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The International Agency for Research on Cancer (IARC), part of the World Health Organization (WHO), has reclassified talc from “possibly” to “probably” carcinogenic and reclassified acrylonitrile from “possibly carcinogenic” to “carcinogenic” to humans.
At a meeting last month in Lyon, France, 29 scientists from 13 countries that make up the IARC working group upgraded talc to “probably carcinogenic to humans” (Group 2A) based on a combination of “limited” evidence that talc causes ovarian cancer in humans, “sufficient” evidence for cancer in experimental animals, and “strong” mechanistic evidence in human primary cells and experimental systems.
The upgrade supersedes the 2010 classification of talc not containing asbestos or asbestiform fibers and perineal use of talc-based body powder as “possibly” carcinogenic (Group 2B).
Talc containing asbestos category was not re-evaluated and retains its carcinogenic (Group 1) classification.
Since 2010, “more consistent positive associations” for ever-use versus never-use of talc have been reported in pooled cohort studies and case-control studies, including evidence of an exposure-response relationship with frequency or duration of use, the working group wrote in a report in Lancet Oncology, published online earlier this month.
However, the working group said it couldn’t rule out bias from differential exposure misclassification or confounding by asbestos contamination of talc.
Kevin McConway, PhD, with the Open University, Milton Keynes, United Kingdom, helped clarify what the IARC classifications mean.
In the case of talc, “there isn’t a smoking gun” that its use causes any increased cancer risk, explained McConway, in a statement from the UK nonprofit organization, Science Media Centre.
In other words, the experts “aren’t saying that talc cannot cause ovarian cancer in humans,” McConway said. “But they can’t at all be sure that in fact it does cause an increase in risk, at the levels of exposure that might happen in normal use of talc or indeed in any other circumstances of use.”
McConway noted that the change from “possibly” to “probably” carcinogenic “seems mainly to be because more evidence from observational studies has been obtained since the previous IARC publication. But that extra evidence wasn’t considered enough to move talc to the IARC’s highest classification of cancer hazard, which would drop the ‘probably’ and simply say that that the substance can cause cancer.”
Overall, “there’s still a lot of uncertainty here,” McConway concluded. “So really we’re in the rather unsatisfactory position that IARC [experts] say that using talc can probably cause cancer in humans under certain circumstances that they don’t define.”
Paul Pharoah, PhD, DPH, professor of Cancer Epidemiology, Cedars-Sinai Medical Center, London, United Kingdom, found the evidence for the carcinogenicity of talc in humas “extremely weak.”
“Many observational studies have shown an association between talcum powder use in the genital area and ovarian cancer risk,” Pharoah commented via Science Media Centre. “However, such a correlation may easily be explained by other factors that are associated with both talc and with ovarian cancer (so-called confounders). Moreover, the reported associations with different types of ovarian cancer — which are known to be very different in their risk factors and underlying biology — are not consistent with the observed association being causal.”
Overall, “my interpretation of all the evidence is that women who have used genital talc in the past should not worry about their future risk of ovarian or other cancers,” Pharoah said.
The working group’s upgrade of acrylonitrile to “carcinogenic” to humans (Group 1) was based on “sufficient” evidence that it can cause lung cancer, as well as “limited” evidence for bladder cancer.
The evidence stems mainly from studies in workers producing or using acrylonitrile. There was sufficient evidence for cancer in experimental animals and strong mechanistic evidence of key characteristics of carcinogens in experimental systems, the working group said.
Acrylonitrile is used in many industries to make certain plastics, rubbers, and chemicals, and in the past, as a pesticide. It’s also present in cigarette smoke and air pollution.
Members of the IARC working group have declared no competing interests. McConway is a trustee of the Science Media Centre and a member of its advisory committee. Pharoah has received fees from law firms representing Johnson and Johnson for expert witness testimony in ongoing litigation relating to talc use and ovarian cancer.
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