NAC/Proposed 2: 11/2009
ACUTE EXPOSURE GUIDELINE LEVELS (AEGLs)
FOR
MERCURY VAPOR (Hg°)
(CAS Reg. No. 7439-97-6)
PROPOSED

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PREFACE
Under the authority of the Federal Advisory Committee Act (FACA) P. L. 92-463 of
1972, the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous
Substances (NAC/AEGL Committee) has been established to identify, review and interpret
relevant toxicologic and other scientific data and develop AEGLs for high priority, acutely toxic
chemicals.
AEGLs represent threshold exposure limits for the general public and are applicable to
emergency exposure periods ranging from 10 minutes to 8 hours. Three levels — AEGL-1,
AEGL-2 and AEGL-3 — are developed for each of five exposure periods (10 and 30 minutes, 1
hour, 4 hours, and 8 hours) and are distinguished by varying degrees of severity of toxic effects.
The three AEGLs are defined as follows:
AEGL-1 is the airborne concentration (expressed as parts per million or milligrams per
cubic meter [ppm or mg/m3]) of a substance above which it is predicted that the general
population, including susceptible individuals, could experience notable discomfort, irritation, or
certain asymptomatic, non-sensory effects. However, the effects are not disabling and are
transient and reversible upon cessation of exposure.
AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance above
which it is predicted that the general population, including susceptible individuals, could
experience irreversible or other serious, long-lasting adverse health effects or an impaired ability
to escape.
AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a substance above
which it is predicted that the general population, including susceptible individuals, could
experience life-threatening health effects or death.
Airborne concentrations below the AEGL-1 represent exposure levels that could produce
mild and progressively increasing but transient and nondisabling odor, taste, and sensory
irritation or certain asymptomatic, non-sensory effects. With increasing airborne concentrations
above each AEGL, there is a progressive increase in the likelihood of occurrence and the severity
of effects described for each corresponding AEGL. Although the AEGL values represent
threshold levels for the general public, including susceptible subpopulations, such as infants,
children, the elderly, persons with asthma, and those with other illnesses, it is recognized that
individuals, subject to unique or idiosyncratic responses, could experience the effects described
at concentrations below the corresponding AEGL.

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1
2	TABLE OF CONTENTS
3	PREFACE	2
4	LIST OF TABLES	5
5	EXECUTIVE SUMMARY	5
6	1. INTRODUCTION	7
7	2. HUMAN TOXICITY DATA	9
8	2.1. Acute Exposures	9
9	2.1.1. Lethality	9
10	2.1.2. Nonlethal Toxicity	10
11	2.2. Occupational Monitoring	11
12	2.3. Neurotoxicity	11
13	2.4. Developmental/Reproductive Toxicity	11
14	2.5. Genotoxicity	12
15	2.6. Carcinogenicity	12
16	2.7. Summary	13
17	3. ANIMAL TOXICITY DATA	13
18	3.1. Acute Studies	13
19	3.1.1. Rats	13
20	3.1.2. Mice	14
21	3.1.3. Rabbits	14
22	3.2. Repeat-Dose Studies	15
23	3.3. Neurotoxicity	17
24	3.4. Developmental/Reproductive Toxicity	18
25	3.5. Genotoxicity	22
26	3.6. Chronic Toxicity/Carcinogenicity	22
27	3.7. Summary	22
28	4. SPECIAL CONSIDERATIONS	23
29	4.1. Metabolism and Disposition	23
30	4.2. Mechanism of Toxicity	28
31	4.3. Structure-Activity Relationships	28
32

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1	4.4. Other Relevant Information	28
2	4.4.1. Species Variability	28
3	4.4.2. Susceptible Populations	29
4	4.4.3. Concentration-Exposure Duration Relationship	29
5	4.4.4. Concurrent Exposure Issues	30
6	5. DATA ANALYSIS FOR AEGL-1	30
7	5.1. Summary of Human Data Relevant to AEGL-1	30
8	5.2. Summary of Animal Data Relevant to AEGL-1	30
9	5.3. Derivation of AEGL-1 Values	31
10	6. DATA ANALYSIS FOR AEGL-2	31
11	6.1. Summary of Human Data Relevant to AEGL-2	31
12	6.2. Summary of Animal Data Relevant to AEGL-2	31
13	6.3. Derivation of AEGL-2 Values	32
14	7. DATA ANALYSIS FOR AI Xi 1.-3	33
15	7.1. Summary of Human Data Relevant to AEGL-3	33
16	7.2. Summary of Animal Data Relevant to AEGL-3	33
17	7.3. Derivation of AEGL-3 Values	33
18	8. SUMMARY OF AEGLS	34
19	8.1. AEGL Values and Toxicity Endpoints	34
20	8.2. Comparison with Other Standards and Guidelines	35
21	8.3. Data Adequacy and Research Needs	37
22	9. REFERENCES	37
23	APPENDIX A: Derivation of AEGL Values	45
24	APPENDIX B: Category Graph of AEGL Values and Toxicity Data	48
25	APPENDIX C: Derivation Summary	50
26

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LIST OF TABLES
S 1.	Summary of AEGL Values for Mercury Vapor	7
TABLE 1.	Chemical and Physical Properties	8
TABLE 2.	Estimated Human Exposures - Reconstructions of Accidental Exposures3	10
TABLE 3.	Summary of Acute Inhalation Data in Laboratory Animals	15
TABLE 4.	Summary of Repeat-Dose Inhalation Data in Laboratory Animals	16
TABLE 5.	Reproductive, Developmental, and Developmental/Neurotoxicity Studies in Laboratory Animals	19
TABLE 6.	Human Metabolism Studies	24
TABLE 7.	Tissue Concentrations of Mercury	26
TABLE 8.	AEGL-1 Values for Mercury Vapor	31
TABLE 9.	AEGL-2 Values for Mercury Vapor	33
TABLE 10.	AEGL-3 Values for Mercury Vapor	34
TABLE 11.	Summary of AEGL Values for Mercury Vapor	35
TABLE 12.	Extant Standards and Guidelines for Mercury Vapor	36

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EXECUTIVE SUMMARY
Mercury vapor, Hg°, (CAS Reg. No. 7439-97-6) is a colorless, odorless gas generated
from elemental mercury or inorganic mercury compounds such as mercuric chloride. Under
ambient conditions, mercury is a silver-white, liquid metal. Metallic mercury is non-flammable
and only slightly volatile (vapor pressure of 0.002 mm).
Controlled inhalation studies in human volunteers that addressed metabolism used
concentrations that ranged from 0.01 mg/m3 for 7 hours to 0.40 mg/m3 for 15 minutes. No
complaints of ocular or respiratory tract irritation were reported in these studies. Review of past
workplace exposure shows that concentrations of 0.4-2.0 mg/m3 in industry result in mercury
poisoning only after chronic exposure. Concentrations of 1.0-5.0 mg/m3 were not unusual in
mercury mining operations in the past (AIHA 2002). Additional human data were available from
reconstructions of accidental exposures. An estimated airborne concentration of 16 mg/m3 for a
few hours was lethal to an infant; whereas, an estimated concentration of 15 mg/m3 for
0.75 hours failed to induce symptoms of Hg poisoning in high-school-age students.
Animal studies addressed acute and repeat-dose toxicity, neurotoxicity, and
developmental/reproductive toxicity. Death at high concentrations is due to damage to the lungs
which results in respiratory failure. At lower concentrations, mercury vapor is neurotoxic.
Absorbed mercury binds to various proteins, particularly thiol-containing proteins, resulting in
nonspecific cell injury. Developmental studies that addressed neurotoxicity (primarily
spontaneous activity and learning ability) yielded conflicting results. Sufficiently high mercury
exposures can induce developmental toxicity at maternally toxic concentrations. Data were
inadequate to determine the carcinogenic potential of mercury vapor.
Mercury vapor has no odor. At low concentrations, there are no sensory or irritant
warning properties. Therefore, AEGL-1 values are not recommended.
Although maternal exposures were for 2 hours/day for 10 days, a single 2-hour exposure
of pregnant Long-Evens rats to 4 mg/m3 mercury vapor (Morgan et al. 2002) was used as the
point of departure for the AEGL-2. This value is a NOAEL for developmental effects.
Developmental effects including increased resorption, decreased litter size and decreased
neonatal weight were observed at the next higher concentration of 8 mg/m3. Uncertainty factors
for the AEGL-2 were based on a weight of evidence of approach. The following factors were
considered in deriving an interspecies uncertainty factor: the 4 mg/m3 value was a NOAEL for
developmental effects (below the definition of the AEGL-2), the exposures were repeated for 10
days, rodents have a higher respiratory rate and cardiac output compared with humans (resulting
in faster uptake), and human monitoring studies show some effects at concentrations of 0.4 to 2
mg/m3 only with chronic exposure (AIHA 2002). The following factors were considered in
ascribing an intraspecies uncertainty factor: the population of fetuses is considered a sensitive if
not the most sensitive population, the protective action of the placenta in sequestering mercury
[the mean concentration of mercury in the brain of dams exposed to 4 mg/m3 for 10 days was 60-
fold higher than in the fetal brain (Morgan et al. 2002)], and incidences of miscarriages and
stillbirths were unaffected in women chronically exposed to mercury vapor (median air
concentration, 0.09 mg/m3; range, 0.025-0.60 mg/m3), although congenital anomalies were

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statistically non-significantly increased (Elghany et al. 1997). Based on these factors,
interspecies and intraspecies uncertainty factors of 1 and 3 were applied. Application of larger
uncertainty factors, for example 10 or 30 (resulting in 2-hour values of 0.4 or 0.13 mg/m3),
results in values that are inconsistent with the available human data, including the chronic
exposure of pregnant women in the study of Elghany et al. (1997). In the absence of time-scaling
information, the resulting 2-hour value of 1.33 mg/m3 was time-scaled using default n values of 3
and 1 for shorter and longer exposure durations, respectively (NRC 2001).
The AEGL-3 values were based on a single 1-hour exposure of male Wistar rats to 26.7
mg/m3; no deaths occurred during the 15-day post-exposure period (Livardjani et al. 1991). No
clinical signs were observed, but lungs showed edema and necrosis. Extending the exposure
period for another hour (at approximately the same concentration) resulted in 62.5% mortality.
This 1-hour highest non-lethal value of 26.7 mg/m3 meets the definition of the AEGL-3. The
26.7 mg/m3 value was adjusted by a total uncertainty factor of 3 (an interspecies uncertainty
factor of 1 and an intraspecies uncertainty factor of 3) based on a weight of evidence approach.
Larger uncertainty factors result in values incompatible with the overall data. Reversible
behavioral changes were observed in male and female Wistar rats inhaling 17.2 mg/m3 for 2
hours/day for 22 exposures (Beliles et al. 1968). The uncertainty factor of 3 is considered
sufficient to protect susceptible populations. Values derived using an intraspecies uncertainty
factor of 3 are supported by the non-lethal concentrations estimated in accidental exposures [up
to 15 mg/m3 for 0.75 hours [Shelnitz et al. 1988; AIHA 2002)] and measured in occupational
settings [0.4-2.0 mg/m3 (AIHA 2002)]. The resulting 1-hour value of 8.9 mg/m3 was time-scaled
using default n values of 3 and 1 for shorter and longer exposure durations, respectively (NRC
2001). Because the 8-hour time-scaled value of 1.1 mg/m3 appears low in comparison to
accidental non-lethal exposures and is lower than some chronic occupational exposures, the 8-
hour value was set equal to the 4-hour value.
The calculated values are listed in the table below.
S 1. Summary of AEGL Values for Mercur
V Vapor
Classification
10-min
30-min
1-h
4-h
8-h
Endpoint (Reference)
AEGL-la
(Nondisabling)
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
No odor or warning
properties
AEGL-2
(Disabling)
3.1 mg/m3
(0.38 ppm)
2.1 mg/m3
(0.26 ppm)
1.7 mg/m3
(0.21 ppm)
0.67 mg/m3
(0.08 ppm)
0.33 mg/m3
(0.04 ppm)
No fetal effects:
4 mg/m3, 2 hours/day,
10 d-rat
(Morgan et al. 2002)
AEGL-3
(Lethal)
16 mg/m3
(2.0 ppm)
11 mg/m3
(1.3 ppm)
8.9 mg/m3
(1.1 ppm)
2.2 mg/m3
(0.27 ppm)
2.2 mg/m3
(0.27 ppm)
Highest non-lethal
value, 26.7 mg/m3 for
1 h - rat
(Livardjani et al. 1991)
a Mercury vapor is odorless. AEGL-1 values are not recommended because mercury vapor has no odor or warning
properties at concentrations that may cause health effects.
1. INTRODUCTION
Mercury vapor (CAS Reg. No. 7439-97-6) is a colorless and odorless gas generated from
elemental mercury or inorganic mercury compounds. The metallic form of mercury and the

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vapor exist in the zero oxidation state (Hg°). At ambient conditions, mercury is a silver-white,
liquid metal. It is non-flammable and only slightly volatile (vapor pressure of 0.002 mm).
Mercury is practically insoluble in water, but is soluble in many organic solvents (O'Neil et al.
2001; AIHA 2002). Additional chemical and physical properties are summarized in Table 1.
Due to its unique chemical and physical properties such as uniform volume expansion
over its entire liquid range, mercury has many uses. It is used in the chloralkali industry as a
cathode in the electrolysis of brine and in making a variety of scientific instruments and electrical
control devices, as an amalgam tooth filling, and in mining for the extraction of gold (Goyer and
Clarkson 2001). Dental amalgam contains 50% mercury. In the past, liquid elemental mercury
was a common component of thermometers, barometers, and other laboratory measuring devices.
The largest commercial use of mercury in the U.S. (35%) is for electrolytic production of
chlorine and caustic soda in mercury cells. In 1995, U.S. consumption was 463 metric tons
(ATSDR 1999). The use of mercury in batteries, pigments, explosives, and as a catalyst for
production of various plastics has been discontinued in the U.S. (U.S. EPA 1992).
Mercury is a naturally occurring element in the earth's crust. Mercury is mined from
cinnebar ore. World wide, cinnebar is mined by open-pit and underground mining; cinnebar is
no longer mined in the U.S. The primary method of mercury extraction from ore is by heating in
a retort or furnace. Leaching, electrolysis, and electro-oxidation methods have also been used.
Primary mercury production has decreased due to mercury recovery and recycling. Use has also
declined due to legal restrictions; in the U.S., mercurial compounds are no longer used as
biocides in protective coatings and in seed treatment. In 2003, world mine production from the
four major producing countries was 1300-1400 tons (DeVito and Brooks, 2005).
TABLE 1. Chemical and Physical Properties
Parameter
Value
Reference
Synonyms
Metallic mercury vapor;
elemental mercury vapor;
quicksilver (solid form)
AIHA 2002
Chemical formula
Hg
O'Neil et al. 2001
Molecular weight
200.59
O'Neil et al. 2001
CAS Reg. No.
7439-97-6
AIHA 2002
Physical state
Liquid metal; colorless vapor
generated from elemental liquid
mercury or inorganic compounds of
mercury
AIHA 2002
Solubility in water
0.28 inoles/L (insoluble)
O'Neil et al. 2001
Vapor pressure (25 °C)
2 x 10"3 (mm Hg)
O'Neil et al. 2001
Vapor density (air =1)
6.9
AIHA 2002
Saturation concentration in air
13 mg/m3 (1.6 ppm) at 20°C
16 mg/m3 (2.0 ppm) at 22°C
30 mg/m3 3.7 ppm at 30°C
111 mg/m3 (13.5 ppm) at 50°C
AIHA 2002
Liquid density (water =1)
13.5 g/cm3 at 25°C
O'Neil et al. 2001
Melting point
-38.9°C
O'Neil et al. 2001
Boiling point
356.7°C
O'Neil et al. 2001
Flammability limits
Not flammable
AIHA 2002

