
in the gold mines of South Africa. In five of these mines, which also contain uranium, radon
daughter concentrations averaged between 0.1 and 0.9 WL with maximum concentrations
up to 4 WL. The existence ol relatively high concentrations of radon in water and air in
Canadian fluorspar mines has been reported." The authors found individual samples in
unused spaces higher than 10 WL, and estimated that average mine air concentrations
ranged from 2.5 to 10 WL.
14
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REFERENCES
1 Report to Congress by Secretary of tlie Interior. Health and Safety of Metal and Nonmetal Mines
(submitted in response to PL 87 300. 75 Stat. 649). 1963.
'•' Harris. W. B.. <-t nl. Environmental Hazards Associated with Milling I'ranium Ore. AM A Archives
Ind. Healtb. Vol. 20, pp. 365-382. Nov. 1959.
3 Chamberlain. A. C., and Dyson. K. C. The Dose to Trachea and Bronchi From Decay Products.
British Journal of Radiology. Vol. 29. p. 317. June 1956.
* Shapiro. J. An ('.valuation of the Pulmonary Radiation Dosage From Radon and Its Daughter
Products. University of Rochester Atomic Knergy Project. I R 298. April 1954.
' I SPHS Pub. 494. Control of Radon and Daughters in I raniuni Mines and Calculations on
Biologic Effects, 1957.
6 USPHS Pub. 843. Governors' Conference on Health Hazards in Uranium Mines—A Summary
Report, 1961.
7 U.S. Bureau Mines Report RI 6106. Kstimating Daily Exposures of Underground Uranium
Miners to \irborne Radon Radon Daughter Products. 1962.
" Wagoner, J. K., <•! nl. Unusual Cancer Mortality \inong Croup Underground Metal Miners.
\. Kng. J. Med.. Vol. 269. No. 6. pp. 284-289. Aug. 1963.
0 Jacoe. P. W. Occurrence of Radon in Nonuranium Mines in Colo. AM A Archives Industrial
Hygiene and Occup. Med.. Vol. 8, p. 118. 1953.
"' Rabson. S. R.. <1 nl. Experience in Measurement of Radon and Radon Daughter Concentration
S. African Uranium Mines. Proc. IAE\ Symposium Rad. Health and Safety Mining and Milling
Nuclear Materials. Vol. II, pp. 335 347. 1964.
" deVilliers. \. J.. and Windish. J. P. Uung Cancer in Fluorspar Mining Community. Brit. J. Ind.
Med., Vol. 21, p. 94, 1964.
15
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SECTION III
BIOLOGICAL EFFECTS ASSOCI \TED WITH EXPOSI RE TO RADON AND
RADON DAt GIITERS WITH SPECIAL KEFEKENCE TO LUi\<; CANCER
3.1 When the mining of uranium-hearing ores began in the I'nitcd States in 1898 '
it was known that radioactivity was associated with the ore. hut the potential health
hazards from this agent were not suspected until about 1921 when I'hlig J suggested that
the high nurnhers of lung caneers found among Schneeherg miners might he due to ionizing
radiation. From a know ledge of radiobiology and of exposure conditions in uranium mine
environments, one might expect the manifestation of radiation injury in the respiratory
tract to he of three basic types: tumors, atrophy of functional tissue, and increased sus-
ceptibility to other disease agents. Possible effects from external radiation and effects
related to the deposition of dust particles containing radiomiclides on the skin and in the
eyes cannot be excluded. However, these radiation doses are so minor in comparison to
the doses to the respiratory tract that they are not treated in this report.
Animal Experiments
3.2 Animal experiments have demonstrated that doses of ionizing radiation delivered
to the lungs may reduce pulmonary dust clearance,3 produce emphysema,4 cause loss of
pulmonary function.''" and cause pulmonary neoplasia, fibrosis, or changes in bronchial
epithelium.7 ••' These experiments have explored many parameters: (1) species—rats, mice,
rabbits, dogs; (2) radiation type -a, tf, y, x-rays; (3) internal and external radiation; (4)
relationships between response and dose rate or total dose; and (5) tumor types. Such
experiments are useful to the extent that they lead to sufficient insight into the mechanisms
of radiation-induced carcinogenesis to aid in the interpretation of human experience.
However, most of the pulmonary radiation dose received by uranium miners is from alpha
particles. Since these particles have a very short range in tissue and are densely ionizing
(high linear energy transfer), the different spatial distribution of energy absorption in
tissue limits the utility of dose-effect relationships derived from animal experiments using
beta, gamma, or x-rays.
3.3 In general, it has not been possible to produce pulmonary carcinomas in animals
in a systematic way from controlled exposure to radon or radon daughters, although sev
eral attempts have been made.2""2" Some animal experiments using radon alone, in combi-
nation with varying amounts of its decay products, or in combination with ore dusts, pro-
duced metaplasia of the bronchial epithelium and a few pulmonary tumors.13 29~32 The small
number of tumors and inadequate controls in some of the experiments precludes drawing
definite conclusions from them. The administration of plutonium 239 (also an alpha
emitter), in relatively large doses, has induced pulmonary malignancy in addition to severe
lung damage in dogs.1- M M
17
-------
Mortality and Disease Patterns among I'ranium Miners and Others
3.4 It has been known for many years that underground uranium miners are subject
to elevated mortality rates from accidents and from lung disease. Accidental death rates
among uranium miners decreased over a period of years to a low of 1 per million man hours
in 1964, comparable to those of other underground miners." However data presented in
paragraph 1.17 indicate an accidental death rate of 1.93 per million man hours in 1965.
Klevated lung cancer rates have been reported among fluorspar miners,3" iron miners,37 38
United States base metal miners,39 and the gold miners of (iwanda.40 Other authors report
that no increase in lung cancer incidence was observed among South African gold miners,41 "
nor among British coal miners.41 The pneumoconioses (including silicosis with accompany-
ing emphysema and cor pulrnonale) found among most groups of miners may have pre-
disposed them to pulmonary infections such as pneumonia and tuberculosis. The incidence
of disability from silicosis and related chest diseases appears to have been markedly reduced
by the use of modern industrial hygiene techniques, the most important of which is
ventilation.44
3.5 Several methods have been used to express epidemiological findings in published
papers. The two given below will be the principal ones used in this report:
1. "Incidence" is the number of cases of a disease appearing in a stated population per
unit of time. Vital statistics are normally reported in terms of annual incidence. Since lung
cancer has a high case fatality rate, the incidence of mortality is practically the same as the
incidence of the disease.
2. "Mortality ratio" relates the number of observed deaths to the number that would
be expected in the same population if the mortality rates derived from the vital statistics
records in large |x>pulations were applicable. Calculation of the "expected" value must
take into account such variables as age, sex, race, and years at risk. Of all the malignant
diseases, lung cancer is the most common cause of death among males in the United States.
The incidence of lung cancer in the general population increases rapidly after age 40 and
reaches a peak around age 60.4i
U.S. I ranium Miners
3.6 In 1950 the USP1IS, in cooperation with other Federal and State agencies, initi-
ated a program to evaluate the health problems inherent in the uranium mining industry.
Miners were enrolled in the study if they volunteered for at least one physical examination
and provided social and occupational data in sufficient detail to allow followup of their
health status. Small numbers of men were examined in 1950, 1951, and 1953. During 1954,
and later, attempts wen- made to examine as many men as could be located and would
cooperate. It has been estimated that in 1957 and 1960, 90 percent of the men working in
the industry in the areas visited were examined.46
3.7 Kstimates were made of each man's exposure to radon daughters, expressed in
WLM (see para. 2.7), on the basis of his occupational history and measurements of radon
daughter levels in mine air. Where such measurements were not available, the probable
value was estimated from measurements made in mines of similar location, depth, ore
type and grade, and ventilation arrangements. The occupational history, including identin-
18
-------
cation of particular mines and when the individual worked in each, depends on individual
recall rather than payroll records.
3.8 For various reasons the PHS study group does not represent the entire
mining population or a random sample thereof. This complicates the evaluation of the
number of lung cancers that would be expected in the group from causes unrelated to
exposure to radon daughters in the mine atmosphere, since a bias resulting from the
voluntary method of selection is |>ossible. This possibility was examined by the PHS in a
previous analysis, and it was concluded that this factor, although present, did not affect
the general conclusions of that analysis."
3.9 The total study group consists of approximately 5,000 underground miners,
uranium mill workers, and other types of aboveground workers in the industry (both white
and nonwhite) who have had at least one physical examination under the program. After
examining the composition of the group enrolled in the PHS study from different view-
|K)ints, the analysis made for the Federal Radiation Council focused on a subgroup of
1,981 white miners who started underground uranium mining before July 1, 1955.
3.10 The number of lung cancers observed in the subgroup selected for analysis was
compared with the number that might be expected based on examination of vital statistics
records of the male population of the States in which the miners worked (see par. 3.5).
For purposes of the analysis, person-years were divided into categories according to in-
creasing exposure to radon daughters expressed as cumulative WLM (see table 6).
3.11 As an example of the procedure, consider a subject who was born in July 1900,
and started underground uranium mining in July 1950 in a mine where the atmosphere
was estimated to contain 10 WL of radon daughters in 1950 and each year thereafter.
He mined full time until December 1957 and has not mined since. In July 1954 this miner
was first examined by a PHS team and thereby entered the study group. For August and
each succeeding month in 1954 he was assigned one person-month at risk in the age group
50-54, WLM category C (360-839 WLM) and the category for less than 5 years after he
started underground uranium mining. This individual was in comparable categories during
the first 6 months of 1955. However, in July 1955 he was removed from the age group
50-54 and changed to age group 55-59 years. Also, this individual was removed from the
category of less than 5 years since he started mining to the category of 5-9 years since he
started mining. The 12 months at risk in 1956 and the first 6 months of 1957 did not alter
the category designations for this particular individual. In July 1957 the calculated WLM
value reached 840 and thereafter the person-months were assigned to category D (840-
1,799 WLM) where he remained until the cutoff date for the analysis (June 1965). If the
individual had died during this period, the month of death would have been used to deter-
mine the last person-month at risk.
