ACN 074 448 829
On May 22, 1989, EPA proposed to delete the current MCL for silver (Ag), because the only potential adverse effect from exposure to silver in drinking water is argyria (a discoloration of the skin). EPA considers argyria a cosmetic effect since it does not impair body function. Also, silver is seldom found at significant levels in water supplies and drinking water has never been identified as the cause of argyria in the United States. While the health effects of silver may only be cosmetic, many home water treatment devices use silver as an antibacterial agent. These devices may present a potential contamination threat when used in a system. Therefore, EPA proposed (54 FR 22062) an SMCL for silver at 0.09 mg/l based on the skin cosmetic effect called argyria. EPA also asked the public to comment on the selection of an uncertainty factor (UF) in the alternate calculation of SMCL, assuming an oral absorption factor of 4 percent.
Public Comments
A total of six individuals or organisations provided comments in response to the proposed rule regarding silver. All commentaries agreed that the MCL for silver (0.05 mg/l) should be deleted. Several commentaries agreed with EPA's proposal of an SMCL for silver. Other commentaries disagreed with EPA's proposal, citing the following reasons for support:
In response to a specific question posed in the Federal Register Notice on the selection of a UF for the alternate calculations of the SMCL, different opinions were expressed. Several commentaries suggested using an uncertainty factor of 2 in support of 25 mg/l, while one proposed to keep the SMCL at the current MCL of 0.05 mg/l.
EPA Response
EPA has decided a SMCL of 0.1 mg/l is needed to protect the general public from the cosmetic effect of argyria (from lifetime exposure to silver). While the health effects of silver may only be cosmetic, many home water treatment devices use silver as an antibacterial agent, thus presenting a potential contamination threat when such devices are used in a system. Therefore, EPA has decided to keep the SMCL at 0.1 mg/l to protect the welfare of the general public from the cosmetic effect of argyria.
EPA is proposing to use the same data base as before to calculate the SMCL for silver. Assuming 1g of silver by I.V. will cause argyria in the most sensitive individuals (Gaul and Staud, Am. Med. Assoc. 104:1387-1390, 1935; Hill and Pillsbury, 1939) and assuming an oral absorption rate of 4 percent (Furchner et al., Health Physics 15:505-514, 1968), a lifetime exposure of 70 years, and a UF of 3, an SMCL of 0.1 mg/l is derived. For more detail, see the following derivation of SMCL.
Derivation of SMCL for Silver
The cosmetic DWEL is calculated assuming 1g of silver administered I.V. will produce a mild argyria in the most sensitive individuals (Gaul and Staud, 1935; Hill and Pillsbury, 1939). Assuming 4 percent absorption of silver (Furchner et al., 1968) following oral exposure, the I.V. dose corresponds to an oral dose of 25g (1g/0.04 = 25g). This dose is then averaged over a lifetime, assumed to be 70 years:
Based on an adult body weight of 70Kg, this corresponds to 14ug Kg/day (978 ug /day / 70 Kg=14 ug /Kg/day).
Step 1 - Cosmetic RfD Derivation
Where:
14ug Ag/kg/day = Lowest Observed Cosmetic Effect Level based on argyria.
3 = uncertainty factor.
An uncertainty factor of 3 was applied for the following reasons. First, a 10-fold uncertainty factor is usually applied to human data to account for interspecies variability. However, since this derivation has already included sensitive individuals, a 10-fold uncertainty factor is not warranted. Second, an uncertainty factor less than 10 (i.e. 3) is sufficiently protective since the estimated dose causing argyria within one to three years is being apportioned over a lifetime. Finally, the effect is based on argyria, which is considered a cosmetic effect, and not an adverse health effect.
Step 2 - Cosmetic DWEL Derivation
= 164 ug /1 (rounded to 200ug/1)
where:
4.7 ug Ag/kg/day = cosmetic RfD.
70 kg = assumed body weight of an adult.
2l/day = assumed water consumption by an adult.
The cosmetic DWEL is derived on the assumption that 100 percent of the silver intake comes from drinking water. As estimated by the World Health Organisation (WHO, 1980), the upper bound of intake level of silver from food is 20 to 80 ug per day; from air it is essentially negligible. Therefore, the SMCL for the cosmetic effect of silver can be calculated by subtracting the amount obtained in food.
Step 3 - SMCL
= 0.12 mg/l (rounded to 0.1 mg/l or 100 ug /l)