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Conversion factors*
1 ppm = 8.20 mg/m3
AIHA 2002

1 mg/m3 = 0.122 ppm

*In this document, values reported in ppm have been converted to mg/m3 for consistency.
2. HUMAN TOXICITY DATA
Mercury vapor is odorless (ATSDR 1999).
Mercury vapor emitted from amalgam dental fillings is the major source of mercury
exposure of the general public. Mercury excretion correlates with the number of "faces" of the
amalgam fillings. Other sources of mercury include natural degassing of the earth's crust, food,
and occupational exposure. Diet represents about 1.66 nmol/day compared with fillings,
15-85 nmol/day. Quantities emitted from the earth's surface are believed to be equal to those
from industrial applications (WHO 2003; Goyer and Clarkson 2001).
Exposure of humans to extremely high concentrations of mercury vapor may produce an
acute, corrosive bronchitis and interstitial pneumonitis (Goyer and Clarkson 2001). At sublethal
concentrations, symptoms include tremor or increased excitability. Chronic exposure can result
in a variety of neuromotor effects including tremor, ataxia, weakness and erethism, characterized
by withdrawal, depression, sensory and sleep disturbances, and emotional lability (Newland et al.
1996). Increased excitability, tremors, and gingivitis have been associated historically with
inhalation of mercury vapor and exposure to mercury nitrate in the fur, felt, and hat industries.
Acrodynia (pink disease) has been reported in infants exposed via contaminated clothes and
carpeting (ACGIH 1996). Mercury was once a common additive in teething powders and other
baby products, and sensitive infants developed skin rash.
Studies on the toxicity of mercury vapor have been reviewed by Friberg and Vostal
(1972), U.S. EPA (1995), ACGIH (1996), ATSDR (1999), AMA (2002), and WHO (2003).
2.1. Acute Exposures
2.1.1. Lethality
Concentrations lethal to humans have been reconstructed from accidental exposures
(Table 2). These accidents resulted from heating of metallic mercury. Death in all cases was
attributed to respiratory failure. Three reports that included fatalities were summarized by AIHA
(2002) as follows. Eight employees were accidentally exposed during recovery of tons of warm
mercury spilled from a ruptured generator (Tennant et al. 1961). The exposure duration was 3-5
hours and the atmospheric concentrations were estimated at 18-30 mg/m3 (2.2-3.7 ppm) based on
mercury in autopsy samples as well as the concentration of mercury in saturated air in a warm
room. All eight employees became ill, and one died. In the second report, torches were used in a
confined space to cut mercury-contaminated pipes (Eto et al. 1999; Kurisaki et al. 1999; Asano et
al. 2000). All 27 employees became ill and three died. The duration of exposure was 2-3 hours
for the three fatalities and the concentration was estimated at 43 mg/m3 (5.2 ppm) based on a
simulation of the accident. The simulation was consistent with mercury found in autopsy tissues.
In one other case, liquid mercury was accidentally spilled on a hot stove. All three exposed
individuals became ill; the only fatality was an infant (Campbell 1948). According to the study

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author, different medical intervention would have been applied if the exposure to mercury had
been reported when the infant was admitted to hospital.
2.1.2. Nonlethal Toxicity
Data on acute exposures are available only from reconstruction of accidents and
metabolism studies (See Section 4.1 for metabolism studies). AIHA (2002) described two
instances in which exposures to estimated concentrations of 13 mg/m3 (1.6 ppm) for 3-5 hours or
16 mg/m3 (2 ppm) for <8 hours produced cough, dyspnea, and chest tightness (Seaton and
Bishop 1978; McFarland and Reigel 1978). In a third report, high-school-age students exposed
to an estimated 15 mg/m3 (1.8 ppm) for 0.75 hours failed to exhibit signs of acute Hg° poisoning
(Shelnitz et al. 1988). Exposure of the students was verified by long-term measurements of
urinary mercury. Two additional reports of gold ore processing in a home resulted in illness, but
no fatalities. The estimated concentration was 16 mg/m3 and the duration was estimated at a few
hours.
TABLE 2. Estimated Human Exposures - Reconstructions of Accidental Exposures"
Exposure Situation
(Reference)
Estimated
Exposure Time
Estimated Concentration
Number Exposed
(Number ill,b fatalities)
Recovery of tons of mercury
from ruptured generator
(Tennant et al. 1961)
3-5 h
18-30 mg/m3 (2.2-3.7 ppm)
8 (8, 1)
Torches used in confined space
to cut contaminated pipes (Eto
et al. 1999; Kurisaki et al.
1999; Asano et al. 2000))
2-3 h
43 mg/m3 (5.2 ppm)°
27 (27, 3)
Liquid mercury poured on
stoved (Campbell 1948)
"few h"
16 mg/m3 (2 ppm)
3 (3, 1 [infant])
Home gold ore processing11
(Haddad and Stenberg 1963)
"few h"
16 mg/m3 (2 ppm)
2 (2, 0)
Home gold ore processing11
(Hallee 1969)
"few h"
16 mg/m3 (2 ppm)
5 (5, 0)
Workers exposed to vaporized
mercury in airflowe (Seaton and
Bishop 1978)
3-5 h
13 mg/m3 (1.6 ppm)
4 (4, 0)
Broken thermostat on hot oven
in small building (McFarland
and Reigel 1978)
<8 h
44.3 mg/m3 (5.4 ppm)°
9 (6, 0)
Mercuric oxide heated in
unventilated school laboratory"1
(Shelnitz et al. 1988)
0.75 h
15 mg/m3 (1.8 ppm)
23 (0, 0)
a Data taken from AIHA 2002.
b Symptoms summarized in text.
0 AIHA (2002) reported an estimated concentration of 16 mg/m3 (see footnote d).
d The minimum of the computed air concentration (mass of vaporized mercury divided by volume of enclosed area)
and the theoretical air saturation concentration for mercury vapor (16 mg/m3) at the known or assumed air
temperature of 22°C.
e Mercury vaporized (estimated at 100 mL) at 90°C, blown into a pressure chamber at 500 ft3/min, temperature of
walls 260 °C, then into chamber (7.2 x 2.4 x 1.8 m) containing workers at 20°C. Twenty hours later, 2 mg/m3
measured by Drager tube.

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Milne et al (1970) reported that four workmen became ill for up to a month after exposure
to mercury while repairing a mercury-contaminated tank. Individual exposures depended on task
performed, and ranged from 2.5 to 5 hours. A simulation of the accident while repairing a
similar tank showed mercury concentrations of 1.1 to 2.9 mg/m3 in the breathing zone of the
workmen. During the actual exposure, a nearby small steam heating pipe was inadvertently left
turned on, likely increasing the mercury concentrations in the tank.
2.2.	Occupational Monitoring
Personal sampling of two workers during maintenance operations at a chloralkali plant
showed TWAs of metallic mercury vapor of 0.152 mg/m3 (0.019 ppm) and 0.131 mg/m3
(0.016 ppm) (Barregard et al. 1992). Total exposure times were 20 and 23 hours, respectively,
over two days. On the third day both subjects were exposed to an average concentration of
0.190 mg/m3 (0.023 ppm) for 8 hours. Stationary sampling in the room showed a TWA of
0.060 mg/m3 (0.007 ppm). For an additional 7 workers, concentrations in work rooms were
between 0.010 and 0.400 mg/m3 (0.001 and 0.049 ppm). Two of the workers (not clearly stated
which ones) had used respiratory equipment during 15% of the working hours. Symptoms were
not addressed, but maintenance operations appeared to be routine.
IARC (1993) summarized occupational exposure to mercury in various industries and
occupations. Mean air concentrations ranged up to 0.20 mg/m3 (0.024 ppm). IARC pointed out
that these general air samples are usually lower than personal samples. Biological monitoring
(blood and urine samples) reflected the exposure concentrations. A review of occupational
monitoring studies by WHO (2003) suggests that an increased prevalence of subclinical
symptoms such as slight objective changes in short-term memory or tremor may occur with long-
term exposure to mercury vapor at levels >20 /ig/m3 (0.0024 ppm).
AIHA (2002) summarized past workplace exposures. Concentrations in the range of
0.4-2 mg/m3 (0.05-0.25 ppm) in industries have resulted in chronic mercury poisoning. Air
concentrations of 1.0-5.0 mg/m3 (0.12-0.61 ppm) were not unusual in mercury mining operations
in the past.
2.3.	Neurotoxicity
Effects of mercury vapor on the central nervous system have been well documented
(Goyer and Clarkson 2001). As noted, high, acute, non-fatal concentrations produce tremor and
increased excitability and can induce pneumonitis. At lower concentrations, symptoms may be
non-specific. Specific concentrations at which these symptoms occur were not provided.
2.4.	Developmental/Reproductive Toxicity
Studies addressing the effect of mercury vapor on human reproduction and development
were summarized by ACGIH (1996), ATSDR (1999), and AIHA (2002). No numeric acute-
duration exposure data were available. Retrospective studies on human fertility were evenly
divided between those showing an effect and those showing no effect. Developmental data were

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similarly inconclusive. Exposure concentrations were not provided in most studies. A 28-year
study of pregnancies in 57 women chronically exposed to inorganic mercury vapor (median air
concentration, 0.09 mg/m3; range, 0.025-0.60 mg/m3) did not reveal a significant difference from
27 controls regarding miscarriages or stillbirths (Elghany et al. 1997). The overall fetal death
rate was similar to that of New York state and national levels for the same period. Although
statistically non-significant, the incidence of congenital anomalies including clubfoot and
extrophy of the bladder was higher in Hg-exposed workers than in unexposed workers (4.2% vs
0%). But, there was no apparent concentration-response relationship for the congenital
anomalies (concentrations of 0.025 to 0.23 mg/m3); however, the study authors noted the small
size of their study population and suggested that additional studies, either a larger, retrospective
or prospective epidemiological study, be conducted. Health habits of the pregnant women were
not assessed. Inorganic mercury concentrations were monitored using a portable, ultraviolet
absorption mercury vapor meter. Samples were taken in the area of the workers faces.
According to AIHA (2002), case studies of accidental exposures suggest that infants and
toddlers are at greater risk than older children and adults of developing severe and progressive
lung injury. Many of these studies did not cite concentrations. In some cases, infants may have
experienced higher exposure while crawling on the floor. Campbell (1948) reports that an infant
that died from mercury poisoning was close to a hot stove on which mercury was vaporized.
Symptoms of fatigue, nausea, and abdominal cramps were reported in two adults in the
household.
The concentration of total mercury in brain and kidney of 18-19 terminated fetuses and
14-15 deceased infants was analyzed by Lutz et al. (1996). The mean concentrations of mercury
in the brain of fetuses and infants were 5 and 6 |ig/kg (ng/g) wet weight (range, <2-23 |ig/kg).
The mean concentrations in the kidney of fetuses and infants were 9 |ig/kg (range, <5-34 |ig/kg)
and 12 |ig/kg (range, 3-37 |ig/kg), respectively (data summarized in Table 7). There was a
tendency of increasing concentration of mercury in the fetal kidney but not in the brain with
increasing number of amalgam fillings in the mothers. The mercury in the tissue samples was
not speciated, and, based on results of other studies, the authors indicate that most of the mercury
in the brain may be methyl mercury. These mean values for mercury in the human brain are
similar to that found by Fredriksson et al. (1996) for rats exposed in utero to 1.8 mg/m3 during
PND2-3.
2.5.	Genotoxicity
Evidence for the genotoxicity of mercury vapor is limited. Cytogenetic monitoring
studies of occupationally-exposed workers produced mixed results, but the overall findings in
studies with appropriate control groups found no convincing evidence that mercury adversely
affects the number or structure of chromosomes in human somatic cells (ATSDR 1999; U.S.
EPA 1995).
2.6.	Carcinogenicity
Chronic toxicity/carcinogenicity studies were summarized by ACGIH (1996) and the U.S.
EPA (1995). ACGIH used an A4 classification - not classifiable as a human carcinogen - for

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inorganic forms of mercury including metallic mercury. The U.S. EPA in their Integrated Risk
Information System (IRIS) considered the evidence for both animal and human carcinogenicity
inadequate and assigned a classification of D - not classifiable as to human carcinogenicity. The
U.S. EPA identified a LOAEL for systemic effects of 0.025 mg/m3 (0.003 ppm) for chronic
(8-hours/day) exposures. The critical effects were hand tremor, increases in memory disturbance,
and slight subjective and objective evidence of autonomic dysfunction. A NOAEL was not
identified.
2.7. Summary
Data on human exposure to mercury vapor are limited to accidental exposures and
occupational monitoring. Concentrations from the reconstructed accidental exposures were
based on symptoms, estimated releases, room size, urinary mercury excretion, and, in cases of
death, tissue concentrations at autopsy. Estimated exposures of 3-5 hours to 18-30 mg/m3 (2.2-
3.7 ppm) and 2-3 hours at 43 mg/m3 (5.2 ppm) killed some workers (AIHA 2002). Exposures to
concentrations of 13-16 mg/m3 (1.6-2 ppm) for several hours caused cough, shortness of breath,
and chest tightness (Seaton and Bishop 1978; McFarland and Reigel 1978; AIHA 2002). One
case report of exposure to 15 mg/m3 (1.8 ppm) for 0.75 hours found no overt symptoms (Shelnitz
et al. 1988). An estimated concentration of 16 mg/m3 was not lethal to adults, but one infant died
following exposure of a "few hours."
Occupational studies indicate that exposure to mercury vapor concentrations of 0.4-
2 mg/m3 (0.05-0.25 ppm) may induce symptoms of mercury intoxication but only after repeated
exposure over several weeks or longer. Signs of Hg poisoning have not been reported for
exposure to these concentrations for hours or days (AIHA 2002).
Human developmental studies indicate a potential for an increase in fetal anomalies as a
result of mercury vapor exposure, but the limited studies to date have not shown a statistically
significant relationship. Additionally, the American Industrial Hygiene Association reported that
the young child may be at greater risk for developing severe and progressive lung injury (AIHA
2002). Mercury vapor does not appear to be genotoxic. Studies are too limited to allow a
conclusion concerning its carcinogenic potential.
3. ANIMAL TOXICITY DATA
3.1. Acute Studies
Acute studies with both lethal and non-lethal endpoints are summarized in Table 3.
3.1.1. Rats
Groups of 32 male Wistar rats inhaled (whole-body) analytically-determined
concentrations of 26.7 mg/m3 (3.25 ppm) for one hour or 27.0 mg/m3 (3.29 ppm) for two hours
(Livardjani et al. 1991). Sacrifices took place from 1 to 15 days postexposure. No rats exposed
to 26.7 mg/m3 for one hour died and no clinical signs were evident. Animals inhaling 27.0
mg/m3 for two hours showed dyspnea and 20 died within 5 days of exposure. Microscopic

