Allergic contact dermatitis is an immune system-related reaction to an allergen that has come into contact with your skin. It causes a delayed reaction: a rash that appears a few days after exposure, and then can last for weeks. “When we see allergic contact dermatitis from clothing, it’s usually from disperse dyes,” says Dr. Susan Nedorost, a professor of dermatology at Case Western Reserve University and director of the dermatitis program at University Hospitals Cleveland Medical Center. Disperse dyes are primarily used in synthetic clothing materials like polyester and nylon, Nedorost says. And they may be present at higher levels in a brand-new, unwashed article of clothing.

Nedorost says that sweating and friction can cause disperse dye to leach out of clothing. Synthetic workout gear—the shiny, stretchy, water-repelling materials that are so popular nowadays—are often the culprit when she treats people for allergic contact dermatitis. “If a patient comes in and has a rash around the back of the neck and along their sides around their armpits, the first question I ask is what they wear when they work out,” she says.

It’s not clear how common disperse-dye allergies are among the general public. But there is one way to limit your risk for bad reactions: “By washing new clothing, you might remove a little extra dye and so have a lower exposure,” Nedorost says.

In very rare cases, taking this step could even prevent the development of a new allergy. If enough of the dye leached onto a skinned knee or other open wound, she says, that could activate the immune system and create a lasting sensitivity.

Allergic rashes aren’t the only health issue associated with clothing chemicals. In a 2014 study, a group of researchers from Stockholm University in Sweden tested 31 clothing samples purchased at retail stores, and that were “diverse in color, material, brand, country of manufacture, and price, and intended for a broad market.” They found a type of chemical compound called “quinoline” (or one of its derivatives) in 29 of the 31 samples, and the levels of this chemical tended to be especially high in polyester garments. Quinoline is used in clothing dyes, and the U.S. Environmental Protection Agency has classified it as a “possible human carcinogen” based on some studies linking it to “tumor-initiating activity” in mice—though the agency also states that no human studies have been conducted to assess the cancer-causing potential of quinoline.

Ulrika Nilsson, a member of the Stockholm University group and a professor of analytical chemistry, also calls out nitroanilines and benzothiazoles, two more chemical compounds that turn up in clothing and that lab and animal evidence has linked to potential adverse health effects, including cancer. While some of these chemicals may remain locked away in the fibers of your clothing, others may slowly work their way out onto your skin or into the air you breathe as your clothing ages and degrades. Unfortunately, Nilsson says, “these chemicals are so far not well studied regarding skin uptake or related health effects” in humans, so it’s not clear whether exposure to these chemicals in your clothing could make you sick.

David Andrews, a senior scientist with the nonprofit Environmental Working Group who has investigated the use of chemicals in the textile industry, says clothing is often treated with stain-repellents, color-fasteners, anti-wrinkle agents, softness-enhancers, and any number of other chemical treatments. Clothing manufacturers don’t have to disclose any of these to customers, and many of the chemicals, including a popular type of waterproofing chemical called fluorosurfactants (often referred to as PFAS), have little or no research backing their safety. Not only could these chemicals pose health risks to people, but they also end up in the air and water supplies, where they could do further harm.

“It’s always in your best interest to wash clothing before wearing,” he says. Nilsson agrees, saying washing new clothes “reduces the content of chemicals,” especially residual chemicals that may be left over from the manufacturing process.

But even so, that doesn’t prevent clothing chemicals from breaking down and leaching out of your clothing and onto your skin or into the air your breathe. And, unfortunately, there’s no easy way to point people toward clothing items that may be safer, Andrews says. Some of the research on clothing suggests synthetic materials may be treated with more chemicals than natural fibers such as cotton. But there’s really no label indicator or certification that signals a garment is chemical-free, he says.

“What’s maddening for the consumer is that you buy a shirt that says ‘100% cotton,’ and yet you’re given no information about any of the chemicals or additives that have been used.”