Last month, I took a road trip to North Carolina. The area had just experienced an extremely wet and rainy spring, and the Airbnb I was staying in had been unoccupied since Covid-19 halted almost all travel in March. When I unlocked the door, a putrid smell hit my nose immediately, like a wet beach towel left too long in a hot car. I was now sharing my rental house with some sort of mold.

The pandemic has forced all sorts of buildings to sit empty for long periods of time. As people venture back into their homes, schools, and offices again, they may also find an unwelcome surprise inside. The Centers for Disease Control and Prevention warns people who are reopening buildings to watch out for potential hazards like mold and legionella, the bacteria that causes Legionnaires’ disease. Greg Bukowski, CEO of the mold inspection and removal firm Moldman USA, says he’s seen an uptick in customers in the Chicago and St. Louis areas where his company is based. “Homes that have been unoccupied for months have a high likelihood of having water intrusion issues and subsequent mold issues,” he says. Water intrusion can come from something like a roof or plumbing leak, or high humidity as a result of leaving the air conditioning off.

This is not a new phenomenon, of course. Vacation homes and foreclosed properties often harbor mold. New construction techniques may be somewhat to blame: Because homes are now tightly sealed for energy conservation, they may be poorly ventilated and susceptible to issues like mold. Every year, some unlucky school districts return in August or September to find classrooms full of the stuff, says Jason Earle, the founder and CEO of 1-800-GOT-MOLD?, a mold inspection and removal firm based in the New York City area. Oftentimes, he says, maintenance staff shampoo the carpets at the end of the school year and then turn air conditioning units off to save on utility costs, inadvertently creating a perfect environment for mold to thrive.

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Fungi need moisture and food to grow. They will eat almost any organic substance, from cardboard and wood to ceiling tiles and upholstery. What I inhaled were the airborne byproducts of its metabolic processes, or what the US Environmental Protection Agency says are “microbial volatile organic compounds.” I personally prefer to call them fungi farts.

Aside from producing a nasty smell, exposure to mold can also cause unpleasant side effects for people who are sensitive to it, like stuffy nose, coughing, and sore throat. If you’re immunocompromised, you may be more vulnerable to these and other symptoms, says Naresh Magan, a professor of applied mycology at Cranfield University in England. For parents, the most serious issue to be concerned about is childhood asthma: A number of studies have found a link between mold exposure and the condition.

That doesn’t mean all mold is scary or harmful. Humans are constantly breathing in a plethora of different fungi and other microbes; usually they just don’t realize it. “There are thousands of mold spores in the air,” says Magan. If you wash a piece of fruit, for example, and then put the runoff water into a petri dish, “you will find loads of bacteria, yeasts, and filamentous molds,” he says. The world is really just a gigantic terrarium full of microscopic creatures ready to be inhaled. But if the concentration of mold spores in the air becomes too high, like inside a mold-contaminated building, it can cause an adverse health reaction.

If you return from quarantining at a loved one’s place to find that mold has turned your home into its home, it should be removed. While some companies will sell you testing kits to identify the exact species, the process is not necessary, according to the CDC. “The health effects of mold can be different for different people so you cannot rely on sampling and culturing to know if you or a member of your family might become sick,” the agency’s website advises.