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examination of the lung tissue revealed alveolar edema, hyaline membranes, and occasional
fibrosis.
3.1.2.	Mice
Groups of 16 C57B16 wild-type and metallothionein-null mice inhaled 0 or 5.5-6.7 mg/m3
(0.67-0.82 ppm) of mercury vapor for 3 hours, and the mice were killed at 1, 24, 72, or 168 hours
post-exposure (Yoshida et al. 1999a). Metallothionein was induced in the lungs, kidneys, and
brain. Liver and kidney function, as determined by glutamate oxaloacetate transaminase and
glutamate pyruvate transaminase activity, blood urea nitrogen, and serum creatinine, were all
within normal limits.
Twenty-four male ICR mice inhaled an analytically-determined concentration of 9.8
mg/m3 (1.2 ppm) mercury vapor for 1 hour; groups of six animals were sacrificed at 1, 24, 48,
and 120 hours after exposure (Shimojo et al. 1996). Protein in broncho-alveolar lavage fluid
(BALF) was used as a marker of lung injury. Protein in BALF increased with time: 169% of the
control value at both 1 and 24 hours and 441% of the control value at 48 hours. At 48 hours after
exposure, BALF contained large amounts of hemoglobin, indicating pulmonary hemorrhage.
Superoxide dismutase, an antioxidant enzyme, increased following exposure, but returned to a
near control level by 120 hours postexposure. Mortality was not addressed.
3.1.3.	Rabbits
Rabbits were exposed to mercury vapor at an average analytically-determined
concentration of 28.7 mg/m3 (3.5 ppm) for 1 to 30 hours (Ashe et al. 1953). Fourteen rabbits
were used, with one or two at each exposure duration. The longer exposure durations were
conducted intermittently, over periods of several days. Surviving animals were sacrificed on the
6th day after exposure ended. Two rabbits exposed for one hour showed mild to moderate
damage to the brain, kidney, heart, and lung (not further described). A two-hour exposure (2
rabbits) resulted in marked cellular degeneration of the kidney and brain. Exposure for >4 hours
resulted in moderate to marked cellular degeneration of the heart, liver, and lungs and severe
damage to the kidney and brain. Severe damage was characterized as nearly complete
destruction with widespread necrosis. One rabbit inhaling Hg° for 6 hours/day for 5 days (total
of 30 hours) died near the end of exposure.

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TABLE 3. Summary of Acute Inhalation Data in Laboratory Animals
Species
Concentration
(ppm)
Exposure
Time
Effect
Reference
Rat
26.7 mg/m3
(3.25)
27.0 mg/m3
(3.29)
1	h
2	h
No deaths; mild lung lesions;
no breathing difficulties
Death of 20/32; severe lung
lesions; dyspnea
Livardjani et al. 1991
Mouse
5.5-6.7 mg/m3
(0.67-0.82)
3 h
Normal liver and kidney
function
Yoshidaetal. 1999a
Mouse
9.8 mg/m3
(1.2)
1 h
Increased protein content of
broncho-alveolar lavage fluid
Shimojo et al. 1996
Rabbit
28.7 mg/m3
(3.5)
1 h
>4 h
Mild organ damage
severe organ damage
Ashe et al. 1953
3.2. Repeat-Dose Studies
Recent repeat-dose studies are summarized in Table 4. Older studies are described in the
following text, but, because of design flaws and outdated analytical methodology, are not
included in Table 4. Data from repeat-exposure studies that addressed neurotoxicity are
summarized in Table 4, but are discussed in Section 3.3. Studies in Table 4 are arranged first by
species and then by increasing concentration. Data from repeat-exposure studies that addressed
pregnancy outcome are included in Table 5 and are discussed in Section 3.4.
Two dogs exposed 8 hours/day for 40 days to 1.89 mg/m3 (0.23 ppm) showed no clinical
signs (Fraser et al. 1934). Six dogs exposed 8 hours/day to 15.29 to 20.06 mg/m3 (1.86-2.44
ppm) died within 1-3 days and two of six dogs exposed daily for 8 hours to 12.55 mg/m3 (1.53
ppm) died within 2-3 days. The remaining dogs developed shortness of breath, weakness,
vomiting, and diarrhea. Although well-conducted for the time, this study suffers from outdated
analytical methodology and the reported exposure concentrations are not reliable.
Groups of 25 female Long-Evans rats were exposed nose-only to 4 mg/m3 (0.49 ppm) 2
hours/day for up to 10 days (Brambila et al. 2002). Groups of 5 treated and 5 control rats were
sacrificed on days 1, 5, 10, 17, and 37. Microscopic examination of kidneys failed to reveal
tissue damage in the kidney (the only organ examined). No clinical effects were described and
no deaths were mentioned at sacrifice on day 37.
No clinical signs other than slight body weight loss (data not provided) were observed in
male and female brown Norway rats (groups of 7) exposed to a measured concentration of 1
mg/m3 (0.12 ppm) of mercury vapor for 24 hours/day, 7 days/week, for 5 weeks or 6 hours/day,
3 days/week for 5 weeks (Warfvinge et al. 1992). One rat in the continuous exposure group died
of kidney damage after 4 weeks of exposure. Neurological examinations were not performed.
The mercury exposure induced an autoimmune disease (not otherwise described) (Hua et al.
1992).
No deaths occurred in a group of female Wistar rats exposed to 1 mg/m3 (0.12 ppm), 24
hours/day for 28 days (Gage 1961). After 10 days of continuous exposure, a steady-state was
achieved in that elimination of mercury in the urine was equal to absorption.

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TABLE 4. Summary of Repeat-Dose Inhalation Data in Laboratory Animals
Species
Concentration
(ppm)
Exposure
Duration
Effect
Reference
Rat, male
Wistar
0.48 mg/m3 (0.06)
5 h/d, 4-5 d/wk,
8 wk
"Irritable" behavior during
weeks 6-8; microscopic
changes in nervous system
Schionning et al. 1998;
Sorensen et al. 2000
Rat,
Norway
1 mg/m3
(0.12)
24 h/d,
7 d/wk,
5 wks
One death (of 7 rats) after 5
weeks of exposure
Warfvinge et al. 1992
Rat, female,
Wistar
1 mg/m3
(0.12)
24 h/d,
28 d
All rats survived; weight
loss; lethargy by the 7th day;
slight tremors when handled
at termination
Gage 1961
Rat, male,
(strain
unidentified)
3.0 mg/m3 (0.37)
3 h/d, 5 d/wk,
12-42 weeks
Reversible tremors at 18
weeks; except for kidney
changes, normal
histopathology at sacrifice
Kishi et al. 1978
Rat, female,
Long-Evans
4 mg/m3
(0.49)
2 h/d,
10 d
No deaths, no kidney
damage
Brambila et al. 2000
Rat, male and
female Wistar
17.2 mg/m3
2 h/d, 5 d/wk,
30 days
Reversible behavioral
changes (increase in escape
response and response time
latency at 15 days; decrease
in avoidance response
Belilesetal. 1968
Mouse,
C57B16
0.06 mg/m3
(0.007)
8 h/d,
23 wk
Neurobehavioral effects
(increased activity)
Yoshida et al. 2004
Mouse,
C57B16
0.1 mg/m3
(0.12)
4.1 mg/m3
(0.50)
I	h/d, 3 d/wk,
two wk,
followed by 30
min/d, 3 d/w,
II	wk
No toxic signs and no deaths
Yasutake et al. 2004
Mouse,
C57B16
6.6-7.5 mg/m3
(0.80-0.91)
4 h/d,
3d
No deaths, no lung lesions
Yoshida etal. 1999b
Rabbit, male
4.0 mg/m3 (0.49)
6 h/d, 4 d/wk,
13 wk
Slight tremors and clonus at
13 weeks
Fukuda 1971
No deaths occurred and no signs of toxicity were evident in wild-type or metallothionein-
null mice exposed to 0.1 mg/m3 (0.1 ppm) for 1 hour/day, 3 days/week for two weeks, followed
by exposure to 4.1 mg/m3 for 30 minutes/day, 3 days/week for 11 weeks (Yasutake et al. 2004).
Metallothionein was induced/increased in the brains of wild-type mice. Mercury could be
detected histochemically in the brains of both strains of mice, but no pathological change was
observed. When tested at 12 and 23 weeks, increased activity and decreased passive avoidance
were observed in female OLA129/C57BL6 mice following exposure to 0.06 mg/m3 for 8
hours/day for 23 weeks (Yoshida et al. 2004). These effects were greater in metallothionein-null
mice.
Groups of 14 C57B16 wild-type and metallothionein-null mice inhaled 0 or 6.6-7.5 mg/m3
of mercury vapor for 4 hours/day for 3 consecutive days (Yoshida et al. 1999b). Seven mice
from each group were sacrificed 24 hours after the first exposure. Clinical signs were not

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described. No wild-type mice died over the 3-day period. Two and eight metallothionein-null
mice died within 24 hours after 2 and 3 days of exposure, respectively. Pulmonary histology
showed congestion, atelectasis, and mild to moderate hemorrhage in the alveoli of both wild-type
and metallothionein-null mice. The lesions were more severe in the metallothionein-null mice
and, for both groups, became more pronounced as exposure duration increased. With increasing
exposure duration, metallothionein increased in the lungs of wild-type mice, but not in the lungs
of metallothionein-null mice.
A group of 16 rabbits inhaled 6.0 mg/m3 (0.73 ppm) mercury vapor for 7 hours/day, 5
days/week (Ashe et al. 1953). Exposures continued for up to 12 weeks. None of the rabbits
died. A rabbit exposed for one week showed mild to moderate damage of the kidney, liver,
brain, heart, and lung. In most cases, damage to the kidney and brain became more severe with
longer exposures and was described as ranging from marked cellular degeneration with some
necrosis to nearly complete destruction with widespread necrosis.
3.3. Neurotoxicity
Neurotoxicity studies are discussed below. Developmental neurotoxicity studies are
summarized in Section 3.4 and Table 5.
Adult male Wistar rats inhaled 0.48 mg/m3 of metallic mercury for 5 hours/day, 4 or 5
days/week over an 8-week period (Schionning et al. 1998; Sorensen et al. 2000). During the last
2-3 weeks of exposure, the treated rats became "irritable" and food and water intake were
decreased. Analysis of changes in the nerve tissue was by microscopic "stereological"
observations. The peripheral nervous system was little affected. The mean cross section of
myelin associated with nerve fibers in the dorsal nerve roots was significantly reduced (by 20%)
and there was a tendency towards a reduction in axon area of myelinated nerve fibers in the
dorsal nerve roots and in the total numbers and mean volume of some cell types. The central
nervous system (cerebellum) showed reductions in numbers of Purkinje and granule cells and
volume of the granular cell layer.
Reversible behavioral changes were observed in male and female Wistar-derived rats
exposed to 17.2 mg/m3 (2.1 ppm) mercury vapor for 2 hours/day, 5 days/week for 18-22
exposures over 30 days (Beliles et al. 1968) and 14 male Wistar rats exposed to 3.0 mg/m3 (0.37
ppm) mercury vapor for 3 hours/day, 5 days/week for 12 to 42 weeks (Kishi et al. 1978). In the
first study, six female albino rats were used in avoidance escape studies (five controls) and seven
pairs of male rats were used in reflexive fighting studies (five control pairs). Mercury exposure
produced an increase in escape response latency and a decrease in avoidance responding when
tested during the exposure. For female rats, the increase in escape response began about day 15
of exposure. Male rats showed an increased spontaneous fighting response late in the exposure.
During the last 5 days of exposure, female rats in the avoidance escape study showed a fine
tremor and some weight loss. The behavioral changes were reversed during a 60-day recovery
period. At autopsy following exposure, two of three female rats that exhibited behavior changes
had histological changes in the medulla oblongata of the central nervous system, consisting of
perivascular cuffing by lymphocytes. These lesions were less severe in rats sacrificed after the
recovery period. No lesions were observed in the lung, liver, or kidney. Male rats also showed

