When we’re on the treadmill, we’re more likely to be thinking about what SZA track to queue up or whether we’re going to make it another mile than what’s in our workout clothing. But your favorite sports bra or well-worn pair of leggings is likely to be made from synthetic fabrics like Spandex, nylon and polyester, all of which are essentially plastics. These materials are made from petrochemicals and are often formulated with harmful chemical additives like phthalates and bisphenols. Now, new research shows sweat leaches chemical additives from plastics and those chemicals are then available to be absorbed through our skin. What did the study find? The study focused on a class of compounds called brominated flame retardants (BFR), which are used to prevent burning in a wide range of consumer products including fabrics, and are linked to adverse health effects such as thyroid disease, hormonal disruption and neurological issues. Researchers at the University of Birmingham found that because sweat contains oil, and oil has a lipophilic chemical nature that encourages the chemicals in plastic to dissolve and diffuse, the oil in your body can leach chemicals from the plastics you touch. In short, oily substances in our sweat “help the bad chemicals to come out of the microplastic fibers and become available for human absorption”, says Dr Mohamed Abdallah, an associate professor in environmental science at the University of Birmingham, and the principal investigator of the study. The Birmingham team focused on flame retardants, which are added to some fabrics but not particularly associated with sportswear, and ran test cases based on how much sweat and plastic interface when people are just sitting around at home. Further research is required to establish the type and quantity of chemicals a sweaty gym-goer would absorb from their synthetic workout wear and the gym environment. But Abdallah says the study implies that other chemical plastic additives, like bisphenols (which have been found at up to 40 times the safe limit of exposure in items from popular sportswear brands), phthalates and PFAS, “may leach out into sweat and become available for dermal absorption”. These findings can be “logically extrapolated in terms of someone who is running and sweating intensely”, he notes. Essentially, the more you sweat, the more chemicals you could absorb. Why does this matter? Previously, researchers have tended to focus on our exposure to plastic through diet, but the Birmingham study raises awareness that humans can be exposed to plastic chemicals through our skin, too. And because harmful chemicals in plastics bioaccumulate – or accrue slowly and stick around in our bodies – repeated and multi-source exposure can result in having high concentrations of chemicals inside us, potentially contributing to health effects. New research published in the journal Environmental Pollution last month found a total of 25 flame retardants in the breast milk of 50 US mothers; every sample included certain flame retardant compounds which the US began phasing out a decade ago due to known harms. Another study this August found increasing rates of cancer among Americans under 50, particularly women, with gastrointestinal, endocrine (including thyroid) and breast cancers rising fastest. Although the exact cause of this surge remains undetermined, experts speculate that exposure to a wide range of harmful pollutants and carcinogenic chemicals is a contributing factor. Alden Wicker’s 2023 book, To Dye For: How Toxic Fashion is Making Us Sick, delves into the many ways we’re exposed to chemicals through our clothing. According to Wicker, workout wear, especially clothing marketed as “sweat-wicking” or water-repellent, often contains PFAS, the notoriously carcinogenic and pervasive family of “forever chemicals”. Due to industrial contamination and widespread use in everything from frying pans to toilet paper, you can find some quantity of PFAS just about anywhere, but they can appear in high concentrations in clothes treated with certain high-performance, water-resistant finishings. “Things like Gore-Tex, that’s just pizzazz,” she says. “That’s just a type of PFAS coating.” Wicker notes that polyester fabrics are usually colored with disperse dyes – a family of chemical colorants used on plastics that are known skin irritants, particularly for children and people with sensitive conditions like eczema. And that’s just the toxins we know about. “Research shows that many more chemicals are present in everyday plastic products than ‘just’ the notorious ones, such as [flame retardants],” says Martin Wagner, a biologist studying the effects of plastics on humans at the Norwegian University of Science and Technology, who is unaffiliated with the Birmingham study. “The latest estimates indicate that there are more than 13,000 plastic chemicals. Since most of these are not investigated for their health or environmental impacts, there is an urgent need to improve the safety of plastics,” he wrote in an email. Opaque supply chains mean that even manufacturers aren’t aware of everything that is in their plastic products. “Most fashion brands do not know who is dying and finishing and manufacturing their materials,” says Wicker. “And those suppliers will be incentivized to do things as cheaply as possible, to get cheaper chemicals from uncertified sources that could be contaminated with heavy metals and other hazardous substances.” What should I do about it? An easy way to avoid exposure to these chemicals is to wear clothing made of sustainably produced, minimally processed natural textiles, which don’t contain the toxins associated with plastic materials. Check fabric labels for items that are predominantly organic cotton, hemp or merino wool (some percentage of elastane or Lycra in workout wear is almost inevitable, for stretch). Seek out meaningful designations from third-party textile certifiers like the Global Organic Textile Standard (Gots) and OEKO-TEX, and visit brand websites to see if they make an effort to list their suppliers; they best know where their products come from, down to their dye houses and mills. California’s Proposition 65 requires businesses to provide warnings about significant exposures to chemicals that cause cancer, birth defects or other reproductive harm. But stricter federal legislation is needed to ensure consumer products do not contain harmful chemicals regardless of which state they are sold in. One recent brand survey found that 72% of respondents would buy plastic-free sportswear if it was readily available. Yet, even with the best intentions, “it can be expensive to get rid of all your synthetics and overhaul your wardrobe”, says Wicker. “I would say do it a little bit at a time unless you have chronic health issues, serious concerns or you react to some of these materials.” Abdallah says he minimizes synthetic fabrics in his home and works out wearing natural fibers like cotton. “Why be exposed to these chemicals at even low levels?” he says. “Why not avoid the risks?”