Doctors are warning patients not to wear items containing metal when coming in for magnetic resonance imaging (MRI) procedures, as the machine will react, heating up and potentially causing light to moderate burning, MarketWatch reports.

Wearers may not realize it, but there could be small threads of metal in their athleisure clothing. The metals, usually silver, are woven into the garments to prevent them from stinking when the wearer sweats during exercise. Metal heats up in MRI machines, so hospitals and radiologists are now having patients wearing such clothing change into cotton or paper gowns.

There are a number of reports of people getting burned in this way. One report from 2012 described an 11-year-old girl getting second-degree burns after her undershirt reacted to the machine while she was sedated inside of it.

Many manufacturers make clothing with silver thread in it. Lululemon was specifically cited in a case from 2014, when a woman pressed the emergency stop button on an MRI machine after she felt a burning sensation. She was wearing the company’s yoga pants, which had the metallic thread in them.

Some signs in hospitals are specifically calling Lululemon out by name in their bans of athleisure, according to MarketWatch.

It’s not just Lululemon — in fact, all clothing that states that it is “anti-microbial” or “anti-bacterial” should be avoided, as it likely also uses this metallic technology. It’s hard to spot with the naked eye as the technology improves, so the only way to know for sure is by looking at the tag. When in doubt, go for a standard t-shirt.