Most people don’t think about where an item of new clothing has been before it comes into their possession. Even with garments that label their provenance, many include materials that were made in one country, dyed in another and stitched together in a third, each with varying laws about allowable levels of chemical use.

One expert, Donald Belsito, a professor of dermatology at Columbia University Medical Center in New York with a specialty in contact dermatitis, explains how lice can linger in fabric and why washing before wearing—maybe even more than once—should be mandatory.

Itchy, scaly, red
There are two major culprits when it comes to allergens in new clothing: dye and formaldehyde resin.

Most synthetic textiles are colored with azo-aniline dyes, which can cause a severe skin reaction akin to poison ivy in the small population of people allergic to them. For others, reactions to dyes are less extreme, and may result in slightly inflamed, dry, itchy patches of skin, Dr. Belsito says.

Until much of the dye is rinsed out—usually in more than one washing—some wearers might notice red, itchy, scaly rashes, “especially near the areas where there is friction or sweating, like the waist, neck and thighs and around the armpits,” he says. Those who are genuinely allergic, however, “may need to avoid the allergen entirely,” says Dr. Belsito, noting that the dye may stick around indefinitely and continue to trigger the allergy.

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