Last week I discussed the fact that for most chemicals
(i.e., those NOT identified as carcinogens) quantitative risk at any
exposure including the exposure limit is NOT estimated as part of any
documentation describing the toxicity of those chemicals. In the case of identified carcinogens, however, a
quantitative level of risk IS estimated such that some finite risk is assigned
for every exposure down to zero. The “allowable”
exposure for members of the general public is often set at around 1 in
100,000 or 1 in 1,000,000 extra or additional risk of getting cancer during a
lifetime of exposure at this level. The “acceptable” level set for occupational
exposures to carcinogens is typically set at exposure limits which render an estimated risk around 1 in 1,000 extra or additional
risk of cancer. The natural questions are: Where did the one in 1,000
come from? Why is there such a large difference
between workers and the general public?
Shouldn't we be just as protective of workers as we are members of the
general public? The answers are
interesting and the subject of this week’s blog.
The conventional wisdom is that the 1 in 1,000 number came out of the US Supreme Court decision on the carcinogenic risk assessment and statutory (OSHA) exposure limit for benzene. Considering a 1 in a billion risk Justice
Steven’s wrote that it “clearly could not be considered significant”. One the other hand, he offered that a one in
a thousand risk of dying from inhaling vapors containing benzene “might well”
be considered significant (emphasis added).
Hence the 1 in a 1,000 level of putative risk became the de facto
standard for occupational carcinogens.
It turns out that the level of actuarial risk of death from occupational
or on-the-job injury in the United States is somewhat higher. See the table below which is perhaps 10 year
old data from the US Bureau of Labor Statistics. These are the actuarial working lifetime risk of death per thousand in various job classifications:
- All private
industry -- 2.1
- Manufacturing -- 1.6
- Mining -- 10.8
- Construction -- 5.9
- Finance -- 0.6
- Commercial
Fishing -- 61
Readers who have watched "The Deadliest Catch" on TV will appreciate the last entry.
From my perspective, a putative risk at 1 in 1,000 for cancer using a linear dose response model described in my previous blogs fits
in reasonably well with these actuarial risks. Other reason that folks have given to
rationalize a higher risk for workers than the general public includes the fact
that workers are normally healthier than individuals in the general populations
which includes very old, very young and some critically and chronically
ill individuals. Another factor
that is not typically stated but perhaps implied is that workers are deriving
some financial benefit from their exposure and should thus bear some higher level risk. Whatever the reason, the difference appears
to be well inculcated as a societal norm at least in the US.
The "gut check" for me is whether I would allow or advise my son or daughter to be occupationally exposed at any such an exposure limit - that is, an exposure that would present a putative risk of 1 in 1,000. The answer to that question depends on the data and model used to make that determination. Given a reasonable data set, model and a clear toxicological rationale by the experts, I would. The problem is that these elements are often not present - which will be a topic included in next week's blog.
Next week's blog will be "Uncertainty is the Bane of the Risk Assessment Process".
