I HAVE A QUESTION REGARDING THE United States of America Environmental Protection Agency Recommendations, From Federal Register, Vol., 56, No. 20, 30 January 1991, p3573, Silver: 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: Lifetime 25 g* ---------------------- = 978ug/day 25,5 50 days 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 14 ug Ag/Kg/day Cosmetic RfD = ---------------------- 4.7 ug Ag/kg/day 3 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 4.7 ug Ag/kg/day * 70 kg Cosmetic DWEL = --------------------------------------- 21/day = 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 (0.0047 mg/kg/day) (70kg) -0.08 mg/day SMCL = ------------------------------------------------------------ 2l/day = 0.12 mg/l (rounded to 0.1 mg/l or 100 ug /l)
1.) What does MCL, SMCL, DWEL and Rfd STAND FOR? 2.) I understand all of the formulas until we get to step 2, "Cosmetic RfD Derivation" and step 3, "Cosmetic DWEL derivation." Why is it divided by 21 days?