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infiltration of lymphocytes in the central nervous system. The concentration of mercury was
measured with a vapor analyzer.
In the second study (Kishi et al. 1978), male rats (two groups of seven, strain unspecified)
exposed to 3.0 mg/m3 (0.37 ppm) mercury vapor for 3 hours/day, 5 days/week for 12 to 42 weeks
exhibited behavioral changes and gained less weight than a control group. When tested during
the weeks of exposure, rats showed a decline in avoidance response and an increase in escape
latency. The time to onset of these effects varied from 12 to 39 weeks. Tremors were first
observed during the 18th week of exposure. All rats recovered to a normal rate of responding and
normal latency within 12 weeks after termination of exposure. Rats sacrificed at the end of
exposure showed slight degenerative changes in the tubular epithelium of the kidney, but no
changes in the lung, liver, or brain. Tissues of rats sacrificed after recovery were normal.
Slight tremors and clonus were observed in two of six male rabbits that inhaled an
analytically determined average concentration of 4.0 mg/m3 (0.49 ppm) mercury vapor for 6
hours/day, 4 days/week for 13 successive weeks (Fukuda 1971). These signs were observed at
the end of the weeks of exposure.
3.4. Developmental/Reproductive Toxicity
Reproductive and developmental toxicity studies including developmental neurotoxicity
studies and fetal and neonatal tissue concentrations of mercury are summarized in Table 5.
Mercury vapor penetrates the maternal placental and blood-brain barrier of the developing
fetus. Mercury associated with metallothionein accumulates in the placenta. Following in utero
exposure, mercury concentrations in fetal organs are lower than those of respective maternal
organs (Khayat and Dencker 1982; Yoshida et al. 1986; Brambila et al. 2002).
Pregnant squirrel monkeys were exposed to either 0.5 or 1.0 mg/m3 (0.06 or 0.12 ppm) of
mercury vapor for either 4 or 7 hours/day (Newland et al. 1996). Exposures were conducted for
5 days/week between weeks 3 and 22 of gestation, for a total number of exposure days ranging
from 63-79 (gestation period of 154 days). None of the six tested offspring was prenatally
exposed over exactly the same period of gestation. Offspring were selected from "uneventful"
pregnancies. The offspring were 0.8 to 4 years of age when evaluated in a lever-press test.
Exposure-related increases in performance variability and longer lever-press duration were
reported, with one offspring displaying "erratic" behavior, but results were extremely variable
among and between the controls and exposed groups.
Soderstrom et al (1995) exposed groups of eight pregnant Sprague-Dawley rats to 1.5
mg/m3 for 1 or 3 hours/day, either early in pregnancy (gestation day [GD] 6-11) or late in
pregnancy (GD 13-18). There was no effect on body weight of the dams or neonates, and clinical
signs and development of the offspring observed at postnatal day 2-3 were normal. A group of
offspring were sacrificed on PND 2-3 for examination of mercury in different areas of the brain.
Additional groups were sacrificed on PNDs 21 and 60, and areas of the brain were analyzed for
nerve growth factor and its low- and high-affinity receptors (mRNA coding for nerve growth
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1	concentrations in brain of the neonate control and 1-hour and 3-hour exposure groups were 1, 4,
2	and 11 ng/g wet weight, respectively (it is assumed this is for both early and late pregnancy
3	exposures). Following the higher exposure during early pregnancy, nerve growth factor was
4	increased in the hippocampus area of the brain and decreased in the basal forebrain and septal
5	area on PND 21. mRNA was reduced in the basal forebrain. The authors suggested neuronal
6	damage and disturbed trophic regulation during development.
7
8
TABLE 5. Reproductive, Developmental, and Developmental Neurotoxicity Studies in Laboratory Animals
Species
Concentration/
Duration/Age
Effect
Reference
Reproductive
Rat
(1)	Pre-breeding: 1, 2, or
4 mg/m3 2 h/d, 11
consecutive days;
(2)	1 or 2 mg/m3 8 days
prior to and 8 days after
breeding
(1)	Estrous cycle and hormone level changes
secondary to general toxicity at 4 mg/m3;
(2)	No significant effect on pregnancy rate or
pregnancy maintenance
Davis et al. 2001
Developmental/Neurotoxicity
Monkey
0.5, 1.0 mg/m3,
4 or 7 h/d
between gestation wk 3
and 22
Exposure-related, but variable performance
in lever-press test in adulthood
Newlandetal. 1996
Rat
1.5 mg/m3,
1 or 3 h/d
GD 6-11 or 13-18
No clinical signs; no developmental effects;
nerve growth factor increased or decreased
in different areas of the brain
Soderstrom et al. 1995
Rat
1.8 mg/m3,
1 or 3 h/d
GD 11-14 or 17-20
No clinical signs and no effect on maturation
endpoints of offspring. Neurotoxicity tests
indicated reduced ability to adapt;
hypoactivity at 3 months and hyperactivity at
14 months, but no effect 11 months later; no
effect on swim maze test
Danielsson et al. 1993
Rat
1.8 mg/m3,
1.5 h/d,
GD 14-19
No effect on clinical signs or developmental
markers of offspring; hyperactivity in
spontaneous motor activity and latencies in
some maze tests at 4-5 months of age;
mercury concentration in brain at 2-3 days
postpartum: 5±2 ng/kg wet weight
Frederiksson et al. 1996
Rat
1, 2, 4, or 8 mg/m3,
2 h/d,
GD 6-15
Concentration-related increases in fetal
tissue concentrations of mercury after 5 and
10 days; maternal and fetotoxicity at 8
mg/m3 (dams became moribund by PND 1);
decreased weight gain (7%) in the dams in
the 4 mg/m3 group
Morgan et al. 2002;
Brambila et al. 2002
Rat
4 mg/m3,
2 h/d,
GD 6-15
No effect on responses of peripheral nerves,
somatosensory (cortical and cerebellar),
auditory or visual modalities when tested on
PNDs 140-168
Herr et al. 2004
Rat
0.05 mg/m3,
1 or 4 h,
neonatal days 11-17
No clinical signs; changes in motor activity
and learning in one test, tested at 2-6 months
of age
Fredriksson et al. 1992
9 GD = gestation day.

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Groups of 12 pregnant Sprague-Dawley rats inhaled 0 or 1.8 mg/m3 (0.22 ppm) mercury
vapor for 1 hour (defined as the low-dose group) or 3 hours (high-dose group) during GD 11-14
and 17-20 (Danielsson et al. 1993). These concentrations were not maternally toxic. On days 2-
3 postpartum, three offspring from each group were sacrificed and the brain, liver, and kidney
were analyzed for mercury. There were statistically significant dose-related elevated amounts of
mercury in all organs analyzed except for brain in the low-dose group.
Additional groups of offspring in this study (Danielsson et al. 1993) were examined for
maturation endpoints as well as activity and learning. There were no differences in body weight,
maturation variables, or clinical signs between exposed and control offspring. Treated offspring
were hypoactive for spontaneous motor activity when tested at 3 months of age but hyperactive at
14 months. In a radial arm maze learning test (for a food reward), prenatally exposed rats
showed retarded acquisition of spatial learning. However, there were no differences between
control and exposed groups in learning ability in a swim maze. In a test of habituation to a novel
environment, prenatally exposed rats showed a reduced ability to adapt. When tested at 11
months after the initial tests, there were no differences in activity among the control and exposed
groups of offspring.
Fredriksson et al. (1996) exposed 12 pregnant Sprague-Dawley rats to 1.8 mg/m3 (0.22
ppm) mercury vapor for 1.5 hours/day during GD 14-19. Additional groups were administered
methyl mercury by gavage (2 mg/kg/day over GD 6-9) or both methyl mercury and mercury
vapor at the described doses. Compared to a control group, treated offspring (all groups) showed
no differences in clinical parameters or developmental markers. Surface righting reflex and
negative geotaxis were unaffected within a few days after birth. When tested at 4-5 months of
age, offspring treated with mercury vapor alone showed hyperactivity in spontaneous motor
activity (locomotion, rearing, and total activity) and longer latencies in swim and radial arm maze
tests. Results in some tests were conflicting as latency was shown in the swim maze on the
second test day of two consecutive days of testing but not on the first day. Exposure to
methylmercury did not alter these functions, but coexposure to methylmercury and mercury
vapor increased the mercury vapor-induced deficits.
Pregnant Long-Evans rats were exposed nose-only to analytically-determined
concentrations of 0, 1, 2, 4, or 8 mg/m3 of mercury vapor for 2 hours/day from GD 6 through 15
(Morgan et al. 2002). Initial groups consisted of 25 rats, with sacrifices on GD 6, 10, and 15 and
1 week after the last exposure. Maternal toxicity, described as concentration-related decreases in
weight gain and mild nephrotoxicity (increased urinary protein and alkaline phosphatase, but not
glucose), were observed in rats exposed to 4 and 8 mg/m3. Dams exposed to 8 mg/m3 exhibited
mild tremor and unsteady gait (day of onset not stated), and were euthanized in moribund
condition on postnatal day (PND) 1. There was no histopathological evidence of toxicity in
maternal lung, liver, or kidney of any rats exposed to 1,2, 4, or 8 mg/m3 at GD 6 or 15, or
PND 1. Metallothionein and glutathione ^'-transferase activity levels were increased in the
kidney, lung, and brain of maternal animals (measured only in the 4 mg/m3 group), but not in the
tissues of neonates. [Glutathione transferase activity was higher in pregnant rats than in a group
of concurrent control non-pregnant rats (Brambila et al. 2002)]. Developmental effects were

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confined to the 8 mg/m3 group and consisted of increased resorption, decreased litter size, and
decreased PND 1 neonatal weight. Neonatal organ weights were not affected by exposure.
Mercury levels in brain, liver, and kidney of neonatal rats increased with increasing number of
exposure days and increasing exposure concentration. At the end of the 10-day exposure,
concentrations were highest in the fetal liver followed by the kidney and brain. In
transplacentally-exposed neonates of dams exposed to 4 mg/m3, mercury accumulation in the
kidney was approximately 1000-fold less than in the kidney of dams. The authors stated that
adverse effects of mercury on developmental outcome occurred only at a concentration that
caused maternal toxicity.
The effect of mercury vapor exposure on neuronal function of the above gestationally-
exposed rats was reported by Herr et al. (2004). The study was limited to effects on offspring of
pregnant Long Evans rats exposed nose-only to 0 or 4 mg/m3 for 2 hours/day on GD 6 through
15. The authors stated that this is approximately a maximal tolerated dose for the dams.
Offspring (one female and one male per dam) were tested between postnatal days 140-168 using
a battery of sensory evoked potentials. Peripheral nerve action potential, nerve conduction
velocity, somatosensory evoked responses (cortical and cerebellar), brainstem auditory evoked
responses, pattern evoked potential, and flash evoked potential were quantified. None of the
evoked responses were significantly altered. On PND 1, fetal brains contained approximately 20
ng/g total mercury (controls, approximately 1 ng/g).
Fredriksson et al. (1992) exposed groups of 10 neonatal Sprague-Dawley rats to an
analytically-determined concentration of 0.05 mg/m3 (0.006 ppm) for 1 hour (low exposure) or 4
hours (high exposure) daily from days 11-17 of age. This period of time is considered a period of
rapid brain growth in rats. Rats were tested for behavioral changes at 2-6 months of age. Tests
included spontaneous motor activity (locomotion, rearing, and total activity) and learning (radial
arm maze and swim maze). No clinical signs were observed during the exposure days and there
was no weight change. Tested motor activity was variable between ages (two and four months)
and over the one-hour observation time (broken into three 20-minute sessions). At two months
of age, rats exposed to the low dose showed no difference in motor activity compared with
controls; whereas, the rats exposed to the high dose showed an increase in locomotion and total
activity and a decrease in rearing. At four months of age, the low-dose rats showed increased
locomotion and total activity and decreased rearing; the high-dose rats showed a marked
hypoactivity in all three motor activity tests. In the radial-arm maze, tested over 2 days at 6
months of age, rats showed a concentration-dependent impairment in learning. Learning
improved over the two-day test for the control and low-exposure groups, but not for the high-
exposure group. There was no difference among control and treated groups in the swim maze
test.
Female reproductive toxicity was studied by Davis et al. (2001) by exposing female
Sprague-Dawley rats, nose-only, to 0, 1, 2, or 4 mg/m3 mercury vapor for 2 hours/day for 11
consecutive days. A two-hour exposure to 4 mg/m3 for 11 days resulted in a 22% weight
decrease relative to controls (weight decreases following 1, 4, and 7 days were 1, 4, and 8%,
respectively). Estrous cycles were slightly prolonged in the 2 and 4 mg/m3 dose groups, and
serum estradiol and progesterone levels were significantly decreased and increased, respectively,
in the 4 mg/m3 group compared to controls. These effects were attributed to body weight loss

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and general toxicity. In an evaluation of pregnancy outcome, female rats were exposed to 1 or 2
mg/m3 of mercury vapor, 2 hours/day for 8 days prior to breeding or 8 days after breeding. There
was no effect on pregnancy rate or numbers of implantation sites.
3.5.	Genotoxicity
Genotoxicity studies were reviewed by ATSDR (1999). Most of the studies addressed
the genotoxicity of organic mercury compounds. Studies with inorganic mercury were usually
performed with mercuric chloride. Mercuric chloride was not mutagenic in the Salmonella
typhimurium plate incorporation assay (strains TA98, TA102, TA1535, and TA1537). However,
mercuric chloride can damage DNA in rat and mouse embryo fibroblasts. A dose-related
increase in chromosome aberrations was observed in the bone marrow of mice administered
mercuric chloride orally at doses up to 4.4 mg/kg, but not in mice injected intraperitoneally at
similar doses. In vitro, mercuric chloride increased chromosome aberrations in Chinese hamster
ovary cells. From these studies, ATSDR concluded that, although the data are mixed, inorganic
mercury compounds have some genotoxic or clastogenic potential.
3.6.	Chronic Toxicity/Carcinogenicity
Ashe et al. (1953) reported that there was no histopathological evidence of respiratory
damage in 24 rats exposed to 0.1 mg/m3 of mercury vapor for 7 hours/day, 5 days/week for 72
weeks. The U.S. EPA considers the evidence for both animal and human carcinogenicity
inadequate (U.S. EPA 1995). Their classification is D - not classifiable as to human
carcinogenicity.
3.7.	Summary
Recently conducted studies show that a 1-hour exposure of mice to 9.8 mg/m3 mercury
vapor resulted in lung damage (Shimojo et al. 1996); whereas, a 2-hour exposure of pregnant rats
to 8 mg/m3 was a NOAEL for lesions of the lung, liver, and kidney when examined immediately
after exposure (Morgan et al. 2002). The 2-hour exposure to 8 mg/m3 resulted in a decreased
weight gain compared with controls and mild nephrotoxicity as indicated by urinary enzymes. A
one-hour exposure of rats to approximately 26.7 mg/m3 resulted in lung lesions, but no deaths; a
2-hour exposure to 27 mg/m3 resulted in death of 20 of 32 rats (Livardjani et al. 1991). There
were no lesions of the lung, liver, or kidney in non-pregnant and pregnant female rats exposed to
4 mg/m3 for 2 hours/day for 1 or 10 days (Brambila et al. 2002; Morgan et al. 2002). Kidney
lesions were not found in neonates exposed in utero to 4 mg/m3. Developmental effects were
observed only in association with maternal toxicity, following repeated exposure to 8 mg/m3.
Reversible behavioral changes were observed in male and female Wistar-derived rats
exposed to 17.2 mg/m3 (2.1 ppm) for 2 hours/day, 5 days/week for 22 exposures over 30 days
(Beliles et al. 1968) and 14 male rats (strain unspecified) exposed to 3.0 mg/m3 (0.37 ppm) for 3
hours/day, 5 days/week for 12 to 42 weeks (Kishi et al. 1978). Following a recovery period,
lesions were observed in the central nervous system in some rats in the first study and in the
kidney in the second study.

