When we want new clothes, most of us either go to the department store or online, purchase, and then wear once in our possession. However, most of us skip a crucial step in between buying and wearing — washing our new clothes. Despite being brand new, these clothes contain toxic chemicals such as synthetic dyes and formaldehyde resin, which can lead to allergic reactions and illnesses, if not washed at least twice before use, according to the Wall Street Journal’s video “Do You Need to Wash Clothes Before Wearing Them?”

New clothes may contain material that is made, dyed, and stitched in different countries, with each containing various laws regarding acceptable levels of chemical use. Dye and formaldehyde resin, two common chemicals found in new clothing, can cause slightly inflamed, dry, itchy patches and rashes to a severe skin reaction. Dr. Donald Belsito, professor of dermatology at Columbia University Medical Center in New York specializing in contact dermatitis, told the WSJ these rashes tend to form “especially near the areas where there is friction or sweating, like the waist, neck, and thighs and around the armpits,” while those who are genuinely allergic, “may need to avoid the [azo-aniline dyes] entirely.”

Meanwhile, urea formaldehyde resins, which are used to prevent cotton-polyester blends from wrinkling and limiting mildew, can cause allergic contact dermatitis and irritant contact dermatitis, producing similar symptoms to azo-aniline dyes. A 2010 study conducted by the U.S. Government Accountability Office found some fabrics for sale exceeded the allowable levels of the formaldehyde’s resin in the U.S.

Typically, levels of formaldehyde are touted as safe in the U.S., but countries like New Zealand advise washing new clothes as a means of precaution. According to the Consumer Affairs in New Zealand, “Trading Standards is advising people that it is good practice for new, washable garments to be thoroughly washed and aired, preferably outside, before use, and for new clothes labelled as ‘dry-clean only’ to be thoroughly aired before use.”

There is still no fool-proof way to know what type of germs are living in new clothes, including clothes made with 100 percent natural fibers. For example, lice and scabies can be transmitted from trying clothes on in the store. Lice is more likely to stick on natural fibers than synthetics, increasing the risk of transmission.

Belsito’s rule of thumb is to wash new clothes at least once and then wear. Regardless of the material, he washes his new clothes at least once with a double rinse before wearing them. “In terms of hygiene, it’s a very good thing to do,” he said.