revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 253
2. HEALTH EFFECTS
The average values for A B for all studies ranged from 0.047 to 0.061 (the values for women and men
reported in Sherlock et al. [1984]). The data suggest that the average value of A B for women may be lower
than that for men, and they further suggest that 0.05 may be appropriate for modeling intake in a group of
women (Sherlock et al. 1984). Based on these studies, the best estimate of A B based on the available data is
0.05. Use of a higher value (i.e., 0.06 instead of 0.05) for this parameter would result in a lower MRL, but
the sensitive populations are pregnant women and developing fetuses, making the 0.5 value more
appropriate for the Seychelles study population.
Elimination constant (b). Reported clearance half-times for methylmercury from blood or hair range from
48 to 65 days (Table 2-5). The average elimination constant based on the six studies listed in Table 2-5 is
0.014. The average of the individual values for b reported for 20 volunteers ingesting 42–233 µg Hg/day in
fish for 3 months (Sherlock et al. 1984) is also 0.014. Use of the value 0.014 for this parameter, rather than
0.01 (as used by WHO 1990), results in a higher MRL.
Volume of blood in body (V), and body weight. Blood volume is assumed to be 7% of body weight, with an
increase to about 9% during pregnancy (Best 1961). Data for the body weight of the Seychelles Islands
women were not found. Assuming an average body weight of 60 kg for women, the blood volume is 4.2 L
(60 kg x 0.07 L/kg). The 9% of body weight value is not used because it is not representative of the blood
volume throughout pregnancy. Blood volume does not begin to increase significantly from the 7% prepregnancy
level until around the 27th week of pregnancy. It then sharply rises until week 40 or parturition
(Guyton 1996). To use the 9% value would, therefore, be representative of the blood volume late in
pregnancy (i.e., mid- to late- third trimester), but not throughout most of pregnancy. In contrast, the hair
mercury level to which it is compared represents an average value throughout pregnancy. The use of the 9%
value would result in a higher MRL, and is not considered appropriate in this instance.
Calculation of Exposure Dose
The concentration of mercury in hair is assumed to be 250 times the concentration in blood. ATSDR's
peer-reviewed, published guidance for MRL derivation (Chou et al. 1998) calls for the use of the highest
value at which no adverse effects were observed in the critical study. Using, therefore, the 15.3 ppm mean
maternal hair (taken at parturition) value from the highest exposure group (range, 12–26.7 ppm) in the
Seychellois test population as a NOAEL for the 66-month Seychelles testing (Davidson et al. 1998), the
corresponding methylmercury concentration in blood would be: 1/250 x 15.3 µg/g x 1 mg/1,000 µg x
1,000 g/L = 0.061 mg/L.