PBDEs accumulate as you travel up the food web - eventually reaching the apex predator: us.

Today I’m going to tell you about a chemical that might cause cancer – one I suspect you haven’t heard of before.

These types of stories usually end with a call for regulation – to ban said chemical or substance – but in this case that has already happened. This new carcinogen I’m telling you about is actually an old chemical. And it has not been manufactured or legally imported in the US since 2013. So… why bother?

Well, because in this case, the chemical – or really group of chemicals – polybrominated diphenyl ethers – hereafter PBDEs – are still around… in our soil… in our food… and in our blood.

PBDEs are a group of compounds that confer flame-retardant properties to plastics, and they were used extensively in the latter part of the 20th century in electronic enclosures, business equipment, and foam cushioning in upholstery.

But there was a problem. They don’t chemically bond to plastics – they are just sort of mixed in- which means they can leech out. They are hydrophobic, meaning they don’t get washed out of soil, and, when ingested or inhaled by humans, dissolve in our fat stores making it difficult for our normal excretory systems to excrete them.

PBDEs biomagnify. Small animals can take them up from contaminated soil or water, and those animals are eaten by larger animals which accumulate higher concentrations of the chemicals. This biomagnification increases as you move up the food web until you get to an apex predator – like you and me.

And this is true of lots of chemicals of course. The concern arises when these chemicals are toxic. To date, the toxicity data for PBDEs was pretty limited. There were some animal studies where rats were exposed to extremely high doses and they developed liver lesions – but I am always very wary of extrapolating high-dose rat toxicity studies to humans. There was also some suggestion that the chemicals could be endocrine disruptors – affecting breast and thyroid tissue.

What about cancer? Well, in 2016, the International Agency for Research on Cancer concluded there was “inadequate evidence in humans for the carcinogencity of…” PBDEs.

In the same report, though, they suggested PBDEs are “probably carcinogenic to humans” based on mechanistic studies.

In other words, we can’t prove they’re cancerous…but come on, they probably are.

But finally we have some evidence that really pushes us towards the carcinogenic conclusion, in the form of this study, appearing in JAMA Network open which is a nice bit of epidemiology leveraging the population-based National Health and Nutrition Examination Survey or NHANES.

Researchers measured PBDE levels in blood samples from 1100 people enrolled in NHANES in 2003 and 2004 and linked them to death records collected over the next 20 years or so.

The first thing to note – researchers were able to measure PBDEs in the blood samples. They were in there. They were detectable. And they were variable. Dividing the 1100 participants into low, medium, and high PBDE tertiles, you can see a nearly ten-fold difference across the population.

And, importantly, there were not many baseline variables that correlated with PBDE level. People in the highest group were a bit younger, but had a fairly similar sex distribution, race, ethnicity, education, income, physical activity, smoking status, and BMI. 

This is not a randomized trial, of course – but at least based on this data exposure levels do seem fairly random. Which is what you would expect from an environmental toxin that percolates up through the food chain – they are often somewhat indiscriminate.

This similarity in baseline characteristics between people with low or high blood levels of PBDE also allows us to make some stronger inferences about the observed outcomes. So let’s take a look at them.

After adjustment for baseline factors, individuals in the highest PBDE group had a 43% higher rate of death from any cause over the follow-up period. This was not enough to achieve statistical significance, but it was close.

But the key finding is deaths due to cancer. After adjustment, cancer deaths occurred 4 times as frequently among those in the high PBDE group. And that is a statistically significant difference.

To be fair, cancer deaths were rare in this cohort. The vast majority of people did not die of anything during the follow-up period regardless of PBDE level. But the data is strongly suggestive of the carcinogenicity of these chemicals.

I should also point out that the researchers are linking the PBDE level at a single time point to all these future events. If PBDE levels remain relatively stable within an individual over time, that’s fine, but if they tend to vary with intake of different foods for example, this would not be captured, and would actually lead to an underestimation of the cancer risk.

The researchers also didn’t have granular enough data to determine the type of cancer, but they do show that rates are similar between men and women, which might point away from the more sex-specific cancer etiologies. Clearly some more work is needed.

Of course, I started this piece by telling you that these chemicals are already pretty much banned in the US. What are we supposed to do about these findings?  Well – studies have examined the primary ongoing sources of PDBE in our environment and it seems like most of our exposure will be coming from the food we eat due to that biomagnification thing – high-fat fish, meat and dairy products, and fish oil supplements. It may be worth some investigation into the relative adulteration of these products with this new, old carcinogen.

A version of this commentary first appeared on Medscape.com.