It might sound weird to wash that pile of fresh back-to-school clothes before they have a chance to collect ketchup and grass stains, but according to experts you should always wash newly purchased clothes before you wear them the first time.

Wearing new clothes without washing them could put you at risk for developing allergic contact dermatitis, according to Dr. Susan Nedorost, a professor of dermatology at Case Western Reserve University and director of the dermatitis program at University Hospitals Cleveland Medical Center. “When we see allergic contact dermatitis from clothing, it’s usually from disperse dyes,” says Dr. Nedorost explained to TIME.

photo: stevepb via Pixabay

The allergic reaction can be delayed and cause a rash that can last for weeks, says Nedorost. The disperse dyes are used in synthetic fabrics, like polyester and nylon, and are often concentrated at higher levels in the fabric prior to being washed for the first time. If you’ve ever washed your new clothes only to be dismayed by the color fading, you can understand how the process works.

Sweating, combined with the friction caused by regular bodily movement, can cause the dye that would be minimized through washing to disperse onto your skin and cause a reaction. This is often the case with workout wear according to Nodorost. “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 told TIME.

There isn’t enough data to determine how common this type of allergy actually is, however, there are other reasons to avoid wearing your clothes before washing them. A 2014 study conducted by Stockholm University in Sweden found that, when tested, a majority of retail clothing items off the rack contained a type of chemical compound called “quinoline” which is used in clothing dyes and is listed as a probable human carcinogen by the the U.S. Environmental Protection Agency. Ulrika Nilsson, a member of the Stockholm University group, explains that washing new clothes “reduces the content of chemicals,” especially those like quinoline that may be residual from the manufacturing process.

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Ultimately, a quick wash cycle isn’t much in the grand scheme of the mountains of laundry you’ll do over your parental career, so better safe than sorry.

—Shahrzad Warkentin

 

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