3.12 After this accounting procedure had been completed for each member of the
study group, the person-months were converted into person-years at risk, classified by
calendar year, age group, WLM category, and time after the individual started under-
ground uranium mining. The expected number of lung cancer deaths was then calculated
for the person-years in each of these categories. For example, in 1958 there were 16.59
person-years in age group 45-49 and WLM category D (840-1,799) with 5-9 years after
19
-------
the individuals started underground uranium mining. Hie annual lung cancer rate for
\vhite male* in this age group in Colorado, t'tah, New Mexico, and Arizona combined was
2.827 per 10,000 in that year. The term "lung cancer" is used in this report to designate
cancer of the hing or of other tissues of the respiratory tract. The expected number of lung
2 827
cancer deaths in this particular group is therefore '°-^9xinfUU, or 0.0017. Kxpected
10,000
numbers obtained in this way for all age groups and calendar years in each exposure
category were then added to obtain the expectations shown in the analysis (see table 6).
3.13 V basic requirement for a comparison of the number of lung cancers observed
in the uranium miner group with the rates in the general population is that the same
criteria be used for including a lung cancer death in the study group that were used in
deriving the basis for calculating the expected deaths, i.e.. lung cancer was listed on the
death certificate as the underlying cause of death. The analysis, shown in table 6, is the
same as that appearing in table 8 of the preliminary draft issued May 1967, with three
additional columns (1) the number of person-years at risk. (2) the expected annual lung
cancer mortality per 10.000 miners, and (3) the calculated annual mortality per 10,000
miners. The 93 percent confidence limits for the observed mortality rates are also shown.
This information, expressed in terms of expected and observed rates per 10,000 miners, is
also shown graphically in figure 1.
3.11 The analysis indicates the presence of a clear association between exposure to
radon daughters in mine air expressed as cumulative WLM and the number of lung cancer
deaths in the study group. The 21 deaths observed versus 1.82 expected in exposure
categories I). K. and F, may be compared to 10 deaths observed versus 3.31 expected in
exjmsure categories A. B. and (i. As can be seen from figure 1, all of the lung cancer rates for
categories I). K, and F are significantly above the expected rate. Kven though the observed
incidence for categories A. B, and (.'. is above that expected, only category B is significantly
high and the lack of a progressively increasing incidence fails to support a causal role for
radon daughter exposures at these levels. However, the absence of a steadily increasing
incidence could easily be the result of chance alone, so that the data do not suggest or
exclude the existence of a threshold. Since the quantitative relationship shown in table 6,
and figure 1 may be. in part, a consequence of the way exposure categories Mere selected,
alternative breakdowns of 1,000 WLM categories in the upper range and of 250 WLM
under l.(MM) WLM were considered. These alternative cumulative WLM exposure category
designations for the 1.981 miners in table 6 were estimated from a random stratified sample
of 299 white underground uranium miners without lung cancer and from the 19 cases of
lung cancer. Kxamination of the ratios of cases to estimated number of miners by exposure
categories at the end of 1963 suggests that the first clearly demonstrable excess of lung
cancer with progressively increasing risks at each higher Vi LM level may be in a category
somewhat above l.(MH) WLM. This is hardly surprising in view of the sampling variation
>%hich might commonly occur with the small number of cases in most categories. In any
event the data are not sufficient to indicate an association between exposure to radon
daughters and the subsequent development of lung cancer \\hen the cumulative exposures
are less than about 1,000 WLM.
20
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TABLE 6.—Lung cancer mortality betiveen July 795.1 and June 1965 inclusive—ivhile miners icho bcpan underground uranium mining before July 1955
Exposure
category
A..
B
C.
D.
E
F
Total . .
Estimated
cumulative
WLM
<120
120-359
360-B39
840-1,799
1,800-3,719
>3,720
No. of
miners*
383
421
4%
•too
218
63
1,981
No. of
person -
years at
risk
3,788
3,914
4,036
3,148
1,623
455
16, 964
Years between beginning of underground uranium
mining and deatli from lung cancer
<5
Exp.
no.
0.20
.11
.07
.02
.00
.00
.40
Obs.
no.
1
0
0
0
0
0
1
5-9
Exp.
no.
0.48
.43
.36
.22
.08
.02
1.59
Obs.
no.
0
0
0
2
0
3
._fj
>10
Exp.
no.
0. 37
.50
.79
.77
.58
.13
Obs.
no.
1
5
3
6
9
4
3. 14 i 28
Total
Exp.
no.
1.05
1.04
1.22
1.01
.66
.15
Obs.
no."
2
5
3
8
9
7
5.13 j 34
Calculated annual mortality
per 10,000 miners
Ex-
pected
2.8
2.7
3.0
3.2
4. 1
3. 3
Observed (95% con-
fidence limits)
0.6-
1.1-
1.5-
11.0-
25.4-
61.8-
5.3-
12.8-
7.4-
25.4-
55.5-
153.8-
C3. 0 13.9- 20.0-
19.1
29.8
21.7
50.1
105.2
316.9
C28.()
* By cumulative WLM in underground uranium mines through 1963 (par. 3.32).
" Par. 3.16.
c Average rates.
21
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Figure 1 • Observed and Expected Annual Lung Cancer Mortality per 10,000 Miners
and 95-percen»-Confidence Limits in Relation to Exposure a
320 r
280
240
c
8
200
o
I
160
120
80
[hi
IL* dr
Observed 4
/''
Expected
\
1000 2000 3000 4000 5000 6000
Cumulative WLM
*See par. 3.13 and table 6.
22
-------
3.15 Lung cancer is not an immediate effect of exposure to radon daughters. The
largest number of lung cancers observed in all exposure categories appeared 10 or more
years after the individuals first started mining, even though several of the miners worked
underground only a few years in the uranium industry. For this reason, another sub-
group of 1,434 miners who started mining after July 1, 1955 have not been included in the
present analysis. The estimated exposures of 114 miners in this subgroup at the end of
1963 were 840 WLM or more. No lung cancers have been reported from this higher ex-
posure group versus an expected 0.09, although 3 have been reported in the lower exposure
group of 1,320 miners versus an expected number of 1.7.
3.16 The 34 observed deaths from lung cancer shown in table 6 are those that meet
the requirements for group comparisons mentioned in paragraph 3.13 (I.P., the underlying
cause of death was recorded as lung cancer on the death certificate). Fifteen additional
deaths from lung cancer make a total of 49* observed in the subgroup of 1,981 miners
who began mining before July 1955. These were not included in table 6 for one of two
reasons: (1) the "cause of death" was not listed as respiratory cancer on the death certifi-
cate, or (2) the date of death was after June 1965, the cutoff date for the mortality analysis.
Five of these additional 15 cases occurred in the group of 63 miners whose recorded ex-
posures were greater than 3,720 WLM making a total of 12 in this small group. The re-
maining 10 were about evenly distributed among the other exposure groups. As related to
the years since the beginning of underground uranium mining, but without regard lo the
eApo8ure level, 3 appeared among miners with 5 to 9 years elapsed time after the start of
mining, and 12 after 10 or more years had elapsed. Further evidence of a time factor is
indicated by the death of 26 individuals with lung cancer by the end of 1963, and of 23
thereafter. The 49 cases may be considered as more indicative of the real incidence of lung
cancer in the study group, although there is no generally accepted way to determine the
expected value. Detailed information on the 49 cases will be found in the transcript of the
hearings conducted by the Joint Committee on Atomic Energy between May 9 and
August 10, 1967.
Cell Type of Uranium Miners' Lung Cancer
3.17 Although the malignant nature of the growths found at autopsy in miners was
recognized as early as 1879, it was not until 1926 that they were established as carcinoma.47
As pointed out by Gates and Warren,9 the reason that they were initially thought to be a
form of lymphosarcoma is probably that most of the cancers seen were of the small cell
undifferentiated variety which bears some resemblance to lymphosarcoma. Saccomanno
et a/.,48 found that 57 percent of the lung cancers among United States uranium miners
were of the small cell undifferentiated variety. The majority of these were considered by
the authors to be of the "oat cell" type. The authors also showed that the proportion of
small cell undifferentiated cancers among uranium miners increased with the estimated
exposure to radon daughters. Small cell undifferentiated bronchial carcinoma rarely exceeds
20 percent of lung cancers found among nonminers.48 49 Although small cell undifferentiated
carcinoma or "oat cell" carcinoma of the lung is described as the morphological type
representative of the lung cancers identified in uranium miners, it may be well to call
* For the purjMwes of this discussion, the 34 deaths shown in tahle 6 will be referred to as "lung
caneer deaths" and the 15 additional deaths will be referred to as "canes."
23
-------
attention to the fact that bronchogenic flung) cancers may vary appreciably in cell type
in different parts of the tumor and the diagnosis of a specific cell type related to a specific
causal agent is subject to some caution.
Base Metal Miners
3.18 An elevated incidence of lung cancer has been reported among miners of a
sulfide ore that contained iron, copper, xinc, lead, manganese, arsenic, antimony, calcium,
fluorine, and silver.39 The uranium-thorium content is not known but it was probably about
1 percent of that in normal uranium ore. Wagoner, <t a/., also concluded that chronic exjwsure to radon or radon daughters alone is unlikely
to account for the observed increase, although there appeared to be a higher percentage
of undifferentiated cancers than in the comparison group.50
Fluorspar Mines
3.19 de\ illiers, Windish,36 and Parsons, el a/.,51 have made detailed studies on
Newfoundland fluorspar miners. Mthough uranium minerals could not be identified in the
surrounding rock, radon entered the mine in ground water. The average concentration of
radon daughters in the mine air was estimated to range between 2.5 and 10 W L. The group
studied by deV illiers and Windish contained 630 men who had worked 12 months or more
in the mines over a 29-year period (1933-61). Of 69 deaths reported in the group, 26 were
attributed to lung cancer. This number is 25 to 40 times the number that might be expected
if the lung cancer rates in the study group were compared to those of the adult male popu-
lation of Newfoundland.16 Lung cancer deaths were not noted during the first 19 years of
operation of the mines.
Joachimsthal and Schneeberg Miners
3.20 Although mines in the Joachimsthal and Schneeberg areas have been operated
since about 1500 A.D., the excess mortality was not generally attributed to lung cancer
until the 1920 decade.2 47 " The radon 222 concentrations in the mine air were estimated to
have averaged between 3,000 and 15,000 pCi per liter of air 5I bt (i.e., radon daughter con-
centrations of 30-150 WL at radioactive equilibrium). The percent of deaths attributed to
lung cancer has been reported to vary between 30 and 70." 56 Although knowledge about
the population size and age distribution of the miners is incomplete, it was estimated in the
course of this review that the incidence was possibly more than 20 times what might have
been expected.
