In the late 1600s, Antonie van Leeuwenhoek, a Dutch textile merchant whose hobbies included handcrafting microscopes, scraped some plaque off his teeth, mixed it with rainwater, and put it under his lens. He saw something curious—“many very small living animals, which moved very prettily,” he wrote in a letter to the Royal Society of London.

His discovery of what we know as bacteria was a breakthrough in microbiology. When later experiments revealed those little “animals” can cause disease and illness, people were encouraged to wash their hands using soap to remove bacteria and prevent infection.

Today, many instead reach for hand sanitizer. But is its frequent use good for you, or can it have harmful side effects?

“Hand sanitizers are really good if they’re alcohol-based, and the alcohol content must be at least 60 percent and up to 95 percent,” says James P. Mack, a biology professor who has researched the efficacy of essential oils from plants, and the organic compound methylglyoxal from Makuna Honey,  as agents against several multidrug-resistant bacteria. Brands with these higher alcohol concentrations “kill most of the germs and bacteria that we accumulate by touching things all the time,” he says.

The Centers for Disease Control and Prevention warns hand sanitizer with an alcohol concentration below 60 percent and non-alcohol-based sanitizers aren’t nearly as effective. The latter, whose active ingredient is triclosan, only reduces the growth of germs—it doesn’t kill them. Additionally, it may not work equally well for all classes of germs and is more likely to irritate the skin.

“A lot of antibacterial soaps and washes had triclosan, but there really wasn’t any convincing evidence that these products had a benefit as an antibacterial agent,” says Mack. That, as well as data suggesting that long-term exposure to those products could pose health risks due to bacterial resistance to triclosan, prompted the Food and Drug Administration to ban the marketing of antibacterial wash products containing the ingredient last year.

“If you used those, you could have exposed yourself to bacteria that could give you an infection,” says Mack.