COMMENTARY || Why it’s so difficult to prove what causes cancer
U of A epidemiology expert explains the real-world complexity that is difficult to replicate in scientific studies.
By WARREN KINDZIERSKI
Last month, the Quebec court authorized a class-action suit against two brands of baby powder that alleges that regular use of talc powder by women in their genital area is linked to a higher risk of ovarian cancer. Part of the allegations relate to claims that an ovarian cancer risk from powdered talc use is demonstrated by nearly four decades of scientific studies. Cosmetic talc has certainly been the subject of much scientific debate, study and, increasingly, legal challenge.
However, the cosmetic talc-ovarian cancer link is commonly misunderstood. Published biomedical studies cover both sides, suggesting a talc-ovarian cancer link and showing no link. Even today in prominent journals, letters to the editor—penned by scientists—rage back and forth, defending their studies or attacking the other side’s studies.
Now this is civilized, real science.
This bouncing back and forth of positive versus negative effects between talc and ovarian cancer is referred to as “vibration of effects” by John Iaonnidis, a professor of medicine and of health research and policy at Stanford University. Studies vary depending on how they are done. Why is this? Well, getting scientists to agree on important things like methods, what data to use and how to analyze and interpret effects from subtle human exposures is next to impossible. It would be no problem if one were studying cancer risks in populations receiving large exposures over long durations; but such situations are non-existent.
The truth is that the ability of any biomedical method, epidemiology included, to discriminate cancer risks in people from small exposures to a physical or chemical agent does not exist.
Most cancers are caused by a number of factors. As a result, establishing cancer causation is complex—unless a particular risk factor is overwhelming. Epidemiology studies cannot and do not realistically replicate this complexity, at least not very well. That is why the U.S. National Institutes of Health and the National Cancer Institute list a number of key risk factors for ovarian cancer and talc is not one of them.
The institute states that it is not clear whether talc affects ovarian cancer risk. An expert U.S. cosmetic-ingredient review panel assessed the safety of cosmetic talc in 2015. It thoroughly analyzed numerous studies investigating whether or not a relationship exists between cosmetic use of talc in the perineal area and ovarian cancer. The panel determined that these studies do not support a causal link. They also agreed that there is no known physiological mechanism by which talc can plausibly migrate from the perineum to the ovaries. The news coverage of the lawsuit has been silent on that evidence.
Part of the public’s misunderstanding about talc comes from scientists offering opinions about cancer from small exposures. Too many scientists use weasel words to stretch facts: “This could,” “can,” “may,” “might,” “probably,” “likely” cause cancer. Flimsy so-called evidence from their studies that suffer from vibration of effects and their speculations are voraciously inhaled by naïve journalists. Stretched facts miraculously get reported as facts to the public—or worse, misused for litigation purposes.
The woman’s bathroom is a chemical exposure chamber with literally dozens of cosmetic products used at various times. Both skin contact and inhalation regularly occur with grooming products. However, repeated uses of small amounts of cosmetic talc or any other cosmetic product do not amount to overwhelming exposures despite the claims of some scientists and media. Overwhelming exposures—the ones that cause effects—are those that occur with laboratory rats and mice. Underwhelming exposures are what occur to people in the real world.
It is highly speculative that repeated use of small amounts of cosmetic talc is a definitive cause of ovarian cancer. It is not a definitive cause; it is only suggestive. Prominent organizations such as the U.S. National Cancer Institute and expert panels should make clear statements about such cancer risks, but they do not. Selective methods in epidemiology studies, speculation by scientists and inaccurate reporting by news media are ingredients used to transform weak suggestive evidence from underwhelming cosmetic talc exposure into something that is mistakenly claimed to be harmful for the public.
And that is why we end up with class action suits against cosmetic companies.
This article originally appeared June 22 in the Financial Post.