IARC classifies thiourea as a 2B, that is, “possibly carcinogenic” to human beings. The whole group 2 category of IARC, “possible” or “probable,” rests without proof as to human carcinogenicity. Most of my dealings with the public reveal that with the word “possible,” they have presumed proof or evidence of human carcinogenicity when in fact the opposite exists. Since com mencing production of thiourea in 1938, no public health official, or occupational warning, or requested case study, has raised the issue of its human carcinogenic potential. Even its clinical use, as an antithyroid agent, over 40 years ago, did not raise the issue. In other words, there is no data (i.e. evidence) to implicate thiourea as a human carcinogenic agent.
Thiourea has induced tumors in rodents in some, but not all, experiments. Thus, the understanding of “animal positive”. The irony is that one experiment suggested that thiourea possessed an anti-carcinogenic property.
Every chemical, except caprolactam, on IARC’s list is a suspected carcinogen and that is the reason for its being there.
In conclusion, there is no evidence to convict thiourea as a human carcinogen. Joseph Ruddick, PhD Environmental Health Centre Health Protection Branch Health and Welfare Canada Ottawa, Ont.
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