24
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Relation to Other Agents and Disabilities
3.21 Analysis of medical histories of U.S. uranium miners examined in 1957 and in
1960 showed a greater prevalence of the following complaints which are also observed in
other miners: (1) shortness of breath, (2) persistent cough, (3) history of wheezing or
whistling sounds in the chest, and (1) history of chest pain or pressure in the chest.51 '""
Ventilatorv function studies made during these examinations showed that cumulative
exposure to the atmosphere in uranium mines contributes to a loss in ventilatory function
in a manner similar to that observed in aging and from cigarette smoking.51 s8
3.22 Since a relatively high percentage of I nited States uranium miners smoke
cigarettes, and since cigarette smoking has been associated with an increased incidence of
lung cancer, it is necessary to consider this agent in the etiology of the lung cancers of
uranium miners.1'0 Cl Although I nited Stales uranium miners smoke more than the general
population, they smoke no more than some other occupational groups whose lung cancer
rate is not comparable to that of uranium miners/'-' Since smoking habits appear not to
differ systematically among individuals assigned to the different cumulative exposure
categories in table 6, this variable is not considered important to the comparisons drawn
between groups in that table. The cigarette consumption of the most highly exposed
group was about the same as that of the less exposed groups. Although cigarette smoking
may affect the total occurrence of lung cancer among uranium miners (i.e., cancers ob-
served compared to expected), smoking alone does not explain the trend with cumulative
WI,M. An analysis made by the 1'IIS led to the conclusion that the smoking history of
the uranium miners might increase the expected number of lung cancers from 5.13 as
shown in table 6 to about 7. The data neither prove nor disprove the existence of a rela-
tionship between cigarette smoking and radon daughter exj>osure in the etiology of lung
cancer among uranium miners.
3.23 There is evidence that cigarette smoking contributes materially to nonmalignant
pulmonary impairments to which uranium miners are subject." M Cigarette smoking can
influence lung cancer development among uranium miners in other ways—by changing
the radiation dose distribution (through alterations in ciliary motion or thickness of the
mucus blanket). It might act as a mitotic stimulant to bronchial epithelium which could
reduce latent periods or influence the cell type of carcinoma produced. Although cigarette
smoking seems to increase the incidence of several cell types of carcinoma, the one most
prominently increased is the epidermoid type. This is in contrast to the small cell undif-
ferentiated type which has been rejwrted to be most prominently increased among uranium
miners.48 The possibility that a synergistic relationship may exist between cigarette smok-
ing and exposure to radon daughters needs further study.
3.21 Increased incidence of lung cancer among miners has at times been attributed
to silicosis. However, a number of studies suggest that silicosis by itself does not necessarily
predispose to lung cancer.56 63 M The prevalence of silicosis among United States uranium
miners has been reported to be comparable to that among other United States metal
miners,44 but lung cancer rates of the two groups are quite different.
3.25 The observations described in the previous paragraphs are consistent with the
older observations from Schneeberg and Joachimsthal that many miners had silicosis and
tuberculosis 4l Vl :'9 although I'irchan and Sikl reported that no notable degree of anthracosis
25
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or silicosis was found in the lungs of Joachimsthal miners submitted to autopsy." Miners
have undoubtedly experienced parenchymal pulmonary damage from exposure to the
atmosphere in uranium mines as well as in other types of mines, quite aside from induced
lung cancer. What portion of this parenchymal damage is due to radiation and what por-
tion is due to nonradioactive dust (such as free silica) cannot be determined at this time,
but it seems possible that sufficient exposure to radon daughters may enhance and alter
the fibrogenic response to silica, and may jwssibly exert a fibrogenic effect of its own.
3.26 Among the many items considered in assessing the etiology of the Schneeberg
and Joachimsthal lung cancers was the |>ossiblc role of heredity.50 59 63 Since both mining
areas >\ere relatively isolated, it was felt that centuries of inbreeding might have resulted
in a high lung cancer strain of people as has occurred among certain strains of mice.65""
Although this v\as a possibility, the development of such high rates of lung cancer has never
been observed among isolated nonmining populations. The fluorspar miners mentioned in
paragraph 3.19 also constituted a relatively isolated population, but it was noted that a
nearby similarly isolated community at Crand Banks, which did no mining, had a low
lung cancer rate.36 It has also been noted that high lung cancer rates have been observed
primarily in males of the population. Among U.S. uranium miners, where marked differ-
ences in lung cancer rates have been shown between different cohorts,46 and different
ex|K>sure groups/'" there is no reason to believe that hereditary factors were significantly
different among the groups. There is no basis on which to implicate a common genetic
factor as an important contributor to lung cancer in United States uranium miners.
3.27 The presence of long-lived radioactive materials in the dust of uranium mines M
has given rise to speculation that these may contribute to the lung cancer rate of uranium
miners. The various isotopes of uranium, thorium, and radium have been the principal
radionuclides considered. In uranium ores, the relative abundance of these nuclides is
generally low. Their concentration in mine dust is therefore a function of the richness of
the ore. Uranium determinations on miners' urine have indicated that uranium is not
absorbed in sufficient amounts to present a toxic hazard.™ Thorium isotopes might be
expected to concentrate in lymph nodes, spleen, liver, lung, and bone.73 Kadium isotopes
would be expected to concentrate in l>one. However, no radium could be detected by whole-
body counting in several uranium mill workers who had been heavily exposed to ore dust.70
All the radionuclides in the uranium decay series undoubtedly contribute something to the
radiation dose of uranium miners, and there is some indication that thorium 230 may
have a longer pulmonary retention than its parent uranium.71 73 However, the contribu-
tion by the immediate daughters of radon is so great that the contribution by other radio-
nuclides to the lung dose during the working years is probably negligible by comparison.
3.28 Since carcinogenic hydrocarbons have been observed to be present in the
exhaust from diesel engines,"' there is the possibility that exposure to the exhaust gases
might increase the incidence of lung cancers in uranium miners. There were no diesel
engines in the Schneeberg and Joachimsthal mines during the periods when high lung cancer
rates were noted. Beginning about 1952 a substantial part of United States production of
uranium ore has involved underground diesel equipment. Many of the United States
uranium miners in the present study who developed lung cancer had relatively little
exjK)siire to diesel fumes in their early work experience. In many nonuranium mines diesel
26
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equipment has been used underground for as long as, or longer than in uranium mines.
Gasoline engines are rarely, if ever, used underground.
3.29 Circumstantial evidence appears to rule out diesel exhaust as an important
agent responsible (or the observed increase of lung cancer rates among uranium miners.
However, since diesel exhaust contains the same type of carcinogens as cigarette smoke,~J
it is entirely possible that diesel smoke might contribute in the manner discussed above
with respect to cigarettes (pars. 3.22 and 3.23).
Discussion and Conclusions
3.30 Respiratory impairment of several varieties has, for many years, been a recog-
nized hazard of underground mining. Analysis of available evidence permits the conclusion
that sufficient exposure to radon 222 and its short-lived radioactive daughters in the mine
atmospheres is associated with an increased incidence of lung cancer. The highest incidence
of lung cancer is occurring now in the group of miners (1) who worked in mines in which
the average concentration of radon daughters was usually higher than 10 WL; (2) whose
total cumulative exposures ranged upward from about 1,000 WLM; (3) who started
mining uranium ore more than 10 years ago; and (4) who were moderate to heavy cigarette
smokers. These observations suggest that additional cases of lung cancer can be expected
to develop in the study group.
3.31 The USPIIS epidemiological study was designed to identify possible etiologic
agents that might be implicated as a reason for the high incidence of lung cancer observed
among the uranium miners. Observations on the United States uranium miners who
started underground mining before July 1955 clearly demonstrate that an association
exists between exposure to radon daughters and a higher than expected incidence of lung
cancer when cumulative exposures are more than about 1,000 WLM. The degree of risk
at lower levels of cumulative exposure cannot be determined from currently available
epidemiological data. It is prudent to assume, however, that some degree of risk exists
at any level of exposure, even though effects may not now be evident at the lower levels
of cumulative exposure.
3.32 A review of the epidemiological data conducted by the USPHS after the pre-
liminary report 8 was issued in May 1967, and testimony provided to the Joint Com-
mittee on Atomic Knergy, amplify the previously described uncertainties in the exposure
data and their interpretation.74 These include:
1. The early measurements were very infrequent (sometimes less than once a year)
and sampling sites were selected for purposes of control.
2. Information about the mines in which each individual had worked, including the
year and the number of months in each, was obtained from an interview with each miner
several years later.
3. The information in table 6 is based on estimates of the months worked in under-
ground uranium mines only. Many of the miners also had extensive experience in other
underground mines where radon daughters were present, but at lower concentrations.
Present evidence suggests that underground uranium mining may account for less than
one-half of the total exjH>sure to radon daughters that might be associated with the com-
plete mining experience of the individual when the \\'I,M assigned to underground uranium
27
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mining is in exposure categories A or B (i.e.. less than 360 WI,M).'4 The addition of ex-
jx>sure experience in other underground mines could result in some redistribution of the
34 lung cancer cases in table 6.
4. The miners in the study group started underground uranium mining before 1955,
when radon daughter concentrations up to several hundred WL were commonly found in
individual samples. Other factors taken alone (i.e., ore dust, blasting gases, and other
noxious agents) do not seem to be implicated as etiological agents for the lung cancers
found in underground uranium miners. However, these factors, together with smoking, can
influence any quantitative relationship between ex|M>sure to radon (laughters and the
probability of developing lung cancer by their individual or collective effects on lung tissue.
3.33 Several dosimetry models have been developed in an attempt to derive a numeri-
cal estimate of the effective radiation dose to the lung that might result from exposure to a
given concentration of radon and radon daughters. Details of some of these models are
presented in the appendix of this report. The differences among the various models and the
uncertainties in the basic- radiological parameters that must be used are too large for the
models to be acceptable at the present lime as a basis for estimating the risk of radiation-
induced careinogenesis. Inference of risk drawn from epidemiological studies of lung
cancers associated with exposure to various concentrations of radon and its daughter
products appears to be the most satisfactory basis for evaluating the associated lung cancer
risk at the present time, although no reliable quantitative statement can be made if
cumulative exposures are less than about 1,000 WLM.
28
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32
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SECTION IV
CONTROL CAPABILITIES IN URANIUM MINES
1.1 Several procedures are available to control exposure of miners to radon and its
radioactive daughters in mine atmospheres. These include: (1) removing radon and its
radioactive daughters from mine areas by air replacement, (2) inhibiting the diffusion of
radon into the air of the work areas from the surrounding rock or from abandoned work-
ings, (3) limiting the time individuals can work in areas with an excessive air concentra-
tion of radon-radon daughters (work force management), and (I) reducing the concen-
tration of radon daughters in the inspired air from that prevalent in the mine atmosphere.
