The Ministry of the Environment recently proposed new standards for “how clean is clean” at contaminated sites in Ontario. The justification document is nearly six hundred pages long, but seems admirably transparent and quite well justified by the supporting science.
One thing that concerned me, though, was that the proposed criteria are all based on the assumed life span of a “composite adult” of 76 years. In contrast, female life expectancy in Canada is considerably longer. Even at birth, Canadian women can expect to be 83, and the older we get, the longer we are likely to live. The “composite” adult therefore does not seem to be sufficiently protective for women. Is this another example of the old tendency to consider only men as normal?
The eminent toxicologist, Dr. Ron Brecher, quickly pointed out the error of my ways:
For most RA parameters, a higher value is more protective (e.g., breathing rate, exposure duration, soil ingestion rate, drinking water ingestion rate, amount of skin exposed, etc.). Not so for lifespan.
The length of a lifetime is used only for amortizing cancer risk for less-than-lifetime exposure. The shorter the amortization period, the GREATER the average daily dose. Therefore, using 76 years is MORE conservative than using a higher value for less-than-lifetime exposure. For lifetime exposure, the exposure period is equal to the amortization period, so the choice of lifetime doesn’t matter. You can verify this yourself:
Suppose a person gets a dose of 70 mg/kg/day for 10 years. The lifetime average daily dose for a man who lives 76 years would be 10/76 x 70 mg/kg/day. The lifetime average daily dose for a woman who lives 83 years would be 10/83 x 70 mg/kg/day. 10/76 (about 0.13) is greater than 10/80 (about 0.12)
This shows that the choice of lifespan makes very little difference in the dose calculation (1% in the 76- vs 83-year example), with the SHORTER lifespan leading to a slightly higher estimate of lifetime average daily dose (hence risk). So the shorter lifespan is more conservative.
The broader underlying concern in your email seems to be about how gender differences might come into play, and whether these criteria protect men and women equally (or perhaps smaller and larger adults equally). Given that, I thought I should also talk about the selection of body weight:
Body weight is like lifespan in that a LOWER body weight is more conservative than a higher one, since dose is expressed on a body weight basis. So a 65 kg woman would have a HIGHER dose per unit body weight than a 75 kg man (I don’t know where the 75 kg men all are; my weight is around 90 kg and I’m reasonably fit and, I think, pretty average in terms of physique). Usually, RAs would employ the average body weight of males and females (I think it’s around 72 kg, but didn’t have time to verify this).
You may have already noticed that the MOE does consider an adult female in developing new standards (body weight of 63.1 kg; p. 30 of new rationale document).
Thank you, Ron! One less thing to worry about….