Adequate Uncertainty Factor for Exposure Limit

Lawrence Segal am879 at FreeNet.Carleton.CA
Thu Oct 19 16:41:29 EST 1995

Brian C. Lee (bclee at writes:
> A liver carcinoma is considered benign?  What kind of cell type was it?  
> 25-fold sounds pretty low.  Was there any human exposure data to show 
> a non-carcinogenic exposure level?

Sorry, I was mistaken in my initial post.  The 75 ppm LOEL in males rats
was for adenomas; carcinomas and adenomas were elevated at 375 ppm.  The
current occupational exposure level is 10 ppm.  The proposal was to reduce
that to 3 ppm in light of the new data, but that seems inadequate given the
LOEL for liver adenoma in male rats.

> : I am currently reviewing information concerning a proposed occupational
> : exposure limit for an industrial chemical.  The substance is a
> : non-genotoxic carcinogen, which has been tested in both mouse and rat
> : chronic/onco studies.  In males of both species, statistically significant
> : increases in liver adenomas/carcinomas were observed at exposure levels of
> : 75 ppm.  The company that manufactures the substance is proposing a TWA
> : occupational exposure limit of 3 ppm.  This provides a 25-fold
> : uncertainty factor applied to the LOEL in the rodent studies.  The company
> : considers this to be an adequate safety factor, because the liver tumours
> : were benign, there was no significant increase in mortality at 75 ppm
> : relative to control, and the latency period was long.  

Lawrence Segal
Ottawa, Ontario, CANADA

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