4.2 Ventilation of underground work areas is the most common procedure. Natural
draft ventilation is rarely adequate to provide the needed rate of air supply. Therefore.
mechanical movement of the large volumes of air needed into or out of uranium mines is
now the general practice. Portable air ducts and auxiliary fans are commonly used to
conduct fresh air from its supply source through passageways to deliver the air to work
areas in all types of underground mining. The incoming air dilutes and removes air fouled
by dust, drill exhaust, blasting fumes and gases, and exhaust gases and heat from internal
combustion engines. Ventilation is also necessary for comfort and health in underground
mines of all kinds. In uranium mines these same considerations hold, but in addition
ventilating air serves to remove radon and radon daughters as well as the dust and noxious
gases.
1.3 Currently the most practical means for removing radon and its daughters from
mines is to remove the contaminated air to the outside and replace it with fresh air. Mine
ventilation procedures must take into account the size, number, and complexity of mine
workings and the level of radon and its daughter products present within these workings.
The concentration of radon and its daughters in mine atmospheres, if not diluted and
carried away, can build up to very high levels (i.e., hundreds of WL). Continuous planned
ventilation is much more effective than an intermittent supply or exhaust of air, or solely
relying on ventilation by natural means.
1.1 The general considerations involved in ventilation control include:
1. Fresh air supply should be channeled through the mine passageways in such a
manner that it would avoid mixing with contaminated air. The displaced air should be
promptly exhausted aboveground at a distance from the air intake.
2. Old workings not needed for other purposes should be sealed off to inhibit the
release of radon into the work areas. This will also decrease the concentration of radon
daughters that can build up in the work areas.
3. Ventilation systems must be promptly altered or modified as development proceeds.
1. Air velocities through mine areas where men work must be kept within practical
comfort limits.
33
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•1.5 The volume of fresh air that ean be .supplied to underground mine workings is
subject to physical limitations on the air velocity and pressure within the portable air
ducts, and the velocity of the displaced air through exhausting passageways. In regard
to the former, the power requirements to move a unit volume of air through a duct of a
given cross-sectional area are rejmrted to increase with the third power of the volume.
The air velocity in passageways directly affects the miners.1 There are valid objections to
working in a high velocity air flow, especially when the temperature is low. Underground
work crews, particularly those working continuously in passageways, object to being
subjected to uncomfortably high air flow rates, even though the air temperature is main-
tained at a reasonable level.
1.6 Mines that are developed with foreknowledge of the ore body extent and with
engineering design for mining and ventilation generally provide mechanical equipment to
move air into and out of the mine through multiple openings with the least possible dilu-
tion or recirculation. A Bureau of Mines survey 2 has related radon daughter levels in
underground uranium mines to the various ventilation practices; that is, with fans blowing
or exhausting, with fresh air intake via the main mine entrance, or with auxiliary ventila-
tion holes and the total volume of fresh air supplied. The results do not indicate any
systematic differences among these procedures. Regardless of the system, however, multiple
exits are advantageous in shortening air retention time.
4.7 Prior to 1960 mine surveys were infrequent and in many cases the results were
not representative of the average concentration of radon daughters in the mine air. Since
1960 State agencies and the major mining companies have conducted systematic moni-
toring in underground mines. Tables 4 and 5 (sec. II) show that a significant reduction in
the higher concentrations of radon daughters in mine air has been achieved since I960.3
Although the number of mines reporting average concentrations of 1 WL or less is un-
changed, average concentrations of 10 WI, or more in mines have been virtually eliminated.
The percentage of mines rej*orting average radon daughter concentrations in the range of
1 to 3 \VI, has increased. Public statements by mine association and company officials in
States having most of the underground uranium mines suggest that the improvement is
continuing.4 f'
1.8 A meaningful comparison of mine ventilation cost from mine to mine is difficult
because the evolution of ventilation systems is intimately associated with and conditioned
by individual mine development. While it might be supposed that the cost of ventilation
in uranium mines would be greater than the cost for ventilation in nonuranium mines, or
that the cost of ventilation in mines with a lower grade ore would be less than that for
those with a higher grade ore, this is not necessarily the case. Many other cost-affecting
features may be of greater significance than air supply alone. These include such factors
as depth below surface, lateral extension of workings, arrangement of passageways, num-
ber and size of mine openings, and so forth. Because of the general lack of specific information
in this area, special analyses were necessary to evaluate the cost-effect of ventilation rate.
\ nmr.ber of uranium mining companies have carried out and rejK>rted on mine ventilation
cost studies at the request of FRC staff (see tables 7 and 8). These estimates are intended
to illustrate the general magnitude of cost in a few selected mines and are not applicable
to the industry as a whole.
34
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1.9 One study concerns 11 of the larger underground mines that produced more
than 20 percent of the national total of uranium ore. These mines have all been in operation
for 6 to 7 years, and it seems likely that they could continue in operation for several more
years if. after the end of the Government procurement program in 1970. there is a con-
tinuing favorahle market. These mines are individually profitable enterprises and, in
general, they exemplify predevelopment planning for advantageous operation, including
the extra ventilation necessary for radon daughter control. They are similar in operation
and depth from the surface. The reported estimates are for the group as a whole.
1.10 To obtain a general estimate of an "exposure index" in these 11 mines the fol-
lowing procedures were used. The concentrations of radon daughters in the mine air were
determined by taking 10-minute air samples at about monthly intervals at underground
work locations. The results of these determinations were then correlated with the time of
occupancy by various categories of mine workers to give approximate time-weighted
average exposures. In this way, an "exposure index" related to the WL was derived for
each of the various work categories. These average exposures were then weighted by the
number of persons involved in the respective categories to reach a "mine index" value
representing the average exposure in the mine. The average mine index value for the 11
mines was reported as about 2 WL in 1965.
1.11 Total costs are reported for the 11 mines collectively in terms of "investment
costs" and "annual operating costs." Based on this cost experience covering 6 or 7 years.
the mining companies have also derived estimates of what these costs might be for two
hypothetical cases: (1) the investment cost and operation expense necessary to provide
the nominal ventilation that would have been needed for these mines without considering
radon control (the prevailing mine index value would have been about 10 WL), and
(2) the investment and operating costs that would be entailed in the further reduction of
the prevailing mine index to 1.0 WL.
1.12 With these estimated costs, the extrapolated "10-year" cost of mining for opera-
lion of mine indexes of 10 WL and 1 WL were estimated. This extrapolation is intended to
provide some appreciation of the way total cost might vary under progressively more
stringent control requirements. These figures will differ from the actual costs as follows:
1. Ore bodies that do not last for 10 years will have less than the extrapolated 10-year
operating cost and more than the stated investment cost, since the investment would have
to be repealed when a new ore body is opened.
2. Ore bodies that do allow operation for 10 years will require additional inveslmenl
and operaling cosls as the present operations are extended. These costs cannot be estimated
at the present time because the rate of new development in particular localions cannot
be predicted thai far in advance. The relationships are shown in table 7.
1.13 Another study concerns estimates for each of 3 mines where conditions differ
from the group of 11 mines as to geology, depth, extent of workings, productive capacity.
arrangement of passageways, number of openings, and so forth. Furthermore, the economy of
operations is so different that the data reported for these 3 mines are not comparable to the
group of 11 mines discussed above, and perhaps not even comparable among themselves.
Collectively, the annual production of uranium ore of the three mines has been at a rate
of about 2 percent of the national total. This study presents ventilation investment costs
35
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and ventilation operating costs over a 6-year period of continual improvement effort to-
gether with current concentration values of radon daughters in all mine areas of concern.
It should he noted that air concentration values are rej>orted rather than a mine index.
The WL values reported in tahle 8 are averages for 1965 and represent the present state of
control development. The costs are then compared with c -/responding costs that would
have heen incurred if the ventilation had been limited to that which would have been
needed in these mines without considering radon control. It has been estimated that the
radon daughter concentrations would then have been in the range of 5 to 20 W'L with this
limited ventilation.
TABLE 7.- I'entilation cost estimates—// mine stud\
Past experience
Kstimated ventilation costs
without radon control
Additional cost radon control
from 10 WL to 2 WL
Estimated additional cost to
Investment
cost
< )per. cost
(10-yr. est.)
Total cost
(K)yrs.)
Millions of dollars
3.9
2.0
7.9
11.8
2. 8 , 4. 8
1.9 5.1
reduce from 2 WL to 1 WL. . . j 1.5
6.0
7.0
7.5
Kstimated mine
index WL
» 2
"10
1
Total cost to control at 1 WL--10 years.
19.3
•Composite mine index for 1965.
b Kstimate of what the composite mine index would be with normal ventilation practices.
TABLE 8. — I enti'ation cost estimates- -3 mine study
-
Investment
rost
( >per.
(6-vr.
cost
est.)
Total cost
(6 >rs.)
Vvcragc
concentration
WL
'in i r .1 n.._
361
321
75
757
63
66
06
135
120
85
50
255
21
18
04
43
481
406
125
1,012
84
84
10
178
M.4
M.5
•1.5
" 5 to 20
Past experience
Mine A
Mine B
Mine C
Total
Kstimated for case of minimum
tvntHation
Mine A
Mine B
Mine C
Total
• Vverape ^ I. concentrations in 1965.
'' Kstimate of what the average \X I. concent rations would IM- with normal ventilation practices.
36
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1.14 This study indicates that radon daughter control at 1.5 \\ I, in these mines is
being achieved at a total cost of ahout $834,000 over a 6-year period. The relation of
ventilation cost to tons of ore produced for the group of three mines is ahout 1.5 times the
corresponding relation in the 11 mine group because of differences in size and other factors.
Inhibiting the Diffusion of Radon
4.15 Sealing off old workings not needed for other purposes is common practice and
is effective, but the use of various types of coatings sprayed on the mine walls has proved
to be ineffective in reducing the diffusion oi radon from the walls into the work areas.
Variations in barometric pressure have been shown to have an inverse effect on the rate of
radon emanation into mines and, therefore, on the concentration of radon and radon
daughters in mine air.7 This suggests that an applied overpressure might be used to ad-
vantage in some mines for reducing the release rate of radon into mine air. Kxperiments
designed to augment the normal atmospheric pressure in mines with an overpressure
approximately equal to the usual range of atmospheric pressure variation have been
performed. A pressure of 0.6 cm of mercury above normal reduced the rate of radon release
to mine air by a factor of about 5. It has also been reported that if the pressure can be
lowered in a nearby underground area—removed perhaps a hundred feet from the mine
workings—the resulting flow of air into the rock can induce a flow of radon away from the
work spaces. The practical value of these suggestions for mine application remains to be
demonstrated on an engineering scale. However, an electrical analog system that permits
analysis of the air flow in various mine configurations at different simulated air pressures
has been developed.8 Many of the.primary factors can be investigated cheaply without the
necessity of large scale engineering experimentation.
