IARC classifies carbon black and titanium dioxide as “possibly
carcinogenic to humans,” based on sufficient evidence in laboratory
studies but for which studies of people exposed have not been conducted. The IARC working group declined to limit the classification
of titanium dioxide to ultrafine, implicitly rejecting the lung overload hypothesis.
Mechanistic hypotheses that minimize human risk compared with
laboratory animals may legitimately be applied in the exposure-response phase of risk assessment but should not be used to preclude
a range of potency estimates. However, it’s legitimate to question
where these hypotheses are coming from and whether public health
agencies are funding research that explores competing hypotheses,
which may lead to increased estimates of low-dose potency.
Meanwhile, practitioners should take to heart the lesson that certain materials thought to be inert or “safe” have turned out to have
considerable toxic potential, and even considerable potency, when
subjected to appropriate bioassay testing. The reference concentrations for respirable PSLT-nuisance particulate should be closer to
100 µg/m3 than to the 5,000 µg/m3 enforced by OSHA.
Franklin Mirer, PhD, CIH, is a professor in the Environmental and Occupational Health
Your Laboratory Guide to Cleaner, Healthier Air
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