As long as you avoid cigarettes and secondhand smoke, you’re in the clear, right? Unfortunately, not quite: Research has shown that thirdhand smoke—AKA the nicotine and other chemicals from tobacco smoke that cling to indoor surfaces like clothes, bedding, furniture, and walls long after the literal smoke dissipates—might have dire consequences for your health too.

Simply airing out rooms won’t do the trick for removing smoke residue; only regularly cleaning or laundering affected surfaces and items gets rid of it, Mayo Clinic reports. And that’s what makes thirdhand smoke so detrimental: It lingers, and people can be exposed to the dangerous components of tobacco simply from touching or breathing around objects upon which trace amounts of it reside.

“The best way to reduce exposure risk is to have a smoke-free environment—especially where you live.” —Dr. Humberto Choi

In the past year, two studies have illustrated thirdhand smoke’s negative effect on mice—both increasing their risk of lung cancer and liver damage and diabetes—and another revealed just how long smoking residue stayed on casino walls and carpets after smoking was no longer allowed. (Spoiler: After three months, only 90 percent of thirdhand smoke was gone.) Thirdhand smoke has even been shown to damage human DNA.

And now the latest study, published in the journal Science Advances, uncovered something even more disturbing: A nonsmoking classroom was coated in residue from airborne tobacco particles that traveled there. The study analyzed the air composition in an unoccupied classroom that had been smoke-free for more than two decades and found that 29 percent of the air mass was made up of thirdhand-smoke chemicals, according to Newsweek.

“It shows that just because you’re in a nonsmoking environment, it doesn’t mean you aren’t exposed to tobacco,” lead study author Peter DeCarlo, PhD, told the Washington Post. “That Uber car you jump into, the hotel room you stay in, even a classroom where smoking hasn’t been allowed for decades: These are places where you are often exposed to a lot more than you expect.”

It’s certainly not comforting to know that we’re exposed to health threats outside of our control (especially if you have small children who are crawling around and putting things in their mouths). But research about thirdhand smoke is still relatively minimal, so until we know more and are thus able to develop smarter defense solutions, the best safety practice you can implement is to make sure no one smokes in your home and stay on top of cleaning your space.

“The best way to reduce exposure risk is to have a smoke-free environment—especially where you live,” pulmonologist Humberto Choi, MD, told the Cleveland Clinic.

Hopefully, as more widespread knowledge about thirdhand smoke becomes available, we’ll gain more control over how to prevent it from affecting our health.

Here’s why toxic cleaning products can be as harmful as smoking. Or, find out the three most important lifestyle changes to make before getting pregnant.