Work Force Management
4.16 Although limiting a miner's occupancy time in relatively high concentrations
of radon or its daughter products has not been a normal practice in uranium mining, this
procedure has been used in various activities of the nuclear industry when men routinely-
work in radiation fields of varying intensities. Since the concentrations of radon daughters
vary widely with location and with time at a single location in a mine (from 1 WL in well
ventilated areas to several hundred WL in stagnant areas), it has been difficult to establish
an accurate record of the time-weighted average exposure for each worker. However,
reasonable estimates of the average values are possible, and improvement in the evalua-
tions can be expected in the future. Controlling a miner's exposure to radon daughter
concentrations by controlling the time he is allowed to work in different atmospheres should
generally be feasible and not too restrictive, provided the radon daughter concentrations
are known and are not too high.
4.17 It is normal practice to control mine operations primarily on the basis of esti-
mated exposures to radon daughter concentrations found in a working area at the time of
sampling. The Colorado Bureau of Mines, for example, assumes that in a small mine with
less than 15 men an individual miner will not spend more than 50 percent of his time in
areas with significant concentrations of radon daughters. In larger mines, where most of
the men stay underground for a full working shift, the men are assigned an exposure value
of 75 percent of the average value for the mine.
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4.18 The basic philosophy of control in Colorado is that if the maximum radon
daughter concentration in working areas is kept within acceptable limits, the individual
worker will be adequately protected without reference to mine averages or time-weighted
exposure calculations. New Mexico also limits maximum concentrations in mines, but places
more emphasis on estimates of time-weighted exposures in its control program. Both
regulatory programs are derived from recommendations made in 1960 by Committee \-7
of the I SA Standards Institute. Acceptable limits, as recommended in the Institute's
standard are:
(a) "If the 13-week weighted average exposure of the workers to radon daughters, is
less than the MFC. the conditions may be considered to be controlled and no action is
necessary.
(b) "If samples in any working area show a concentration of radon daughters exceed-
ing the MFC. but less than three times this level, sufficient additional measurements shall
be taken to determine the worker's weighted average exjK>sure for 13 weeks.
(c) "If samples show a concentration of radon daughters more than 3 times the MFC,
but less than 10 times this value, corrective action shall be initiated.
(d) "If samples show a concentration of radon daughters greater than 10 times the
MFC, immediate action shall be taken to reduce the worker's exposure and correct the
condition."
The MFC referred to in the Institute's recommendations is equivalent to 1 WI, of radon
daughters in the mine atmosphere.
4.19 Integrating personnel exposures to a varying radiation field by providing a
personal dosimeter, such as a film badge, is standard practice for controlling exi>osurc to
external sources in the nuclear industry. A film badge capable of recording the alpha
particles emitted from nuclides in mine air has been developed by the mining industry
and its reliability is being tested. With proper calibration, such a device may have utility
as an integrating dosimeter. The availability and use of a reliable integrating dosimeter
may permit more effective estimates of exposure than can be achieved by calculation of
time-weighted average exposures. In addition, a detection system permitting continuous
measurement and remote readout of radon daughter concentrations in various mine areas
are under development.
1.20 The possibility that the quantities of the longer-lived lead 210 (RaD) or
polonium 210 (RaF) in various body tissues could serve as an index of the total quantities
of radon and radon daughters taken in has been investigated. Concentrations of these
radionuclides in blood, excretion in urine, and deposition in hair, teeth, and bone have
been examined; however, present information does not permit correlation of these quantities
directly to lung dose.
Respirators
4.21 Reducing the concentrations of radon daughters in inspired air, as compared
to the concentrations in the mine air by the use of respirators, is an obvious exposure con-
trol technique. Available estimates suggest that an ordinary surgeon's mask could give a
2- to 5-fold reduction. More efficient filters with low impedance might achieve a 5- to 100-
fold reduction. Respirators would be primarily effective for the partial removal of radon
daughters. Radon concentrations would not be affected. Therefore, respirators can be
38
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useful as ancillary devices to reduce exposure when the radon daughter concentrations in
the mine atmosphere are relatively high. The primary prohlem with respirators now in use
is that it is very difficult to assure that they are used and that tliev are worn properly,
even if the design of the respirator is appropriate to mining operation.
1.22 The use of respirators or self-contained breathing devices in United States
uranium mines has been generally reserved for emergencies or contiol of silica dust. It has
been reported that respirators that reduce the radon daughter products in inhaled air are
worn by uranium miners in some other countries.9 Since it is accepted that there should
be more stringent control over the internal radiation exposure received by uranium miners,
the Council staff strongly recommends that there be a concerted effort to improve respira-
tory devices. It is imperative that they be reliable and that they be acceptable to the
miners. In most cases, underground uranium miners wear additional equipment, such as
the hard hat, miners' lamp and battery (which may weigh up to 6 pounds), safety boots,
and in some cases safety lines. In addition to his regular equipment a miner may also have
to carry manv different pieces of equipment and tools. His work sometimes requires over-
head work, barring of loose rock, crawling in cramped spaces, and climbing ladders. If a
respiratory device is to be imposed as an additional requirement it must not interfere with
the miner's vision or with his freedom of motion to such an extent that it results in a net
increase in his probability of incurring a serious accident. Furthermore, since oral communi-
cation is of major importance in mines, any respirator design should take this into account.
1.23 Use of respirators by miners should not deter mine management from providing
adequate ventilation in their mines. Ventilation remains the primary means bv which
radon daughter concentrations can be controlled. If the basic principles of ventilation
engineering and design are closely followed, radon daughter concentrations in mines can
be significantly reduced.
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REFERENCES
1 Coleman. K. I)., t'l til. Haclon and Radon Daughters Hazards in Mine Atmospheres Investiga-
tions on Supplemental Control. Mill Quarterly, />«r. 1956.
•'Report to Congress l>y Secretary of the Interior. Health and Safety of Metal and Noninetal
Mines (submitted in response to PL 87 300. 75 Slat. 649). 1963.
3 Colorado Bureau of Mines \nnual Report, 1965.
* Homney. Miles P. Utali Mining Assoe. Testimony, p. .'573. U.S. Congress. Senate. Committee on
l,alx>r and Public Vlelfarc. To establish a 1'Vileral Metal and Nonmctallie Mine Safety \ct: Hearings
before Subcommittee on l,alx>r. 89tb (Congress. 2d session.
5 Swent, L. W. Homestake Mining Co.. N. Mex. Testimony, p. 408. I .S. Congress. Senate, Com-
mittee on Labor and Public Welfare. To establish a Federal Metal and Nonmetallic Mine Safety Act:
Hearings before SulH-ommittee on Labor. 89th Congress, 2d session.
'' Heamer. R. U . Wyoming Mining Assoc. Testimony, p. 474. I'.S. Congress. Senate. Committee on
Labor and Public Welfare. To establish a Federal Metal and Nonmetallic Mine Safety \ct: Hearings
before Subcommittee on Labor, 89th Congress, 2d session.
' Schroeder, (». I... F.vans. H. I)., and Kramer. II. W. F.ffect of Applied Pressure on IJadon Charac-
teristics of an Underground Mine Fnvironment Trans. Society of Mining F.ngineers. March 196(».
"MIT Annual Progress Heporl (1966). MIT 952-3. Hadiiim and Mesothoriiim Poisoning and
Dosimetry and Instrumentation Techniques in Applied Radioactivity.
9 Proc. IAFA Symposium on Radiological Health and Safely in Mining and Milling of .Nuclear
Materials, Vienna, p. 32, 1964.
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SECTION V
SUMMARY AND RECOMMKNDATIONS
5.1 To provide a Federal policy on human radiation exposure, the Federal Radiation
Council was formed in 1959 (Public Law 86-373) to ". . . advise the President with respect
to radiation matters, directly or indirectly affecting health, including guidance for all
Federal agencies in the formulation of radiation standards and in the establishment and
execution of programs of cooperation with States." The present report fulfills this responsi-
bility with regard to a Federal policy relating to the protection of underground uranium
miners against deleterious health effects resulting from the inhalation of the radioactive
daughters of radon 222. Although primary emphasis has been placed on underground
uranium mining, the guidance contained in this report may be applied, as necessary, to
any type of mine where similar concentrations of radon daughter products may be found.
5.2 This report emphasizes the radiation hazard associated with the inhalation of
radon daughters as they occur in the air of underground uranium mines. Although other
sources of radiation exposure occur in connection with underground mining, and potential
radiation effects are not necessarily confined to the respiratory system, they are considered
of secondary importance as compared to the possible effects resulting from the inhalation of
radon daughters.
5.3 In prior reports the Federal Radiation Council has expressed the philosophy
that guidance for radiation protection involves achieving a balance between the risk of
radiation-induced injury and the benefits derived from the practice causing the exposure
to radiation. An implicit part of such a balance is a necessity for considering the relation
between the difficulties involved in reducing the radiation exposure by a given amount and
the risk that might be associated with that amount of exposure.
Evidence of Radiation Hazards in Underground Cranium Mines
5.1 Available information on the radiological factors involved in the underground
mining of uranium ore has been carefully examined. The findings of immediate interest for
establishing radiation protection guidance are:
I. It has been observed that underground uranium miners have a higher incidence of
lung cancer than is found in the male population in the same geographic area. Continued
exposure to the radioactive decay products of the naturally occurring gas radon 222 has
been implicated as an important cause of this increased incidence. The decay products of
radon 222 (radon daughters) of interest are: polonium 218, lead 214, bismuth 211, and
polonium 211. For puqwses of this report, the radiation dose from radon itself is not
considered.
2. The principal radiation hazard is associated with the inhalation of mine air con-
taining radionuclidcs that irradiate lung tissue nonuniformly. The most serious result is
43
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the development of lung cancer, which generally does not appear for 10 to 20 years after
the individual started uranium mining.
3. In most domestic underground uranium mines the measurement of radon daughter
concentration is obtained by using a specific field method. The results are then compared
to the "Working Level" (WL), a unit defined as any combination of radon daughters in
1 liter of air that will result in the ultimate emission of 1.3 • 10' McY of potential alpha
energy. The concentration of radon daughters in air of unventilated underground uranium
mines ranges from a fraction of a WL to several hundred WL. Kxposure to radon daughters
over a period of time may he expressed in terms of cumulative "Working Level Months"
(WLM). Inhalation of air containing a radon daughter concentration of 1 WL for 170
working hours results in an exposure of 1 WLM.