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Developmental neurotoxicity tests with rats involving 6-8 daily 1-4-hour exposures at
concentrations as low as 0.05 mg/m3 (0.006 ppm) were associated with altered spontaneous
motor activity and decrements in learning tasks (Fredriksson et al. 1992; 1996; Danielsson et al.
1993). The study with squirrel monkeys (Newland et al. 1996) clearly shows effects but does not
provide sufficient information about the shape that a dose-response relationship might assume,
and it did not identify a no-effect level.
Genotoxicity studies, usually conducted with mercuric chloride, demonstrated mixed
results; none of the studies used the inhalation method of administration. According to ATSDR
(1999), inorganic mercury compounds may have some genotoxic or clastogenic potential. No
recent chronic toxicity/carcinogenicity studies have been conducted with laboratory animals.
4. SPECIAL CONSIDERATIONS
4.1. Metabolism and Disposition
A review of animal and human studies indicates that 70-80% of inhaled mercury is
absorbed into the lungs, primarily in the alveolar-interstitial region (Leggett et al. 2001; AIHA
2002). Deposition in the lungs is controlled by the rate of breathing (Hayes and Rothstein 1962).
As an uncharged monatomic gas, mercury is highly diffusible and lipid soluble. The lipophilic
vapor is readily absorbed into the bloodstream; from there it diffuses to all tissues in the body
(Goyer and Clarkson 2001). In erythrocytes it is rapidly oxidized by cytosolic catalase-hydrogen
peroxide to mercuric mercury (Hg+2). Mercuric mercury is highly reactive and rapidly combines
with intracellular ligands such as sulfhydryls, potentially disrupting enzymes and proteins
essential to normal organ function. It may also form complexes with glutathione which are then
secreted in the bile. The presence of mercury in the body induces thiol-containing proteins, the
metallothioneins. Induction of metallothioneins (regulated by at least two genes, MT-1 and MT-
II) is believed to be a protective mechanism. Metallothioneins are induced in the kidney and
placenta of rat dams exposed during gestation (Brambila et al. 2002; Morgan et al. 2002) and in
fetal rat brain following in utero exposure (Aschner et al. 1997).
A small amount of elemental mercury may be transported to tissues including the brain
where biotransformation takes place. Elemental mercury traverses the blood-brain barrier more
readily than mercuric ions. Mercury is absorbed by all tissues, but the primary organs of
deposition are the brain and kidneys (Goyer and Clarkson 2001). Deposition in tissues is linearly
related to exposure time and atmospheric concentration. Clearance half-times of inhaled mercury
by human subjects range from 1.7 days for the lungs to 64 days for the kidney region (Hursh et
al. 1976). Mercury absorbed by human skin was estimated to be 2.2% of that absorbed by the
lung (Hursh et al. 1989). Excretion is in the urine and feces; only a small portion is excreted in
the expired air.
The biokinetics of inhaled mercury vapor (Hg°) have been studied in human subjects.
These studies are summarized in Table 6 and blood concentrations are summarized in Table 7.
Exposure concentrations and durations ranged from 0.10 mg/m3 (0.007 ppm) for 7 hours
(Teisinger and Fiserova-Bergerova 1965) to 0.40 mg/m3 (0.049 ppm) for 15 minutes (Sandborgh-
Englund et al. 1998). No adverse effects were reported in these studies. The most recent, well-
conducted study in which known extraneous sources of mercury were minimized is described.

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Additionally, a monitoring study is included in Table 7. Blood mercury in control subjects is
generally below 50 ng/mL and urine mercury is generally below 25 |ig/L (Goldwater 1972).
TABLE 6. Human Metabolism Studies
Concentration
(Subjects)
Exposure
Duration
Observation
Reference
0.050-0.35 mg/m3
(3 male, 1 female
subjects)
5 mina
70-85% absorption
Nielsen Kudsk 1965
0.06-0.11 mg/m3
(5 male subjects)13
14-24 min
74% retention; 7% expired; tissue half-
lives; lung, 1.7 days, head, 21 days
Hurshetal. 1976;
Cherianetal. 1978
0.06-0.40 mg/m3
(5 male subjects; 3
ingested 65 mL
ethanol)b
11-21 min
73% retention; lower Hg retention
(55%) following ingestion of ethanol
Hurshetal. 1980
0.4 mg/m3
(9 males, females;
light physical
exercise at 50 W)°
15 min
rapid absorption; 67% retention; 7.5-
12% loss by expiration in 3 days; blood
plasma half-life of 10 days
Sandborgh-Englund et
al. 1998
0.10 mg/m3
(4 subjects); 0.20
mg/m3 (1 subject)0
7 h
76% retention; urinary excretion
(, :g/24 hours) approximated non-
exposed values
Teisinger and Fiserova-
Bergerova 1965
0.01-0.106 mg/m3;
mean 0.04 mg/m3
(10 workers)
8 h, 5 d
blood Hg concentration of 2.0 , -g/dl:
urinary Hg concentration of 50 , 'gig
creatinine
Roelsetal. 1987
a Exhaled air was collected for 5 minutes; it is assumed the exposures were also for 5 minutes.
b Subjects inhaled through a mouthpiece.
c Nasal inspiration and oral expiration.
Sandborgh-Englund et al. (1998) described the absorption, blood levels, and excretion of
mercury vapor after a single exposure in humans. Nine healthy volunteers, two males and seven
females, ages 19-53, inhaled 0.4 mg/m3 (analytically determined range of 0.365 to 0.430 mg/m3)
of mercury vapor through a reverse valve mouthpiece for 15 minutes. None of the subjects had
amalgam fillings, and fish had been excluded from the diet for the previous month. The subjects
exercised on a bicycle ergometer at 50 W. The median retention was 69% of the inhaled dose
(range 57-73%). During the following three days, 7.5-12%> of the absorbed dose was lost by
exhalation, with a median half-life of mercury in expired breath of two days. Absorption by
blood and plasma was rapid, followed by a bi-exponential decline in both media. Substantial
inter-individual variation was observed in the area under the concentration-time curves for blood
and plasma. About 1% of the absorbed dose was excreted via the urine during the first 3 days
after exposure, and an estimated 8-40%> was excreted during 30 days. Total mercury in blood
(plasma and erythrocytes combined) increased from a baseline value of 6.0 nmol/L (range 4.0-7.6
nmol/L) to a maximum concentration of 7.4 nmol/L (median net value; 25-75 percentile of 6.6-
8.9 nmol/L). Blood samples were taken from 15 minutes up to 8 hours after the beginning of the
exposure. The maximum blood concentration was reached 5 hours after the beginning of
exposure. Baseline blood samples were composed of primarily methyl mercury.

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Jonsson et al. (1999) used the data of Sandborgh-Englund et al. (1998) to model the
kinetics of mercury vapor in humans. A four-compartment model, including two depot
compartments to account for retention in lungs and kidneys, respectively, gave the best fit to the
data. The median half-time in the respiratory depot compartment was estimated at 1.8 days; the
median half-life in the excretion depot was estimated at 63 days.
Barregard et al. (1996) observed two phases of elimination in workers following an initial
high unquantified exposure to Hg vapor. Workers were also exposed to inorganic Hg
compounds and lead. Monitoring followed 2-10 days of exposure, 7 hours/day, after which signs
and symptoms of Hg intoxication appeared. The half-life of the fast phase was 2-16 days and the
half-life of the slow phase was more than a month. The authors attributed the fast phase to
exceedence of the capacity of the Hg binding sites in the kidney.
Roels et al. (1987) monitored workers at an alkaline battery manufacturing plant.
Monitoring was performed daily with personal samplers for 8-hour periods for 5 days.
Atmospheric concentrations ranged from 0.010 to 0.106 mg/m3 (overall mean 0.040 mg/m3).
Blood mercury concentrations taken at the end of the work shift averaged 2.0 //g/dl (20 /ig/L)
and urine samples collected the following morning averaged 50 //g/g creatinine (values estimated
from graphs).
Falnoga et al. (1994) followed the uptake of mercury in tissues and organs of male adult
Wistar rats placed in the working area of a mercury mine for 38 days. Average air concentrations
were 0.57 (0.50-0.70) mg/m3. Blood samples, taken at 4, 8, and 18 hours into the exposure, were
209, 256, and 479 ng/g, respectively (approximately 0.21, 0.26, and 0.48 //g/mL, respectively);
the control value was 11.6 ng/g. Blood concentrations leveled off at 1 to 1.5 days into the
exposure (647-776 ng/g), but were higher on the 38th day of exposure (880 ng/g). Following
exposure of rats to a slightly higher concentration, 1.1 (0.55-1.72) mg/m3 for 17 days, elimination
from organs and tissues was complete in 46 days. The authors described uptake into the blood as
irreversible zero order kinetics and elimination from the blood by irreversible first order kinetics.
The authors stated that several rats died during the exposure to 1.1 mg/m3, but gave neither the
number of rats nor day of death.
Neonatal rats were exposed to mercury vapor, 0.05 mg/m3 for 1 or 4 hours/day, from days
11 to 17 of age; sacrifice took place on day 25 (Fredriksson et al. 1992). Mercury concentrations
(mg/kg) in tissues of the control, 1-hour, and 4-hour exposure groups were: brain, 0.002, 0.002,
and 0.002; liver, 0.017, 0.084, and 1.247; and kidney, 0.063, 0.219, and 6.734, respectively.
These concentrations were measured approximately one week after exposure. Nursing infants
may be exposed via consumption of contaminated breast milk from nursing mothers exposed via
the occupational or diet sources (ATSDR 1999).
Adult Wistar rats of either sex inhaled 0.5, 1.0, or 2.0 mg/m3 for 1, 2, or 3 hours (nine
exposure groups) (Halbach and Fichtner 1993). Tissue concentrations, determined immediately
after exposure, increased linearly with exposure time or with Hg concentration in air. For
example, following inhalation of 0.5 mg/m3 for 1, 2, or 3 hours, brain concentrations were 7, 12,
and 15 |ig/kg, respectively. Following inhalation of 0.5, 1, or 2 mg/m3 for one hour, brain
concentrations were 7, 12, and 18 |ig/kg, respectively. Concentrations were highest in the lung.

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1
2	Although the transport of mercuric ions is limited at the placental barrier by the presence
3	of high-affinity binding sites (Dencker et al. 1983), dissolved mercury vapor easily penetrates the
4	placental barrier and accumulates in fetal tissues. Mercury was elevated in tissues of fetuses of
5	exposed pregnant rats and monkeys, although the magnitude of accumulation was less than that
6	seen in the maternal brain (Fredriksson et al. 1992; Danielsson et al. 1993; Warfvinge et al. 1994;
7	Newland et al. 1996; Morgan et al. 2002; see Section 3.4. for study descriptions). Examples of
8	fetal uptake in animal models are listed in Table 7.
9
TABLE 7. Tissue Concentrations of Mercury"
Concentration
(Subjects)
Exposure
Duration
Observation/Tissue Concentration
Reference
Human Monitoring Studies
0.01-0.106 mg/m3;
mean 0.04 mg/m3 (10
workers - occupational
exposure)
8 h, 5 d
20 /ig/L (blood)
50 ,'g/g creatinine (urinary Hg)
Roelsetal. 1987
Background
(military population
with dental amalgams)
Chronic
2.55 |ig/L (blood, total mercury)
0.54 |ig/L (blood inorganic mercury)
3.09 |ig/L (urine, total mercury)
2.88 |ig/L (urine, inorganic mercury)
Kingman etal. 1998
Background
Exposure: 0.40 mg/m3
(9 healthy volunteers)13
15 min
0.19 |ig/L (blood, inorganic mercury)
1.20 |ig/L (blood, total mercury)
1.48 |ig/L (blood, total mercury)
Sandborgh-Englund et
al. 1998
Background
Fetuses
Infants (<3 months)

Total mercury:
Brain: 5 ng/g; kidney: 9 ng/g
Brain: 6 ng/g; kidney: 12 ng/g
Lutz et al. 1996
Rodents
Rat: 4 mg/m3
Non-pregnant
Pregnant
Neonates
10 d, 2 h/d
GD 6-15
PND21
Kidney (total mercury):
60 ng/g (control: 0.009 ng/g)
86 ng/g (control: 0.019 ng/g)
0.089 ng/g (control: 0.018 ng/g)
Brambila et al. 2002
Rat: 4 mg/m3
Dam
Fetus
10 d, 2 h/d
Brain:
Control: 0.002 ng/g
GD 15: 3 ng/g
Control: 0.001 ng/g
GD 15: 0.05 ng/g
Morgan et al. 2002
Rat: 0.05 mg/m3
days 11-17 of age
1 or 4 h/d;
sacrificed day 25
Brain:
Control: 0.002 ng/g
1 h/d: 0.017 ng/g
4 h/d: 0.063 ng/g
Fredriksson et al. 1992
Rat: 1.8 mg/m3 inutero
GD 14-19,
1.5 h/d
Brain: 5 ng/g (measured PND 2-3)
Fredriksson et al. 1996
Guinea pig: 0.2-0.3
mg/m3
Dam
Neonate
2 h/d, 4 days
Brain:
Control: 29 ng/g
Exposed: 95 ng/g
Control: 19 ng/g
Exposed: 17 ng/g
Yoshidaetal. 1986
10 a Concentrations are wet weight.

