Flame retardants – added to furniture and other products as a safety measure – are in us, as well.

That is ever more worrisome, as links to possible health effects, including developmental delays in children, are discovered.

In 2014, new flammability standards went into effect in California. Supporters said those standards would affect the nationwide market, ultimately resulting in less use of the chemicals. Many have argued that the chemicals have not been proven to prevent fires effectively, anyway.

Some flame retardants already had been removed from the market. But researchers at Duke University and their colleagues recently completed a study showing that levels of newer flame retardants — replacements for the older chemicals — in humans have increased dramatically since 2002.

"One thing we find fairly consistently with flame retardants is that more frequent hand-washing may reduce levels of exposure."
— Kate Hoffman

Lead researcher Kate Hoffman, a visiting assistant professor at Duke's Nicholas School of the Environment, recently spoke to us about the study, which was published online last month in the journal Environmental Science and Technology Letters.

Duke researchers have been studying flame retardants for quite a while. Can you summarize some of the important findings so far?
What we've been focusing on the last few years is understanding what flame retardants people are exposed to, and how they're exposed to these chemicals. That's not something we've reached the finish line on, partly because new flame retardants keep coming out. That's how we've gotten to the class we're looking at now — organophosphate flame retardants. These are detected quite frequently in the foam used in furniture and baby products, including car seats and play mats.

Although these chemicals are used as flame retardants, some also are used in other applications. For instance, one use is as a plasticizer in nail polish. What we have learned so far is that these chemicals are indeed in a lot of products in people's homes, and that people are exposed to them. Research suggests that more than 90 percent of Americans have these chemicals in their bodies.

What led you to this new study on how levels in humans have changed?
We know that beginning in the 2000s, these organophosphate compounds may have started to be more widely used. Older classes of flame retardants were phased out in the early 2000s because of concerns that they were accumulating in the body and that there may be potential health risks. So industry moved toward these compounds as replacements. We wanted to see how people's levels of exposure to those new chemicals might be changing.

What did the study show?
Our work suggested that exposure to these replacement compounds definitely seemed to be increasing. We saw the biggest increases for a specific compound — commonly referred to as chlorinated tris, or TDCIPP, although its official name is tris(1,3-dichloro-2-propyl) phosphate — a chemical that was used in children's pajamas in the 1970s. Because of health concerns, it was phased out of pajamas, but not out of other applications.

Measurements of a breakdown product of chlorinated tris in urine samples from adults were about 15 times higher in 2015 than in 2002. That was a pretty dramatic increase. We also saw increases – but not as dramatic — in another commonly used flame retardant.

The levels of chlorinated tris in children rose fourfold between 2010 and 2015. We also found that children have higher levels of exposure than adults — something we've shown in several studies now. Some of the highest levels we have seen have been in very young infants. We're not sure why that is, although what we saw as the strongest predictor of higher levels was the number of products in the home: car seats, swings, play mats, nursing pillows, and many other padded products.

Overall, we don't know why levels in people are increasing. But certainly, it makes sense because data would suggest its use is increasing in furniture during this same time period. We know people are exposed through dust and air in their homes. This is because the flame retardants don't stay in the products that they're used in. Over time, they migrate into the home environment. If there are higher levels of a compound in your home's dust and air, we would expect your levels of exposure to be higher.

It's important to note that the study was based on more than 800 urine samples, aggregated from 14 earlier studies. It was a unique coming together of a lot of research to answer one question. Designing a single study that would last 13 years would be difficult and costly.

Why is the increase you found a concern? Have health professionals identified a clear link between exposure to flame retardants and specific diseases?
We're just beginning to understand how these chemicals might affect human health. So far, there are very few studies looking at that. But chlorinated tris is considered a human carcinogen by the state of California. Data suggests that exposures to other organophosphate flame retardants could affect thyroid hormone regulation and male fertility. Toxicology studies also suggest these compounds may be related to obesity and may affect children's brain development.

So right now, we're working on assessing the potential human health impacts. It seems pretty clear at this point that the vast majority of the population is exposed to these chemicals on a daily basis, and that exposure has increased in recent years. But we don't know if it translates into a health effect.

We are especially interested in stage of human development that might be more vulnerable to chemical exposures. As I mentioned, we think that infants may have higher levels of exposure, and that's a particularly important time for human brain and immune system development. So we're interested in looking at those groups first to see how these chemicals might impact long-term health. Here at Duke, we are just starting to look at whether higher levels of exposure during pregnancy affects children's later growth and development.

Is there any way people can reduce their exposure?
This is difficult, because flame retardants are in so many products. But one thing we find fairly consistently with flame retardants is that more frequent hand-washing may reduce levels of exposure, since flame retardants often wind up in household dust, which we then touch. It's certainly a good public health practice anyway. And it's an affordable solution for people who might want to reduce their exposure to flame retardants, or other chemicals in the environment in general.