I. Observations on I nited States uranium miners who started underground mining
before July 1955 clearly demonstrate that an association exists between exposure to radon
daughters and a higher than expected incidence of lung cancer when the cumulative
exposures are more than about 1,000 WLM. The degree of risk at lower levels of cumulative
exposure cannot be determined from currently available epidcmiological data. As discussed
in paragraph 3.It. the data do not suggest or exclude the existence of a threshold. It is
prudent, however, to assume that some degree of risk exists at any level of exj>osiire, even
though possible effects may not now be evident at the lower levels of cumulative exposure.
5. The highest incidence of lung cancer is now occurring in the group of miners:
(1) who worked in mines in which the average concentration of radon daughters was usually
higher than 10 WL: (2) whose total cumulative exposures were estimated to range upward
from about 1,000 WLM: (3) who started mining uranium ore more than 10 years ago; and
(4) who were moderate to heavy cigarette smokers. These observations suggest that addi-
tional cases of lung cancer can be expected to develop in the study group.
factors Related to the Evaluation of Benefit and Control Capabilities
5.5 Available information on benefits to be derived from the mining of uranium ores,
difficulties encountered in reducing radon daughter concentrations from previous levels
to current levels, and the additional difficulties that can be anticipated if further reduc-
tions in radon daughter concentrations are required has also been reviewed. The findings
of immediate interest to the derivation of guidance for radiation protection in uranium
mining are as follows:
1. Uranium is currently the basic fuel needed for the development of nuclear energy,
and all projections point to an increasingly important role for nuclear energy in meeting
national electric jM>wer requirements.
2. Lranium mining is an important economic asset to the States in which the ore is
mined. In addition to the value of the ore, mining provides important opportunities for
employment. It is estimated that the work force will vary between 2,000 and 5,000 men in
the next decade.
3. A significant reduction in the concentration of radon daughters in the air of under-
ground mines has been achieved by the industry since 1960. Kstimates of probable ex-
posures of 2.177 miners in a five-State area during the third quarter of 1966 indicate that
about 31 percent of miners were exposed to levels of 1 WL or less; 55 percent were exposed
in the range of 1-3 WL; and about 11 percent were exposed to levels higher than 3 WL.
44
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I. Ventilation with fresh air is presently considered the most feasible technique for
controlling the concentrations of radon daughters in mine air. The highest concentrations
are usually found in the slope areas where some work must he done before ventilation can
be brought directly to the miner's working area. This suggests that there are practical
limits to the degree by which radon daughter concentrations in mine air can be controlled
by ventilation alone, \\ailable information suggests that it may be physically impossible
to maintain radon daughter concentration in all parts of the mine to as low as 1 WI, at all
times.
5. Research studies have been made on possible procedures that might be used to
block the diffusion of radon from rock into mine air. Positive pressure ventilation can be
useful if the rock is porous enough to permit air to flow into it. Coating the rock surface
might be useful if it is capable of blocking the diffusion of radon.
6. The effectiveness, feasibility, and safety of various types of auxiliary respiratory
protective equipment, as they might be used in underground uranium mining, deserve the
most thorough study.
7. It is common practice to limit the highest radon daughter concentrations in which
normal mining operations are allowed without determining the radiation exposure at
lower concentrations. This procedure makes it possible to estimate the maximum exposure
rate, but does not provide the information necessary for estimating a true average exposure,
and hence the radiation risk for any one individual or group.
(riiidanre for Radiation Protection in I nderground Mines
5.6 On the basis of the preceding findings, the Federal Radiation Council has con-
cluded that radon and its radioactive daughter products occur in sufficient concentrations
in underground uranium mines to require actions to control the potential radiation ex-
posure associated with working in such environments. Although primary emphasis has
been placed on underground uranium mining, the guidance contained in this report may
be applied, as necessary, to any type of mine where similar concentrations of radon daughter
products may be found.
5.7 The Council has considered: (1) the apparent relationship between cumulative
exposure to radon daughters and the risk of subsequent development of lung cancer as
shown by presently available epidemiological data; (2) the range of annual exposures
received by various categories of miners now engaged in uranium mining; (3) the techno-
logical problems involved in achieving control to various levels of annual exj>osure, and
tlie ability of present technology as practiced by the industry to reduce radon concentra-
tions to different levels; (4) improvements that might be introduced by application of a
more advanced technology, and the length of time such improvements might take; and
(5) the magnitude of the radiation risk in the light of the other risks that are faced in
underground uranium mining, and the impact of the Council's recommendations might
have on efforts to reduce these risks simultaneously.
5.8 The selection of the proposed standard for the control of radon daughters in
underground mine air must necessarily involve a judgment based on all relevant infor-
mation. In selecting the standards the Council has also considered: (1) the possible magni-
tude of the cumulative radiation exposure that individuals might receive under the practical
application of the proposed standard; (2) the range of individual risk that might result;
45
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(3) the practical difficulties and feasibility of reducing exposures; (4) the change in indi-
vidual risk that could he associated with such reduced e\|M>sures; and (5) various control
standards used by different countries for control ol radon and radon daughter concentra-
tions in underground uranium mines.
5.9 On the basis of the information presented in the preceding sections, the following
guidance is recommended:
1. Occupational exposure to radon daughters in underground uranium mines be
controlled so that no individual miner will receive an exposure of more than 6 Vi I,M in any
consecutive 3-month period and no more than 12 WLM in any consecutive 12-month period.
Actual exposures should be kept as far below these values as practicable.
2. Areas in underground uranium mines, whether normally or occasionally occupied,
be monitored for the concentration of radon daughters in the mine air. The location and
frequency of taking samples should be determined in relation to compliance with recom-
mendation 1.
3. Appropriate records of the exposure from radon daughters in the mine air teceived
by individuals working in uranium mines be established and maintained.
5.10 The Federal Radiation Council recognizes that current mining conditions are
much better than those prevailing 10 years ago. However, it also considers that more
improvement is needed to provide proper control of exposure to radon daughters. Steps
to make improvements should be initiated immediately and made operational as soon as
possible.
Research and Development Needs
5.11 The Federal Radiation Council recognizes that present regulatory procedures
and the presently used technology are not adequate to insure compliance w ith the foregoing
recommendations. The development of the appropriate technology and the modification of
existing regulatory procedures needs to be supported by an applied research and develop-
ment program. The general areas deserving attention include: (1) the development of the
technology directed to the control and more reliable estimate of individual exposures to
radon daughters; (2) registration and compilation of individual exposure records; (3) causal
relationships between varying exposures to radon daughters and subsequent development
of disability; and (4) improvement of mining practices. In addition, continued attention
needs to be given to the development of adequate compensation procedures and the
provision of educational opportunity and training programs wherever needed.
5.12 The technology concerned with the control and estimate of individual exposure
to radon daughters include: (1) the development of more sensitive and more rugged equip-
ment for the measurement of radon daughter concentrations in mine air; (2) the develop-
ment of continous air monitoring equipment; (3) the development of devices permitting
measurement of integrated exposures (personnel dosimetry); (1) the development and
testing of low impedance respiratory protective devices; and (5) a more precise definition
of the composition of the mine atmospheres, including (a) a measurement of radon 222
and each of its first three daughters separately in representative mine air, (b) the distribu-
tion of the three daughters between ions, nuclei, and various larger particle sizes, and
(c) the conditions under which these may be altered by varying modes of ventilation, or
by various designs of respirators.
46
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5.13 The practical determination of what is needed to implement the recommenda-
tions should take cognizance of the following: (1) data needed to indicate compliance with
recommendation 1; (2) an evidentiary record needed to support or deny claims for occu-
pational disability; (3) studies to evaluate possible causal relationships between exposure
to radon daughters and the incidence of lung cancer at the lower cumulative exj>osure
levels; and (1) development of a basis for estimating cumulative occupational exposure
that the average miner might receive in the future under this guidance.
5.11 In regard to causal relationships, epidemiological studies should be continued
on uranium miners and expanded to include other miners who could serve as appropriate
comparison groups. Such studies should include, with proper categorization, low, inter-
mediate, and high radon radon daughter exposure groups as well as a health history
follow up.
5.15 The development of improved mining practices should place the emphasis on
mine-planning as it relates to ventilation, removal of radon daughters from underground
mine air, and reduction of radon emission from the rock into the mine. This program is
primarily directed to the control of the mine air environment.
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APPENDIX
DOSIMKTKIC AM) R ADIOBIOLOGICAL CONSIDKR ATIONS RKLATKD TO
TIIK ANALYSIS OF RADIATION HAZARDS IN URANIUM MINING
I. Since the observed lung cancers appear to arise primarily in the bronchi near the
hilus of the lung, most authors concerned with the dosimetric and radiobiological aspects
of the problem assume the relevant biological target to be the basal cells in the bronchial
epithelium. It is further presumed that the principal radiation dose arises from the alpha
particles (see table 3, sec. II) released by the decay of the short-lived radioactive decay
products of radon on or in the mucous membrane of the bronchi. The small contribution
of alpha particles released from radon gas in the air passages or from radon gas dissolved
in the mucus covering the walls of the bronchial passages and beta or gamma radiation
originating from any of the radon decay products is disregarded in the present analysis.
It is further assumed that, except for the extent that they carry the short-lived daughter
products of radon, the airborne dust particles (par. 2.1) contribute only indirectly to the
risk of lung cancer, but the point is not absolutely established.
2. The immediately relevant nuclides start with polonium 218 and terminate with
polonium 211; the intervening short-lived nuclides come into radioactive equilibrium with
the parent radon 222 in about 3 hours. The formation of lead 210, with its 22-year half-life,
then effectively blocks the series; its formation in the respiratory system is inconsequential.
Bismuth 211 has an alternative mode of disintegration by alpha emission to thallium 210,
which then decays by beta emission to lead 210; however, the fraction of the disintegrations
follow ing this mode is too small to be of consequence in the present analysis.
Range of Alpha Particles in Tissue
3. One of the most im|K>rtant radiobiological parameters is the range of alpha particles
in tissue. Alpha particle ranges as a function of energy are accurately known in air, many
simple gases, and a few solids, notably mica, which can be manipulated in thin sheets.
The range in liquids is estimated by calculations based on the summation of stopping
powers in an aqueous material of unit density. However, the actual ranges in real tissue
depend primarily on the tissue density and the energy of the alpha particle.