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b Mercury measured five hours after exposure (background subtracted); no dental amalgams, fish excluded from diet.
GD = gestation day; PND = post-natal day.
In the study of Morgan et al. (2002), metallothionein and mercury concentrations in
selected tissues of the dam and fetus were provided following inhalation of 4 mg/m3, 2
hours/day, on GD 6-15. On GD 10, metallothionein concentrations were highest in the placenta
of the dam followed by the kidney, liver, and lung. Due to small size, fetal tissue could not be
weighed separate from the placenta prior to GD 10 and brain tissue could not be separated from
the fetus prior to GD 15. On GD 15, the mean mercury concentration in the developing brain of
the fetus was 49 ng/g (approximately 0.05 jug/g). The mean mercury concentration in the brain
of dams was approximately 3 jug/g, and that of the placenta was 2.4 jug/g. The concentration in
the fetal brain was a factor of approximately 60 lower than that of the dam brain. On PND 21,
the concentration in the kidney of neonates exposed in utero to 4 mg/m3 was 0.089 jug/g, whereas
the concentration in the kidney of the control neonate was 0.018 jug/g.
Yoshida et al. (2002) studied the role of metallothionein in maternal-to-fetal distribution
of mercury in pregnant C57B16 mice. Following inhalation of 5.5-6.7 mg/m3 for 3 hours during
late gestation, elevated mercury concentration in the placenta was associated with
metallothionein. The authors suggested that metallothionein plays a defensive role in preventing
maternal-to-fetal mercury transfer. In non-exposed and mercury-vapor exposed mice,
metallothionein levels were highest in the liver followed by the placenta. Metallothionein
concentrations were statistically significantly higher in the lung and kidney of exposed dams
compared with the control group. Mercury concentrations were elevated in the fetuses (whole
body) of exposed dams by a factor of 9-10 compared to fetuses of control dams.
During late gestation, pregnant Hartley strain guinea pigs were exposed to 0.2-0.3 mg/m3
for 2 hours/day for 4-11 days (Yoshida et al. 1986). Dams and offspring were sacrificed
immediately after birth. After 4 days of exposure, mercury in brain of offspring was not
increased over control levels; however, concentrations were increased in lung, liver, and kidney.
After >5 days of exposure, mercury was increased in all offspring tissues.
Between GD 65 and 76, Hartley strain guinea pigs inhaled 8-10 mg/m3 for 150 minutes
(Yoshida et al. 1990). Some dams were sacrificed at 2 hours after exposure and tissue levels of
dams and fetuses were analyzed for mercury. Additional dams were allowed to give birth and
offspring, fostered to unexposed dams, were sacrificed at 5 or 10 days after exposure. Fetal brain
concentrations of mercury were not elevated compared to controls. The highest mercury
concentration was in the liver, largely bound to metallothionein. At 5 to 10 days after exposure,
neonate brain concentrations were elevated 3- to 5-fold compared with current controls. On days
5 and 10, the highest mercury concentrations were found in the kidney, followed by liver, lung,
and brain. The authors suggested that mercury, initially bound to metallothionein in the liver was
redistributed during the neonatal period. In the fetal liver, about 50% of the mercury is bound to
metallothionein (Yoshida et al. 1987).

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4.2.	Mechanism of Toxicity
At high levels of exposure, respiratory failure, cardiac arrest, and cerebral edema ensue
(Goyer and Clarkson 2001; WHO 2003). These consequences have been attributed to the
divalent (Hg+2) mercury which binds to a variety of enzymes and intracellular proteins including
those of microsomes and mitochondria, producing nonspecific cell injury or cell death (Goyer
and Clarkson 2001). The affinity of mercury for ligands containing sulfhydryl (SH or thiol)
groups results in a glutathione complex which is excreted in the bile. The central nervous system
is probably the most sensitive target for elemental mercury vapor exposure. Within the kidney,
the primary toxic effect is on the epithelial cells of the proximal tubules (ATSDR 1999).
4.3.	Structure-Activity Relationships
No data were located relevant to structure-activity relationships.
4.4.	Other Relevant Information
4.4.1. Species Variability
No relevant data on species variability were located. The absorption, distribution, and
excretion of mercury in humans and animals have similar aspects (ATSDR 1999). The rate of
inhalation of xenobiotics including that of mercury vapor is related to the ventilation rate and the
inhaled concentration (Hayes and Rothstein 1962; Medinsky and Valentine 2001). Relative to
body weight, rodents have a much higher respiratory rate and cardiac output than humans [the
respiratory rate of the mouse may be up to 10 times that of the human (Witschi and Last 2001;
Kale et al. 2002)]. Respiratory rate and cardiac output are the two primary determinants of
systemic uptake of volatile gases following inhalation exposure. As a result of the greater
respiratory rate and cardiac output, rodents generally receive a greater overall dose than humans
at equivalent exposure concentrations.
Few data were available to compare concentrations of mercury in human and rodent
tissues. Mercury in brain of 19 human fetuses and 14 infants of less than three months of age at
autopsy averaged 5 and 6 ng/g wet weight, respectively (ranges, 2-9 and <2-23 ng/g, respectively)
(Lutz et al. 1996). Autopsy results of human infants at birth showed total mercury concentrations
in the cerebral cortex of <1 to 20 ng/g wet weight of tissue (Drasch et al. 1994). The variability
among human young is most likely due to differing diets and number of dental amalgam surfaces
in the mothers. Mercury in control fetal guinea pigs and 5- and 10-day old guinea pigs averaged
13±2, 9± 1, and 7±1 ng/g wet weight, respectively (Yoshida et al. 1990). Following in utero
exposure to 10 mg/m3 of mercury for 150 minutes, brain concentrations in guinea pigs of the
respective age groups were 14±5, 27± 1, and 32-33 ng/g wet weight. Fetal levels were measured
two hours post-exposure. The mean concentrations of mercury in GD 15 fetuses of rat dams
exposed to 4 mg/m3 mercury vapor for 10 days was 49 ng/g. Values could not be provided
following a single exposure, but this study as well as other studies show tissue accumulation with
repeat exposure.
Exposure of rats during fetal development comprises a subchronic exposure. Ten days of
exposure for 2 hours/day for a total of 20 hours over a 20 day gestation period (Morgan et al.

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2002) comprises 4% of the rat gestational period; whereas, the same exposure scenario comprises
0.3% of the human gestation period of 270 days.
4.4.2.	Susceptible Populations
Based on a reconstructed accidental inhalation exposure and results of accidental food
poisoning, infants and children are considered the most susceptible members of the population
(ATSDR 1999; AIHA 2002). Mercury (Hg°) rapidly passes the blood-brain barrier and reaches
the fetal brain (Danielsson et al. 1993; Fredriksson et al. 1996). A neurotoxicity study with
neonatal rats involved repeat exposures (Fredriksson et al. 1992). Exposure to 0.05 mg/m3
(0.006 ppm) mercury vapor for 4 hours/day for 7 days resulted in changes in activity and learning
in some neurotoxicity tests but not in others.
Increased mercury uptake of neonates compared with dams may indicate increased
sensitivity. Yoshida et al. (1989) exposed Hartley strain guinea pig dams and 12-hour-old
neonates to 8-10 mg/m3 of mercury vapor for 2 hours. Animals were sacrificed immediately
after the exposure. Catalase activity was determined in blood and liver, and mercury and
metallothionein concentrations were measured in selected tissues. Compared with the respective
control and exposed groups of dams, catalase activity was lower in neonate blood and liver by
factors of 2 (blood) and 5 (liver) in the control group and by factors of 1.3 (blood) and 4 (liver) in
the exposed group. Mercury in plasma of neonates was two to three times higher than
concentrations in maternal plasma, but the concentrations in erythrocytes were similar. Except
for kidney, mercury concentrations in organs of neonates were higher than that of dams, with
brain values of neonates being 12-28% higher. For both control and exposed groups,
metallothionein concentration in the liver was higher in the neonates than in the dams.
In contrast to neonates, fetal tissues of rats accumulate less mercury than the tissues of
mercury-treated dams (Fredriksson et al. 1992; Danielsson et al. 1993; Warfvinge et al. 1994;
Newland et al. 1996; Morgan et al. 2002). Following 10 days of exposure to 4 mg/m3, (GD 15)
total mercury in brains of dams and fetal rats were 3300 and 49 ng/g of tissue, respectively
(Morgan et al. 2002).
Following exposure of nine healthy male and female volunteers to 0.40 mg/m3 for 15
minutes, the maximum blood concentration of mercury did not vary greatly (median, 7.4 nmol/L;
range, 5.9-13.0 nmol/L) (Sandborgh-Englund et al. 1998). The time to maximum blood
concentration varied considerably.
Based on simulations of accidental exposure, a concentration of 16 mg/m3 was fatal to an
infant (Campbell 1948), whereas concentrations of 18-43 mg/m3 were fatal to 4 of 35 adults
(Tennant et al. 1961; Asano et al. 2000).
4.4.3.	Concentration-Exposure Duration Relationship
Studies with animal models show that tissue mercury concentration increases with
exposure concentration and exposure duration (Morgan et al. 2002; Yoshida et al. 1986).
However, no data were available to describe a concentration-response relationship. Exponential

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scaling (ten Berge et al. 1986) was used to derive exposure duration-specific values. It has been
shown that the concentration-exposure time relationship for many irritant and systemically acting
vapors and gases may be described by Cn x t = k, where the exponent n ranges from 0.8 to 3.5. In
the absence of chemical-specific data, an n value of 3 was applied to extrapolate to shorter time
periods, and an n value of 1 was applied to extrapolate to longer time periods to provide AEGL
values that would be protective of human health (NRC 2001).
4.4.4. Concurrent Exposure Issues
Amalgam dental fillings as well as food such as fish constitute major sources of chronic
mercury exposure in humans. Mercury vapor from amalgam dental fillings is a potentially
significant source of exposure to elemental mercury as estimates of daily intake from amalgam
restorations range from 3 to 17 //g mercury/day, with the majority of individuals exposed to 5 jig
mercury/day. Mercury in whole blood ranges from 2 //g/L in the absence of fish consumption to
8 //g/L when 2-4 fish meals/week are eaten. The mercury intake from fish is primarily in the
form of methyl mercury (Brune et al. 1991; Barregard 1993; Sandborgh-Englund et al. 1998;
AT SDR 1999; WHO 2003).
Catalase inhibitors such as ethanol inhibit the oxidation of elemental mercury to divalent
mercury, thus reducing mercury retention. Human subjects that ingested 65 mL of ethanol prior
to exposure to mercury vapor had reduced mercury retention, an increase in the rapid phase of
vapor loss by expiration, increased mercury storage in the liver, a marked reduction in mercury
uptake by the red blood cells, and the abolition of prompt storage of mercury by the lung.
Similar results were seen with mice and rats (Hursh et al. 1980).
5. DATA ANALYSIS FOR AEGL-1
5.1.	Summary of Human Data Relevant to AEGL-1
Exposure to mercury vapor at low concentrations does not induce irritation or other
warning signs. There is no odor.
Controlled human exposures during metabolism studies used low concentrations and
short exposure durations. No adverse effects were reported following exposure of healthy adults
to 0.10 mg/m3 for seven hours (Teisinger and Fiserova-Bergerova 1965) or 0.40 mg/m3 for up to
15 minutes, the latter exposure accompanied by light physical exercise (Sandborgh-Englund et al.
1998).
Under conditions of a reconstructed accidental exposure, high-school-age students
exposed to an estimated 15 mg/m3 for 0.75 hours did not become ill (Shelnitz et al. 1988).
5.2.	Summary of Animal Data Relevant to AEGL-1
Acute studies with animal models used concentrations that induced effects greater than
those defined by an AEGL-1. Repeat-dose studies, including neurotoxicity studies, resulted in
organ lesions; the time of appearance of these lesions could not be ascertained.

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5.3. Derivation of AEGL-1 Values
Mercury vapor is odorless and produces no irritation or early warning signs. Accidental
human exposures (Section 2.1) show that even lethal exposures may be tolerated for several
hours without apparent warning signs. Because there are no signs of notable discomfort or
irritation at low concentrations, and studies that document asymptomatic, non-sensory effects
that meet the definition of an AEGL-1 are not available, AEGL-1 values are not recommended
(Table 8).
TABLE 8. AEGL-1 Values for Mercury Vapor
10-min
30-min
1-h
4-h
8-h
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
6. DATA ANALYSIS FOR AEGL-2
6.1.	Summary of Human Data Relevant to AEGL-2
As noted in Section 4.1, no adverse effects were reported following controlled exposures
of healthy adults to 0.10 mg/m3 mercury vapor for seven hours (Teisinger and Fiserova-
Bergerova 1965) or 0.40 mg/m3 for up to 15 minutes (Sandborgh-Englund et al. 1998). The
latter exposure was accompanied by light physical exercise.
In their summary of occupational monitoring studies, the AIHA (2002) notes that
exposure to mercury vapor concentrations of 0.4-2 mg/m3 (0.05-0.25 ppm) "may result in
symptoms of mercury intoxication after exposures of weeks or longer. Adverse effects have not
been reported for exposure to these concentrations of hours or days." Similarly, chronic exposure
to low concentrations during pregnancy (0.025-0.6 mg/m3) have failed to show an increase in
miscarriages or stillbirths, but congenital anomalies were non-significantly increased (Elghany et
al. 1997). The increase was not concentration related.
Concentrations during accidental exposures are generally estimated. Students exposed to
an estimated 15 mg/m3 (1.8 ppm) in an unventilated high-school lab for approximately 0.75
hours did not become ill (Shelnitz et al. 1988). Based on simulations of accidental exposure, a
concentration of 16 mg/m3 for a "few hours" was fatal to an infant (Campbell 1948), whereas
concentrations of 18-43 mg/m3 for several hours were fatal to 4 of 35 adults (Tennant et al. 1961;
Asano et al. 2000).
6.2.	Summary of Animal Data Relevant to AEGL-2
Pregnant Long-Evans rats exhibited no lesions of the lungs, liver, or kidney after 1, 5, or
10 two-hour daily exposures to 4 mg/m3 mercury vapor, although dams became moribund
following 10 days of exposure to 8 mg/m3 (Brambila et al. 2002; Morgan et al. 2002).
Developmental effects consisting of increased resorptions and decreased litter size were confined
to the 8 mg/m3 exposure group. No effects on pregnancy or offspring were reported in the group
exposed to 4 mg/m3. Following 10 days of exposure to 4 mg/m3, the mean mercury
concentration in the brain of the developing fetus was lower by 60-fold than that in the brain of

