Chemical hand warmers — those little packets that you can purchase at your favorite store for a few bucks — work by producing an exothermic reaction, according to Chemical & Engineering News. Long story short, when you follow the directions on the package, you expose chemicals to each other, and a reaction that produces heat takes place. Indeed, it's quite a lot of heat: they can reach up to 135 degrees Fahrenheit. That's about 20 degrees warmer than what's needed to produce first-degree burns, according to WHNT, and is getting into second-degree burn territory.
What's more, in certain situations, those devices can get even hotter, up to 185 degrees. It's not unheard of for people to suffer gruesome injuries from hand warmers. For example, as Contemporary Pediatrics notes, in 2019 a 9-year-old girl presented in an emergency room with blistering on her skin near her ankle. Doctors determined she'd suffered a second-degree burn from a disposable, chemical hand warmer she'd been using in her boots.
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