1. Large differences in the range for the 6.0 MeV alpha particle from |>oloniiim 218
are reported by different authors; Lea ' calculates a range of 17ju, while Jacobi - gives
55ju. Since most radiological physicists accept the values given by Lea, his values (table 1)
have been used for the calculations in this appendix. It is recognized, however, that what
would appear to be a simple physical parameter, is actually subject to ai\ important degree
of uncertainty.
Linear Energy Transfer Along Alpha Particle Tracks
5. The primary ion density of an alpha particle is relatively high throughout the track
and increases sharply as the energy of the particle falls below 2 MeV. Typical values as
49
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given by Ix>a ' are shown in table 2. If it is assumed that virtually all alpha particles pene-
trating to sensitive tissue in the bronchial epithelium are already depleted to an energy
of about 2 MeV, the effective ionization would all take place near the peak of the Bragg
Curve. More importantly, the possible role of the delta rays (secondary electrons) as con-
tributors to the total ion i/a lion has apparently not been (minted out. At 2 MeV the delta
rays produce about 80 percent as much ionization as the primary alpha particle ionizations.
Because of their lower linear energy transfer and low mass, the delta rays produce a "fuzz"
of ionization around the alpha track whose total track length is two to three times that of
the primary track. The ionization produced by delta rays is considered primarily in relation
to the detailed mechanisms by which ionizing radiation may initiate changes that ulti-
mately lead to injury. It is conceivable that the delta rays could play a significant role in
the physical dose actually delivered to the critical tissue. However, the empirical observa-
tions of biological effects associated with exposure to stated quantities of radon daughters
must inevitably include whatever effects of delta ray ionization are present.
TABLE 1. Kncrgies and ranges of alpha /xirtir/r.s in tissue
\urli
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7. Failla,5 in a report prepared in 1942, proposed a simple model of the bronchus 1.5
centimeters in diameter and 3 centimeters long containing the inhaled radioactive gas.
Corrections were proposed for solubility of the gas in the fluid of the bronchial tube. The
entire radiation dose was assumed to be absorbed in a cylindrical shell of thickness equal
to the "effective range" in tissue. In line with a familiar device of the time, the effective
range was taken to be one-half of the nominal full range in tissue. Failla"s argument was
generalized for all radioactive gases. Some discussions were held at the time on the role of
the "active deposit" in inhaled radon gas. Mitchelle in 1915, using a model very similar to
Fa ilia'ft, included a contribution from radon decay products formed by the radioactive
decay of radon in the bronchus.
8. The first clear identification of the short-lived decay products of radon as the
principal source of radiation dose to the bronchus is attributable to Bale.7 Work at the
University of Rochester by Bale, Shapiro, and others has notably clarified understanding
of the important parameters.8 n Chamberlain and Dyson 12 recognized that polonium 218,
formed by the radioactive decay of radon 222 gas, initially exists as a highly mobile free
ion or atom. In relatively dust-free air it may persist in that form for about 50 seconds.
Free atoms impinge on the walls of the trachea and are trapped with virtually 100 percent
efficiency. The authors demonstrated this deposition in glass tubes using controlled flow
rates, and later in a simulated trachea and main bronchus. The observations were success-
fully related to diffusion theory by the method of Gormley and Kennedy.13
9. With the dust loading in actual mine atmospheres the mean life of the free atoms
may be as short as 10 seconds and rarely as long as 50 seconds. A mean life of 30 seconds
is often assumed for calculation purposes. Chamberlain and Dyson calculated representative
average doses in a cylindrical shell of tissue 45-microns thick at different breathing rates,
and for an atmosphere containing 1,000 pCi radon 222 (1.72 x 107 atoms), 1,000 unattached
polonium 218 atoms, and 100 unattached lead 214 atoms per liter of air. Typical results
are shown in table 3. These calculations assume that only the alpha particle from polonium
218 is effective, other decay products being removed from the site of initial deposition
along the mucus escalator. Also, no correction was made for deposition of polonium 218
free atoms in the nasal passages, although this was estimated at about 30 percent.
TABLE 3.—Average dose rates to the epithelium of the trachea and main bronchi
Minute volume
(liters)
10
20
40
Ix>wer trachea
(mra
-------
1. Polonium 218 in the relevant atmosphere is in radioactive equilibrium with the
radon 222, while the longer-lived lead 21 1 and bismuth 21 I are at 50 percent of equilibrium.
All the radon daughter products are inferential!) attached to dust particles.
2. Twelve percent of the dust particles are fixed in the bronchial passages of the lung
and remain there through the decay of polonium 21 1.
3. The volume of air in the bronchial passages was taken as 100 cc, and the mass of
"uniformly" irradiated tissue was taken as 20 grain*.
12. From these assumptions. Morgan concluded that either an atmosphere of 8.8
pCi of radon 222 per liter of air with the stated burden of associated radon daughters, or
3,500 pCi of radon per liter of air with no burden of daughters, would give a dose of 0.3
rein per 168-hour week to bronchial tissue.
13. According to Stewart and Simpson, the maximum permissible concentration of
radon 222 presented in the International Commission on Radiological Protection (ICRP)
report of Committee II on Permissible Dose for Internal Irradiation16 is a compromise
interpolating between the calculations of Morgan and those of Chamberlain and Dyson.
The ICRP in its latest publication on this subject 17 has not changed its recommendations;
however, a subcommittee is now reviewing the recommendations. The ICRP formula is
given as:
3,000
MPC;, = /. i . /- P^> 2"Rn per liter of air
where/ is the fraction of the equilibrium amount of polonium 218 ions which are unattached
to nuclei. Morgan's case for radon 222 alone is then equated with / = zero, leading to a
value of 3,000 pCi radon 222 per liter of air instead of the 3,500 pCi radon 222 per liter of
air originally calculated by Morgan. Chamberlain and Dyson's case corresponds to/ = 0.1
leading to a value of 30 pCi radon 222 per liter of air.
1 \. \ more recent report by a task group on lung dynamics for ICRP Committee II
contains a valuable compilation of data appropriate for lung deposition calculation.18 The
latter report presents arguments both for and against the inijrartance of free atoms (or
ions), and concludes that "neither the concentrations occurring nor their deposition
tendencies has been established."
15. The two models described next attempt to improve on the Chamberlain and
Dyson model and attribute the principal hazard to the inhalation of radon daughters
attached to respirable dust.
16. Mtshulcr. Nelson, and Kuschner '* chose a reference atmosphere of 100 pCi radon
222 per liter having 200 pCi of total daughter products per liter of air ('i the equilibrium
value). Sixty percent of the daughter products were distributed between free ions (about
150 per liter of air) and those attached to particles of less than 0.1 -micron (/*) diameter.
Forty percent of the daughter products were considered to be attached to airborne particles
greater than 0.1 n in diameter. These larger particles were further subdivided into live
groups of aerodynamic size 0.2, 0.6, 2, 6, and 20/*.
17. Although these subdivisions are arbitrary, they appear to be reasonable. The total
daughter activity of 200 pCi per liter is compatible with the measurements of Tsivoglou.
Aver, and Hobday.1'" The 10-percent attachment to particles greater than O.!M appears to
be compatible with measurements in mines of the Colorado plateau made by the VKC
Health and Safety laboratory,-'1 if the interpretation of the cascade impactor measure-
ments is accepted. The number (150) of free ions per liter of air is said to have come from
52
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the work of Chamberlain and Dyson.12 The number of particles with diameters less than
O.IM was apparently obtained from the difference.
18. The deposition of free ions and nuclei in the lungs was calculated by the general
methods of Shapiro and of Chamberlain and Dyson. Particle dc}K>sition was deduced by
the method of Landahl.22 Separate calculations were made for mouth breathing and nose
breathing. These derivations of the regional de|>osition of radon daughters in the lung
appear to provide a better estimate of the initial deposition than those used in previous
models.
19. A further improvement is the attempt to calculate regional distribution of the
radon daughter product disintegrations. This is based on calculations of mucus flow
throughout the bronchial tree. The end product is an estimate of the number of disintegra-
tions of polonium 218 and polonium 211 separately occurring in the respiratory regions:
(1) trachea, (2) main bronchi, (3) lobar or secondary, (4) segmental or tertiary, (5) sub-
segmental or quaternary, and (6) terminal bronchi or bronchioles. The average regional
dose is computed separately over the depth of penetration of each alpha particle. It is
further assumed that the tissue dose from polonium 218 alpha particles falls linearly to
zero at 47^, while that from polonium 214 alpha particles falls linearly to zero at lln,
each being normalized to its own average dose at midrange. Combined depth dose curves
are thus obtained from which the effective dose at any prescribed depth may be derived.
20. Altshuleretd/.,19 visualized the mucus layer as being In thick, with all the active par-
ticles resting on the free surface. This layer is swept upward by the cilia which are bathed
in a serous layer of the same thickness. Below this lies the bronchial epithelium containing
goblet cells, ciliated cells, and basal cells supported on the basement membrane. The
relevant biological target was considered to be the nuclei of the basal cells, some 7^ above
the basement membrane. The prescribed depth is obtained by measuring the bronchial
epithelium thickness and adding In (i.e., In mucus-\-7n serous layer—7/u above the base-
ment membrane). See figure 1.
21. The thickness of the bronchial epithelium is highly variable. Typical values quoted
by Altshuler rt al. (table 4), came from one subject and included a correction factor of 1.30
for tissue shrinkage. Kngel23 shows sections that arc generally compatible witli the figures
in table 4, although they appear to show even wider variations. The calculated dose is
very sensitive to the estimated thickness of the mucus layer and epithelium thickness;
small changes in the estimated thickness cause large changes in the calculated dose.
TABI.K 4.-—Thickness of bronchial epithelium in different ftarts of the lunfis "
(!alegor\
Main
lx>har
(M)
.N-gmental
Kxrcptionally thin. .
Median thickness.
52 ! 42 29
89 I 63 ! 56
For example, the prescribed depth for the exceptionally thin portions of the segmental
bronchi is taken as 29/u + 7/u = 36/u, which would give a calculated dose of 21 rads per year
for the assumed burden of radon daughters. At the other extreme, the prescribed depth
would be 89ju+7^ leading to a '"/ero" dose in the region of median thickness in the main
bronchus since the maximum range for the |x>lomum 211 alpha particle is only 71/u (see
table 1).