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the dam (0.05 |ig/g vs 3 jug/g). A slight weight loss in the dams following inhalation of 4 mg/m3
for 10 days was attributed to the repeat nature of the exposure. Pregnancies were uneventful in
squirrel monkeys exposed to 0.5 or 1.0 mg/m3 for approximately half of their gestation period,
but offspring tested at 0.8 to 4 years of age showed variable behavioral deficits.
A 1-hour exposure of mice to 9.8 mg/m3 greatly increased the protein content of broncho-
alveolar lavage fluid; hemoglobin was present in the fluid at 48 hours, suggesting lung damage
(Shimojo et al. 1996). No deaths were reported within 5 days post-exposure. Liver and kidney
function were normal in mice that inhaled 5.5-6.7 mg/m3 for 3 hours as determined by sacrifices
up to 7 days later (Yoshida et al. 1999a). Rats exposed to 26.7 mg/m3 for 1 hour showed no
breathing difficulties, but exhibited lung lesions (Livardjani et al. 1991). No deaths occurred
after the 1-hour exposure; but extension of the exposure for another hour resulted in 62.5%
mortality.
In repeat-dose studies, reversible neurotoxicity was observed in rats exposed to 17.2
mg/m3 mercury vapor for 2 hours/day, 5 days/week for 22 exposures (Beliles et al. 1968) or 3.0
mg/m3 for 3 hours/day, 5 days/week for 12 to 42 weeks (Kishi et al. 1978). Rats were held for up
to 12 weeks after cessation of exposure; only minor effects were reported. Developmental
studies with repeated exposures showed variable results and may not be relevant to a single
exposure, as mercury accumulates in the brain with each successive exposure.
6.3. Derivation of AEGL-2 Values
Although maternal exposures were for 2 hours/day for 10 days, a single 2-hour exposure
of pregnant Long-Evens rats to 4 mg/m3 mercury vapor (Morgan et al. 2002) was used as the
point of departure for the AEGL-2. This value is a NOAEL for developmental effects.
Developmental effects including increased resorption, decreased litter size and decreased
neonatal weight were observed at the next highest concentration of 8 mg/m3. Uncertainty factors
for the AEGL-2 are based on a weight of evidence of approach. The following factors were
considered in deriving an interspecies uncertainty factor: the 4 mg/m3 value was a NOAEL for
developmental effects (below the definition of the AEGL-2), the exposures were repeated for 10
days, rodents have a higher respiratory rate and cardiac output compared with humans (resulting
in faster uptake), and human monitoring studies show some effects at concentrations of 0.4 to 2
mg/m3 only with chronic exposure (AIHA 2002). The following factors were considered in
deriving an intraspecies uncertainty factor: the population of fetuses is considered a sensitive if
not the most sensitive population, the protective action of the placenta in sequestering mercury
[the mean concentration of mercury in the brain of dams exposed to 4 mg/m3 for 10 days was 60-
fold higher than in the fetal brain (Morgan et al. 2002)], and incidences of miscarriages and
stillbirths were unaffected in women chronically exposed to 0.025 to 0.6 mg/m3, although
anomalies were statistically non-significantly increased (Elghany et al. 1997). Based on these
factors, interspecies and intraspecies uncertainty factors of 1 and 3 were applied. Application of
larger uncertainty factors, for example 10 or 30 (resulting in 2-hour values of 0.4 or 0.13 mg/m3),
results in values that are inconsistent with the available human data, including the chronic
exposure of pregnant women in the study of Elghany et al. (1997). In the absence of time-scaling
information, the resulting 2-hour value of 1.33 mg/m3 was time-scaled using default n values of 3
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calculations are in Appendix A and values are listed in Table 9 below. A category plot of AEGL
values in relation to toxicity data is shown in Appendix B.
TABLE 9. AEGL-2 Values for Mercury Vapor
10-min
30-min
1-h
4-h
8-h
3.1 mg/m3
(0.38 ppm)
2.1 mg/m3
(0.26 ppm)
1.7 mg/m3
(0.21 ppm)
0.67 mg/m3
(0.08 ppm)
0.33 mg/m3
(0.04 ppm)
The AEGL-2 values are supported by the data of Yoshida et al. (1986). Following
inhalation of 0.2-0.3 mg/m3, 2 hours/day for 4 days, by guinea pig dams, mercury concentrations
were elevated in the dam brain by a factor of 3.3 (95 vs 29 ng/g) whereas the concentration in the
neonate brain was similar to that of the control (19 and 17 ng/g, respectively).
7. DATA ANALYSIS FOR AEGL-3
7.1.	Summary of Human Data Relevant to AEGL-3
Human data are limited to reconstructions of accidental exposures. One such study
indicates that students exposed to an estimated 15 mg/m3 in an unventilated high-school lab for
approximately 0.75 hours did not become ill (Shelnitz et al. 1988). Longer exposures to similar
concentrations resulted in illness, and exposure to higher concentrations for several hours
resulted in some fatalities (see Table 2).
7.2.	Summary of Animal Data Relevant to AEGL-3
Male Wistar rats inhaling 26.7 mg/m3 mercury vapor for one hour and observed for up to
15 days exhibited no clinical signs, but exhibited lung edema and necrosis (Livardjani et al.
1991). Extending the exposure period for another hour (at approximately the same
concentration) resulted in 62.5% mortality. Although the study is old, rabbits exposed to 31.3
mg/m3 for 1 hour and held for 6 days showed no mortality, but mild to moderate undefined
changes in the lung, kidney, brain, and heart were observed (Ashe et al. 1953).
Reversible behavioral changes were observed in male and female Wistar-derived rats
exposed to 17.2 mg/m3 (2.1 ppm) for 2 hours/day, 5 days/week for 22 exposures over 30 days
(Beliles et al. 1968) and 14 male rats (strain unspecified) exposed to 3.0 mg/m3 (0.37 ppm) for 3
hours/day, 5 days/week for 12 to 42 weeks (Kishi et al. 1978). Following a recovery period,
lesions were observed in the central nervous system in some rats in the first study and in the
kidney in the second study.
7.3.	Derivation of AEGL-3 Values
The 1-hour non-lethal exposure of rats to 26.7 mg/m3 (Livardjani et al. 1991) was used as
the point of departure for development of AEGL-3 values. The 26.7 mg/m3 value was adjusted
by a total uncertainty factor of 3 (an interspecies uncertainty factor of 1 and an intraspecies
uncertainty factor of 3) based on a weight of evidence approach. Larger uncertainty factors result
in values incompatible with the overall data. Reversible behavioral changes were observed in
male and female Wistar rats inhaling 17.2 mg/m3 for 2 hours/day for 22 exposures (Beliles et al.

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1968). The uncertainty factor of 3 is considered sufficient to protect susceptible populations.
Values derived using an intraspecies uncertainty factor of 3 are supported by the non-lethal
concentrations estimated in accidental exposures [up to 15 mg/m3 for 0.75 hours [Shelnitz et al.
1988; AIHA 2002)] and measured in occupational settings [0.4-2.0 mg/m3 (AIHA 2002)]. The
resulting 1-hour value of 8.9 mg/m3 was time-scaled using default n values of 3 and 1 for shorter
and longer exposure durations, respectively. Because the 8-hour time-scaled value of 1.1 mg/m3
appears low in comparison to accidental non-lethal exposures and is lower than some chronic
occupational exposures, the 8-hour value was set equal to the 4-hour value. Time-scaling
calculations are in Appendix A and values are listed in Table 10 below. A category plot of
AEGL values in relation to the animal toxicity and human metabolism data is found in Appendix
B.
TABLE 10. AEGL-3 Values for Mercury Vapor
10-min
30-min
1-h
4-h
8-h
16 mg/m3
11 mg/m3
8.9 mg/m3
2.2 mg/m3
2.2 mg/m3
(2.0 ppm)
(1.3 ppm)
(1.1 ppm)
(0.27 ppm)
(0.27 ppm)
The derived values are supported by concentrations estimated during accidental non-
lethal exposures. Concentrations of 15 mg/m3 for 0.75 hours and 16 mg/m3 for a "few hours"
were not lethal to older children and adults (although one infant death was recorded) (AIHA
2002). The derived values are also supported by the study of Beliles et al. (1968) in which
reversible behavioral changes were observed in male and female Wistar-derived rats exposed to
17.2 mg/m3 mercury vapor for 2 hours/day, 5 days/week for 22 exposures over 30 days. A
histological change in the medulla oblongata comprised of perivascular cuffing by lymphocytes
was partially reversible following a recovery period. This lesion may be attributed to the repeat
exposure protocol. No lesions were observed in other tissues.
8. SUMMARY OF AEGLS
8.1. AEGL Values and Toxicity Endpoints
AEGL values are summarized in Table 11. Because mercury vapor has no odor or
warning properties, AEGL-1 values were not recommended.
The point of departure for the AEGL-2 was a single 2-hour exposure of pregnant rats to 4
mg/m3 mercury vapor (Morgan et al. 2002). The exposure to 4 mg/m3 was a NOAEL for
developmental effects in rats. The 4 mg/m3 value was adjusted by a total uncertainty factor of 3
(an interspecies uncertainty factor of 1 and an intraspecies uncertainty factor of 3) based on a
weight of evidence approach and the incompatibility of the derived values with monitoring data
if a larger uncertainty factor is used. Based on mercury uptake by the sensitive developing brain
of the fetus, compared with uptake by the brain of the dam, an intraspecies uncertainty factor of 3
was considered sufficient to protect susceptible populations. In the absence of time-scaling
information, the resulting 2-hour value of 1.33 mg/m3 was time-scaled using default n values of 3
and 1 for shorter and longer exposure durations, respectively.
The point of departure for AEGL-3 values was the 1-hour exposure of rats to 26.7 mg/m3
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significant mortality. The 26.7 mg/m3 value was adjusted by a total uncertainty factor of 3 (an
interspecies uncertainty factor of 1 and an intraspecies uncertainty factor of 3) based on (1) faster
uptake in rodents compared with humans and (2) the incompatibility with monitoring and
accidental exposure data if a larger uncertainty factor is used. The uncertainty factor of 3 is
considered sufficient to protect susceptible populations. The resulting 1-hour value of 8.9 mg/m3
was time-scaled using default n values of 3 and 1 for shorter and longer exposure durations,
respectively.
TABLE 11. Summary of AEGL Values for Mercury Vapor
Classification
Exposure Duration
10-min
30-min
1-h
4-h
8-h
AEGL-l3
(Nondisabling)
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
AEGL-2
(Disabling)
3.1 mg/m3
(0.38 ppm)
2.1 mg/m3
(0.26 ppm)
1.7 mg/m3
(0.21 ppm)
0.67 mg/m3
(0.08 ppm)
0.3 3 mg/m3
(0.04 ppm)
AEGL-3
(Lethal)
16 mg/m3
(2.0 ppm)
11 mg/m3
(1.3 ppm)
8.9 mg/m3
(1.1 ppm)
2.2 mg/m3
(0.27 ppm)
2.2 mg/m3
(0.27 ppm)
a Mercury vapor is odorless. AEGL-1 values are not recommended because mercury vapor has no odor or warning
properties at concentrations that may cause extensive tissue damage.
8.2. Comparison with Other Standards and Guidelines
Standards and guidelines for exposure to mercury vapor are summarized in Table 12.
The AIHA (2002) did not develop an Emergency Response Planning Guideline-1 (ERPG-1)
because mercury vapor is odorless and produces no irritation or other early warning signs. The
ERPG-2 was based on a combination of animal studies (Fraser et al. 1934; Kishi et al. 1978;
Livardjani et al. 1991) and nine occupational studies. The occupational exposures involved
vapor concentrations of 0.05-0.25 ppm (0.41-2.1 mg/m3) which resulted in symptoms in some
workers after exposures of weeks or longer, but not after periods of hours or days. The ERPG-3
was also based on a combination of controlled laboratory studies with animals and fairly
consistent findings from accidental human exposures.
The 1-hour Spacecraft Maximum Allowable Concentration (SMAC) is 0.01 ppm
(0.08 mg/m3), and the 24-hour value is 0.002 ppm (0.02 mg/m3) (NRC 1996). The lung was
considered the target organ for acute exposure. The 1-hour value was based on an estimated
LOAEL of 2 mg/m3 for 5 hours from reconstruction of accidental exposure of 13 workers.
Uncertainty factors of 10 and the square route of 13 divided by 10 were applied; the latter
number was applied to account for the small number of subjects.
The ACGIH (1996) TLV-TWA of 0.025 mg/m3 was based on a combination of factors
including a Biological Exposure Index urine mercury level of 35 //g Hg/g creatinine collected
prior to an 8-hour work shift. The BEI applies only to exposure to elemental and inorganic forms
of mercury and not to organic mercury exposures. The monitoring data of Roels et al. (1987)
suggests that 50 //g Hg/g creatinine is the biological threshold for neurological damage in
workers.

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The NIOSH REL and the OSHA PEL are both 0.05 mg/m3, and the ceiling values
established by NIOSH and OSHA are both 0.1 mg/m3. The NIOSH IDLH is 10 mg/m3. The
IDLH is based on acute inhalation toxicity data in animals in the study of Ashe et al. (1953). All
NIOSH and OSHA values have a skin notation. The German MAK is 0.1 mg/m3 and the Dutch
MAC is 0.05 mg/m3.
The National Academy of Sciences (NRC 1984) developed Emergency and Continuous
Exposure Limits (EEGLs) for Selected Airborne Contaminants. Their 24-hour EEGL of 0.2
mg/m3 was based on the data of Milne et al. (1970). In that study, four workmen became ill after
exposure to an estimated 1.1 to 4.0 mg/m3 for 2.5 to 5 hours. A shorter-term guideline was not
established.
TABLE 12. Extant Standards and Guidelines for Mercury Vapor
Guideline
Exposure Duration
10 min
30 min
1 h
4 h
8 h
AEGL-1
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
AEGL-2
3.1 mg/m3
2.1 mg/m3
1.7 mg/m3
0.67 mg/m3
0.33 mg/m3
AEGL-3
16 mg/m3
11 mg/m3
8.9 mg/m3
2.2 mg/m3
2.2 mg/m3
ERPG-1 (AIHA)a


Not appropriate


ERPG-2 (AIHA)


0.25 ppm
(2.0 mg/m3)


ERPG-3 (AIHA)


0.5 ppm
(4.1 mg/m3)


SMAC (NRC)b


0.01 ppm
(0.08 mg/m3)


PEL-ceiling
(OSHA)0




0.1 mg/m3
IDLH (NIOSH)d

10 mg/m3



REL-TWA,
ceiling (NIOSH)e




0.05 mg/m3,
0.1 mg/m3
TLV-TWA
(ACGIH)f




0.025 mg/m3
MAK
(Germany)8




0.1 mg/m3
11(8)
MAC (The
Netherlands)11




0.05 mg/m3
aERPG (Emergency Response Planning Guidelines, American Industrial Hygiene Association (AIHA 2002)
The ERPG-1 is the maximum airborne concentration below which it is believed nearly all individuals could be
exposed for up to one hour without experiencing other than mild, transient adverse health effects or without
perceiving a clearly defined objectionable odor.
The ERPG-2 is the maximum airborne concentration below which it is believed nearly all individuals could be
exposed for up to one hour without experiencing or developing irreversible or other serious health effects or
symptoms that could impair an individual's ability to take protective action.
The ERPG-3 is the maximum airborne concentration below which it is believed nearly all individuals could be
exposed for up to one hour without experiencing or developing life-threatening health effects.
bSMAC (Spacecraft Maximum Allowable Concentration, National Research Council) (NRC 1996)
SMACs are intended to provide guidance on chemical exposures during normal operations of spacecraft as well
as emergency situations. The one-hour SMAC is a concentration of airborne substance that will not compromise

