Are PFAS 'Forever Chemicals' Causing Thyroid Cancer?
Looking at the science behind the headlines
PFAS - the collective acronym for per- and polyfluoroalkyl substances - are something that we love to hate. They are a class of chemicals used in a lot of industrial applications, such as clothing and plastics, because they have a bunch of useful qualities. They are also increasingly the focus of epidemiological research, because there are concerns about whether PFAS could be causing a range of human health issues.
In addition, PFAS are somewhat worrying because they take a long time to break down in the environment. This has lead the media to dub them ‘forever chemicals’. And, according to recent headlines, these forever chemicals might be giving people thyroid cancer.
Fortunately for all of us, the link between PFAS and thyroid cancer is, at best, incredibly weak. While it’s not impossible that PFAS substances might be causing issues with your thyroid, the evidence thus far is unconvincing and probably not worth worrying about.
The Science
The headlines that have sprung up everywhere about PFAS are based on a new epidemiological investigation published recently in the journal eBioMedicine, an offshoot of the prestigious journal the Lancet. In this study, researchers took a group of people with thyroid cancer who’d had a range of blood tests before their diagnosis, and compared them to some matched controls who had blood tests but no thyroid cancer.
After controlling for a number of factors, the researchers found that for one PFAS, specifically n-PFOS, there was a modest correlation with thyroid cancer. The corrected odds ratio for this association was 1.56, which means that the odds of having thyroid cancer were increased by 56% for people with much higher levels of this substance in their blood.
This finding has led to the many headlines about PFAS causing cancer, but I think it’s worth putting the results into proper context before getting terrified about this all-new cancer risk. Firstly, this study was genuinely tiny - the authors managed to collect data on just 88 people with thyroid cancer and 88 controls. That’s really not enough to do any sort of robust epidemiological analysis - you can generate interesting correlations, but what those correlations mean is anyone’s guess.
But even more importantly, you could easily argue that the results of this study show that most PFAS aren’t associated with thyroid cancer. The authors looked at 18 separate chemicals in the participants’ blood. Of those 18, 10 were undetectable - i.e. there wasn’t any in the blood samples - and therefore excluded from the analysis. Of the remaining 8, there was no strong association for 7 of these, but for the final substance there was a statistically significant correlation.
Now, the authors did in fact control for these multiple comparisons, so there are no serious methodological concerns here, but equally there’s just not much you can reasonably gather from finding a single correlation among a vast ocean of possibilities. You can illustrate this quite nicely by pointing out that the authors tested two isomers of PFOS, and only one was significantly associated with cancer. It could be causal, but there’s really no way to know from a tiny study like this.
Bottom Line
I have nothing bad to say about the authors of this paper, because they ran what is in most ways a pretty decent scientific analysis. The issue really lies with the media, and more importantly how we consume science as a society.
The findings from this new paper are extremely boring, everyday epidemiology. Maybe there’s a correlation there, but you’d need to investigate this in MUCH bigger samples, with more controls, and better data, to understand what that correlation means. It’s not headline news - it’s not even really news. The paper itself is just another note for people who have access to bigger/better datasets that they should look for similar correlations to see what it might mean.
But that’s not how we see scientific research. We can’t accept that most science is just…not that interesting to us on a personal level. And so we get studies like these blown out of all proportion, because “Forever chemicals are giving you cancer” makes a much better headline than “New study implies possible risk - needs more investigation to know more”.
It’s possible that PFAS are giving people cancer. Frankly, given that the category of PFAS includes literally thousands of chemicals, it would not be entirely surprising to find at least one carcinogen in the bunch. And governments around the world are already investigating how likely it is that these chemicals could be harming us.
Equally, it’s not unlikely that this correlation is entirely spurious. There’s a world of possibilities here, including PFAS both causing and preventing cancer, and we won’t know the answer until we have better research.
Not as fun a story as you might’ve read in the headlines, but it has the advantage of being a lot more true.
Good article - of course if a study looks at a lot of things, by chance alone they are likely to find something. Is it worthwhile to do additional studies? Yes, this is potentially serious, but at this point, as you've said, this is very weak evidence.