53
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22. A model which is conceptually similar to that proposed by Altshuler et al. was
proposed at the same symposium by Jacobi.2 However, there are important differences in
detail between the two. These are:
1. Jacobi used a continuous distribution of particle size in aerosols of ordinary au-
ral her than the multicompartment model of Altshuler et a/., leading to different values
for the distribution of radon daughters among different particle sizes. For example, Jacobi
reports 25 percent of polonium 218 as free ions instead of the 9 percent used by Altshuler.
Similarly, Jacobi considers 52 percent of the radon daughters are attached to nuclei of
O.I// diameter or less, versus Altshuler's value of 60 percent.
2. Jacobi assumes that all polonium 218 atoms remain on the surface of the mucus
layer, but that the terminal polonium 211 atoms are distributed in a linear gradient from a
maximum at the mucus surface to zero at the boundary of the bronchial epithelium.
3. Jacobi uses values for the thickness of the mucus layer which are higher than those
used by Altshuler: (1) in the trachea and main bronchi, 80 microns; (2) in the lower bronchi,
20 microns; and (3) in the bronchioli, 5 microns. If Jacobi's values for the thickness of the
mucus layer are correct, the alpha particles from polonium 218 cannot reach the underlying
tissues in the main bronchi. The Chamberlain and Dyson model, by contrast, attributes
practically all of the dose in this region of the lung to the deposition of polonium 218 atoms
in the form of free ions.
4. Jacobi accepts the effective biological target as the basal cell layer in apparent agree-
ment with Altshuler et al. He appears to describe the histological location of this layer as
lOjx below the interface of the goblet-ciliated cells and the mucus layer (depth c—fig. 1).
5. Jacobi used a range of 55/i for the polonium 218 alpha particles instead of the 47p
used by Altshuler.
23. In addition to the principal differences described in the preceding paragraph, the
authors of the two models compute their results for different anatomical subdivisions of the
lung so that a direct comparison of the original calculations cannot be made. However, if
one makes some reasonable assumptions to normalize both models to the same anatomical
subdivisions, at least an approximate value of the dose in rads can be estimated from each
model. The values are shown in table 5 for mouth breathing with a minute volume of 15
liters per minute, a 40-hour week (2,000-hour work year) and a reference atmosphere of
100 pCi of radon 222 per liter plus the selected burden of daughter products (200 pCi of
radon daughters per liter of air).
TABLE 5.—Approximate annual dose (rads) in reference atmosphere
Region
Trachea and main bronchi.
Secondary—quat. bronchi
Bronchioli
Alveolar tissue
Mtchuler et al.
16
20
3 (or more)
0.8
J aoobi
1
22
0.8
0.8
24. Results from these two models agree in the estimate of alveolar dose, because
both involve a calculation which is independent of tissue structure in this region. They
also appear to agree in the estimates of dose in the secondary and quaternary bronchi.
This apparent agreement is fortuitous since it disappears if the same depth to the target
tissue and the same values for the alpha particle ranges are used in both. They disagree
substantially in estimates of the dose to bronchioli and the trachea and main bronchi.
54
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Goblet Cells
a !
b ;
c
/
D
t**>,
Cilia
Strous Fluid
Bathing Cilia
Ciliated Cells
Basement Membrane
Basal Cells
Goblet Cells
Cilia
Mucus Layer
Serous Fluid
Bathing Cilia
Ciliated Cells
Basement Membrane
Basal' Cells
Figure 1. Ixx-ation of the Biological Target
Upper: Stylized cross-section of bronchial epithelium: I) is height ,,f basal cell nuclei above ba^ment
membrane. Target depth-(a + b + c). \hshuler m.xlel: a, Ti. ami I) earl. , m.crons. Kp.thcm.m
thickness (c+I)) variable.
Umer: Typical fol.ling: For ^gmental bronchi Altshuler quotes C-22, l«>. and -77 micron, for thin.
median, and thick parts. Actual sections (r.£., Knpel) show greater variation.
55
-------
25. Haque and Collinson have recently presented another model for calculating the
dose to the respiratory system owing to radon and its daughter products.24 Based on Weibel's
lung model,-' the dissipation of alpha particle energy from the daughter activity deposited
on the walls of the respiratory system was calculated for anatomical locations ahove the
alveoli and at various depths in tissue. The equilibrium daughter activity was calculated
from the deposition rate and the mucus movement. Doses calculated by this method were
found to be highest in the segmental bronchi and were generally higher than those calcu-
lated by Mtshuler «< «/. The annual dose for an exposure equivalent to 0.1 WL, at a deptli
of 30 microns from the top of the mucus layer of the segmental bronchi, was calculated to
be 13.8 rads.
26. These models, which attempt to correct for the defects assumed to exist in the
Chamberlain and Dyson model, appear to represent the best that can be achieved at this
time. The differences between them emphasize the significant uncertainties that exist in
the needed radiological and physiological data and lead to the conclusion that a realistic
relationship among the atmospheric burden of radon and radon daughters, the physical
description of the relevant radiation dose to the lung, and radiation induced carcinogenesis
cannot yet be defined.
Radiation Dose Estimates Associated With the "Working fovel"
27. The chief merit of the WL (described in par. 2.6 of the report) approach is its
attention to the predominant position of the alpha emitters in the decay product chain.
\ simple field method for estimating the concentration of decay products in terms of total
alpha particle emission has been developed and serves as a distinct advantage in the use
of the WL concept. The measurements are not unduly sensitive to the actual ratio of
polonium 218. lead 21 t. and bismuth 211 in the atmosphere. A |>ossiblc disadvantage in
relating such measurements to radiation hazards is the necessary assumption that the
hazard is adequately defined by the total alpha emission alone, and that the relevant dose
is not sensitive to the distribution of the daughters between free ions, nuclei, and other
various particle sizes.
28. The short-lived decay products through polonium 211 are not in radioactive
equilibrium with the radon in actual mine atmospheres. (Generally the first product.
|K)loniurn 218, is close to radioactive equilibrium. The intervening nuclides are typically in
the range of 20 to 80 percent of the equilibrium value, the lower values being associated
with higher ventilation rates (see par. 2.1 of the rej>ort). Using the basic model of Morgan
(par. 11) it was estimated that the average lung dose from inhalation of the short-lived
decay products would be about 20 times greater than that from the radon alone.-'
29. The reference atmosphere of Mtshuler <-t al. contained 200 pCi of radon daughters
per liter of air. or ostensibly a radon daughter concentration of 2t (67 percent) of a W L.
The |M>tential alpha energy calculated from the stated composition of Mtshuler's reference
atmosphere (i.e., 91 pCi L'18Vo. 62 pCi '14Pb, tl pCi ->I4Bi) is 7.1 X 104 MeV or 57 percent of
a V> L as defined.
30. I sing Mtshuler's model, calculations of the dose to different segments of the lung
for nose breathing and mouth breathing, each at a rate of 15 liters per minute, give results
that range between 55 percent and 65 percent of the dose that would be associated with a
radon daughter concentration of 1 WL. It is considered accurate enough to conclude that
Mtshuler's reference atmosphere results in 60 percent of a "Working Ixrvel dose" to the
bronchi, and has a value between 10 and 30 rads a year. In view of the ambiguities of
conversion. Mtshuler's reference atmosphere will be considered, with important reserva-
56
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tions, to produce 20 rads in a normal working year (2,000 hours at a breathing rate of 15
liters per minute) to the bronchial epithelium. Thus the "WLVI" corresponds to — x -—
or 2.8 rads, and exposure for 1 year at a radon (laughter value of one WL would give 33
rads. The mean organ dose would be lower. In the absence of an appropriate factor for the
KBK, the calculated dose cannot be converted to rem.
Discussion and Evaluation
31. Although the models discussed in this appendix appear to be more realistic than
merely averaging over the alpha particle range, there are still serious uncertainties in the
various parameters that must be considered. These include:
1. Distribution of radioactive material in or on the mucus layer. Some authors27
consider the mucus sheet having as many as three layers in laminar flow with no mixing
between them, each layer arising from different regions of the lung. However, the move-
ment of the viscous layer by ciliar brushing implies some disturbance of laminar flow. In
addition, the detection of cancerous or precancerous cells by sputum tests also implies
quite sizable intrusions into the mucous.
2. Uncertainties remain concerning the actual range of alpha particles in tissue, the
role of the delta rays, and the thickness of the mucus and bronchial epithelium.
3. Kngel2S states that the mucous membrane of the bronchi is normally arranged in
folds, which in some instances almost close off separate channels. Although many models
consider a uniform deposition on the surface of a smooth tube, it is reasonable to sup|K>sc
that the true deposition is far from uniform. If so, it is reasonable to suppose that the most
deposition will occur at the crests of the folds where the epithelium is thickest.
t. Observations on mucus flow rate are necessarily practical averages. If the flow
rate differs between the crests and the troughs, the actual distribution of regional dis-
integrations could differ from the computed values in a systematic way related to the local
epithelium geometry.
5. The thickness of the mucus sheet is probably more variable than provided in most
models although this weakness is usually recognized. Mucus secretion is normally consid-
ered to increase as the result of irritation which is not necessarily confined to radiation and
may increase with time in an occupation such as mining. If the mucus sheet is thick enough
the alpha particles, particularly those from polonium 218, cannot penetrate to the depth
of the assumed biological target. This implies that the physical dose to lung tissue follow ing
inhalation of a given quantity of radon and radon daughters may change progressively
with time in a particular individual.
6. The choice of basal cells as the critical biological target is plausible, but injury to
these cells has not been shown to be the source of cancer induction. Presumably, this choice
would relate the radiation injury to a somatic mutation in the basal cell nucleus. At some
later time, a breakdown of control occurs, leading to the development of malignancy. The
long delay time, however, makes proof of such a relation very tenuous. If somatic mutation
is a factor, the determination of the relative biological effectiveness of the alpha radiation
is complicated by redundant formation along any alpha track intersecting a chromosome
and by the delta ray comjx>nent.
7. If either mucus secreting cells or ciliated cells are injured to the ]>oint of impaired
function, an initially small injury may be compounded into a progressively more dis-
advantageous cellular environment without injury of the chromosome. In this regard it
57
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would be of great interest to learn what association there might be between the production
of lung cancer and the ciliary "faults" (islands of squamoiis epithelium or metaplastic
tissue devoid of cilia), \\hich are reported to be found in the lungs of adults and smokers."
8. Finally, there remains a question as to the effects of the doses from radioactive
particles in the lymphatic vessels, lymphoid tissue, connective tissue, and alveolar tissue
immediately adjacent to the broncheolar structures.
58
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60
u s oovtRNMtNi Mnhtisc orna m: o ^76 2*1
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