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the performance of specific tasks by astronauts during emergency conditions or cause serious or permanent toxic
effects. Such exposures may cause reversible effects such as skin or eye irritation, but they are not expected to
impair judgment or interfere with proper responses to emergencies.
cOSHA PEL-TWA (Occupational Safety and Health Administration, Permissible Exposure Limits - Time
Weighted Average) (NIOSH 2005) is defined analogous to the ACGIH-TLV-TWA, but is for exposures of no
more than 10 hours/day, 40 hours/week.
dIDLH (Immediately Dangerous to Life and Health, National Institute of Occupational Safety and Health)
(NIOSH 2005) represents the maximum concentration from which one could escape within 30 minutes without
any escape-impairing symptoms, or any irreversible health effects.
eNIOSH REL-TWA (National Institute of Occupational Safety and Health, Recommended Exposure Limits -
Time Weighted Average) (NIOSH 2005) is defined analogous to the ACGIH-TLV-TWA.
fACGIH TLV-TWA (American Conference of Governmental Industrial Hygienists, Threshold Limit Value -
Time Weighted Average) (ACGIH 1996) is the time-weighted average concentration for a normal 8-hour
workday and a 40-hour workweek, to which nearly all workers may be repeatedly exposed, day after day,
without adverse effect.
gMAK (Maximale Arbeitsplatzkonzentration [Maximum Workplace Concentration]) (Deutsche
Forschungsgemeinschaft [German Research Association] 2005) is defined analogous to the ACGIH-TLV-TWA.
For mercury, category 11(8) indicates an excursion factor of 2, 8 times during the shift.
hMAC (Maximaal Aanvaarde Concentratie [Maximal Accepted Concentration]) (SDU Uitgevers [under the
auspices of the Ministry of Social Affairs and Employment], The Hague, The Netherlands 2000)
is defined analogous to the ACGIH-TLV-TWA. The 15-minute peak is 0.5 mg/m3.
8.3. Data Adequacy and Research Needs
There are no human data with measured concentrations and symptoms that meet the
definitions of the AEGLs. Human metabolism studies used relatively low and presumably safe
concentrations. The metabolism studies in conjunction with estimated concentrations from non-
fatal human accidental exposures were considered in the development of AEGL values. An
AEGL-1 was not recommended because mercury is odorless and without irritation at
concentrations that may be harmful. The AEGL-2 and AEGL-3 values were based on several
rodent studies that used suitable concentrations and exposure durations. The sensitive
developing fetus was considered in development of AEGL-2 values.
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73.

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MERCURY VAPOR	NAC/Proposed 2: 11/2009; Page 45 of 51
1	APPENDIX A: Derivation of AEGL Values
2
3	Derivation of AEGL-1 Values
4
5	Because mercury vapor has no odor or warning properties, AEGL-1 values are not recommended.
6

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MERCURY VAPOR
NAC/Proposed 2: 11/2009; Page 46 of 51
Derivation of AEGL-2 Values
Key Study:
Toxicity endpoint:
Time scaling:
Uncertainty factors:
Calculations:
10-minute AEGL-2:
30-minute AEGL-2:
1-hour AEGL-2:
4-hour AEGL-2:
Morgan et al. 2002
A 2-hour/day, 10-day exposure to 4 mg/m3 mercury vapor was a NOAEL for
increased resorptions and fetal death in pregnant rats. The next higher
concentration, 8 mg/m3, resulted in fetotoxicity.
Cn x t = k where n = 3 and 1 for shorter and longer exposure periods,
respectively (ten Berge et al. 1986; NRC 2001).
Total of 3 (weight of evidence approach)
Interspecies: 1, based on the following factors: the 4 mg/m3 value was a
NOAEL for developmental effects (below the definition of the AEGL-2), the
exposures were repeated for 10 days, rodents have a higher respiratory rate
and cardiac output compared with humans (resulting in faster uptake), and
human monitoring studies show some effects at concentrations of 0.4 to 2
mg/m3 only with chronic exposure.
Intraspecies: 3, based on the following factors: the population of fetuses is
considered a sensitive if not the most sensitive population, the protective
action of the placenta in sequestering mercury (Morgan et al. 2002), and
incidences of miscarriages and stillbirths were unaffected in women
chronically exposed to 0.025 to 0.6 mg/m3, although anomalies were
statistically non-significantly increased.
C/3 = (4 mg/m3)/3 = 1.33 mg/m3
C3 x t = k
(1.33)3 x 120 minutes = (282.32 mg/m3)3 minutes
C1 xt = k
1.33 x 120 minutes = 159.6 mg/m3minutes
C3 x 10 minutes = 282.32 (mg/m3)3 minutes
C = 3.1 mg/m3
C3 x 30 minutes = 282.32 (mg/m3)3 minutes
C = 2.1 mg/m3
C3 x 60 minutes = 282.32 (mg/m3)3 minutes
C = 1.7 mg/m3
C1 x 240 minutes = 159.6 mg/m3minutes
C = 0.67 mg/m3
8-hour AEGL-2:
C1 x 480 minutes - 159.6 mg/m3minutes
C = 0.33 mg/m3

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MERCURY VAPOR
NAC/Proposed 2: 11/2009; Page 47 of 51
Derivation of AEGL-3 Values
Key Study:
Toxicity endpoint:
Time scaling:
Uncertainty factors:
Calculations:
10-minute AEGL-3:
30-minute AEGL-3:
1-hour AEGL-3:
4-hour AEGL-3:
8-hour AEGL-3:
Livardjani et al. 1991
Highest 1-hour non-lethal exposure of rats: 26.7 mg/m3
Cn x t = k where n = 3 and 1 for shorter and longer exposure periods,
respectively (ten Berge et al. 1986; NRC 2001).
Total of 3 (an interspecies uncertainty factor of 1 and an intraspecies
uncertainty factor of 3), based on (1) faster uptake in rodents compared with
humans and (2) the incompatibility of derived values with monitoring data
and accidental exposure data if a larger uncertainty factor is used. The
intraspecies uncertainty factor of 3 is considered sufficient to protect
susceptible populations.
C/3 = (26.7 mg/m3)/3 = 8.9 mg/m3
C3 x t = k
(8.9 mg/m3)3 x 60 minutes = (42298.14 mg/m3)3 minutes
C1 xt = k
8.9 mg/m3 x 60 minutes = 534 mg/m3minutes
C3 x 10 minutes = 42298.14 (mg/m3)3 minutes
C = 16 mg/m3
C3 x 30 minutes = 42298.14 (mg/m3)3 minutes
C = 11 mg/m3
C/3 = 8.9 mg/m3
C1 x 240 minutes = 534 mg/m3minutes
C = 2.2 mg/m3
C1 x 480 minutes - 534 mg/m3minutes
C = 1.1 mg/m3 (see below)
Because of inconsistency with some monitoring data and estimated non-fatal concentrations, the 8-hour
AEGL-3 value of 1.1 mg/m3 was set equal to the 4-hour value (2.2 mg/m3).

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MERCURY VAPOR	NAC/Proposed 2: 11/2009; Page 48 of 51
1	APPENDIX B: Category Graph of AEGL Values and Toxicity Data
2
3
^BGLN^Iles and aiarical Tcidcity Data
IVfercLiyX^pcr	I I
100.0
~
hLrrcn- Dsxrrfcrt
10.0
o
Airral - ISb Bfect
Airral - Dsccrrfcrt
Airral - DssOirg
o
0.1
0.0
0 eo 120 180	300 300 420 480
^	Mnutes
5

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MERCURY VAPOR	NAC/Proposed 2: 11/2009; Page 49 of 51
1 Data:
Category: 0 = No effect, 1 = Discomfort, 2 = Disabling, SL = Some Lethality, 3 = Lethal
Source
Species
mg/m3
Minutes
Category
Comments






NAC/AEGL-1

NR
10
AEGL

NAC/AEGL-1

NR
30
AEGL

NAC/AEGL-1

NR
60
AEGL

NAC/AEGL-1

NR
240
AEGL

NAC/AEGL-1

NR
480
AEGL







NAC/AEGL-2

3.1
10
AEGL

NAC/AEGL-2

2.1
30
AEGL

NAC/AEGL-2

1.7
60
AEGL

NAC/AEGL-2

0.67
240
AEGL

NAC/AEGL-2

0.33
480
AEGL







NAC/AEGL-3

16
10
AEGL

NAC/AEGL-3

11
30
AEGL

NAC/AEGL-3

8.9
60
AEGL

NAC/AEGL-3

2.2
240
AEGL

NAC/AEGL-3

2.2
480
AEGL







Nielsen-Kudsk 1965
human
0.05
0.10
0.20
0.35
5
5
5
5
0
0
0
0
metabolism study
Hurshetal. 1976
human
0.11
0.06
24
19
0
0
metabolism study
Hurshetal. 1980
human
0.06
0.40
21
12
0
0
metabolism study
Sandborgh-Englund
etal. 1998
human
0.40
15
0
metabolism study
Teisinger and Fiserova-
Bergerova 1965
human
0.10
420
0
metabolism study
Roelsetal. 1987
human
0.04
480
0
occupational monitoring
Morgan et al. 2002
rat
4.0
120
0
no fetotoxicity
Livardjani et al. 1991
rat
26.7
27.0
60
120
2
SL
no deaths;
mild lung lesions
62.5% mortality
Shimojo et al. 1996
mouse
9.8
60
1
increased protein -
broncho-alveolar fluid
Ashe et al. 1953
rabbit
28.7
60
2
mild organ damage
2
3

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MERCURY VAPOR
NAC/Proposed 2: 11/2009; Page 50 of 51
1
2
3
APPENDIX C: Derivation Summary
MERCURY VAPOR (CAS Reg. No. 7439-97-6)
AEGL-1 VALUES
10-min
30-min
1-h
4-h
8-h
Not
Recommended"
Not
Recommended
Not
Recommended
Not
Recommended
Not
Recommended
a AEGL-1 values are not recommended because mercury vapor has no odor or warning properties at concentrations
that may cause extensive tissue damage.
AEGL-2 VALUES
10-min
30-min
1-h
4-h
8-h
3.1 mg/m
2.1 mg/m
1.7 mg/m
0.67 mg/m
0.33mg/m
Key Reference: Morgan, D.L., S.M. Chanda, H.C. Price, R. Fernando, J. Liu, E. Brambila, R.W. O'Connor,
R.P. Beliles, and S. Barone, Jr. 2002. Disposition of inhaled mercury vapor in pregnant rats:
	maternal toxicity and effects on developmental outcome. Toxicol. Sci. 66:261-273.	
Test Species/Strain/Number: Rat (pregnant)/Long-Evans/25 (serial sacrifice)
Exposure Route/Concentrations/Durations: Inhalation/0, 1, 2, 4, 8 mg/m for 2 hours/day over GD 6-15
Effects: No fetotoxicity observed after 2 hour/day, 10-day exposure to 4 mg/m . Developmental effects
including resorptions, decreased litter size and decreased PND 1 weight at 8 mg/m3.	
Endpoint/Concentration/Rationale: 2-hour exposure to 4 mg/m was a NOAEL for the definition of the AEGL-2,
i.e., irreversible effects to the fetus.
Uncertainty Factors/Rationale:
Total uncertainty factor: 3 (based on weight of evidence approach)
Interspecies: 1, based on the following factors: the 4 mg/m3 value was a NOAEL for developmental effects
(below the definition of the AEGL-2), the exposures were repeated for 10 days, rodents have a higher
respiratory rate and cardiac output compared with humans (resulting in faster uptake), and human monitoring
studies show some effects at concentrations of 0.4 to 2 mg/m3 only with chronic exposure (AIHA 2002).
Intraspecies: 3, based on the following factors: the population of fetuses is considered a sensitive if not the
most sensitive population, the protective action of the placenta in sequestering mercury (Morgan et al. 2002),
and incidences of miscarriages and stillbirths were unaffected in women chronically exposed to 0.025 to 0.6
mg/m3, although congenital anomalies were statistically non-significantly increased (Elghany et al. 1997).
Modifying Factor: None applied
Animal to Human Dosimetric Adjustment: Not applicable
Time Scaling: Cn x t = k where n is 3 and 1 for shorter and longer exposure durations, respectively
(ten Berge et al. 1986; NRC 2001).	
Data Adequacy: The values are supported by occupational monitoring of pregnant women. A study of
pregnancies in women chronically exposed to mercury vapor (median air concentration, 0.09 mg/m3; range,
0.025-0.599 mg/m3) did not reveal a significant difference from controls regarding miscarriages and
stillbirths, although congenital anomalies were statistically non-significantly increased (Elghany et al. 1997).
8

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MERCURY VAPOR
NAC/Proposed 2: 11/2009; Page 51 of 51
AEGL-3 VALUES
10-min
30-min
1-h
4-h
8-h
16 mg/m
11 mg/m
8.9 mg/m
2.2 mg/m
2.2 mg/m
Key Reference: Livardjani, F. M. Ledig, P. Kopp, M. Dahlet, M. Leroy, and A. Jaeger. 1991. Lung and
blood superoxide dismutase activity in mercury vapor exposed rats: effect of Y-acctylcystcine
	treatment. Toxicology 66:289-295.	
Test Species/Strain/Number: Rat/Wistar/32
Exposure Route/Concentrations/Durations: Inhalation/0, 26.7 mg/m3 for 1 hour, 27.0 mg/m3 for 2 hours
Effects:
1-hour	exposure to 26.7 mg/m3: no deaths over 15-day observation period; lung lesions
2-hour	exposure to 27.0 mg/m3: death of 20/32 rats	
Endpoint/Concentration/Rationale: NOAEL for lethality: 26.7 mg/m for 1 hour meets the definition of the
AEGL-3
Uncertainty Factors/Rationale:
Total uncertainty factor: 3 (an interspecies uncertainty factor of 1 and an intraspecies uncertainty factor of 3),
based on (1) faster uptake in rodents compared with humans, and (2) incompatibility of derived values with
monitoring and accidental exposure data if larger uncertainty factors are used.	
Modifying Factor: None applied
Animal to Human Dosimetric Adjustment: Not applicable
Time Scaling: Cn x t = k where n is 3 and 1 for shorter and longer exposure durations, respectively (ten Berge et
al. 1986; NRC 2001). The 8-hour value was set equal to the 4-hour value.	
Data Adequacy: Although the data base is not large and data on humans consist of clinical studies and estimated
concentrations and exposure durations from accidental exposures, a repeat-dose study with the rat (Beliles
et al. 1968) supports the values. In that study, male and female Wistar-derived rats exposed to 17.2 mg/m3
for 2 hours/day 5 days/week for 22 exposures showed behavioral changes that were reversible following a
recovery period. Organ lesions consisted of lymphocytic perivascular cuffing of the medulla oblongata.

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