Cigarette smoke topped the list in a new study—but what other dangerous toxins could you and your family members be inhaling?

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You may think that if you’re not a smoker, the air inside your house is pretty clean. But in a new study of homes in San Diego, cigarettes were not the only big indoor polluter. Fumes from cleaning products, candles, frying food, and marijuana smoke were also common—and all could be hazardous to residents’ short- and long-term health.

These chemicals may be especially dangerous for low-income households, the authors wrote in the journal PLOS One, since research shows that poorer families are at greater risk of health problems related to air quality.

The study, conducted by researchers at San Diego State University and the University of California San Diego, monitored the indoor air quality in nearly 300 homes for three months. Each household had at least one smoker, and at least one child age 14 or younger lived there as well.

Children, whose lungs are not fully developed and who breathe around three times more per kilogram of body weight than adults, are especially vulnerable to the health effects of secondhand smoke and other air pollutants, the authors wrote in their paper.

“We wanted to see if we could promote a smoke-free home and promote other strategies to reduce children’s exposure to air pollutants,” says Neil Klepeis, PhD, an environmental health research scientist at San Diego State University. “So first we had to look at all of the possible sources where those pollutants could be coming from.”

To do that, the researchers installed two air-particle monitors in each home—one in the area of the house closest to where smoking usually occurred, and one in the child’s bedroom. They also interviewed members of the households to find out when activities like cooking, cleaning, and smoking took place.

The monitors regularly scanned the air for fine particles between 0.5 and 2.5 micrometers in size—a range that includes dust, fungal spores, and automobile emissions. Research has shown that particles of this size can travel deep into the lungs, where they can contribute to breathing issues, heart problems, and other health complications.

Although all of the homes had smokers, not all of them smoked inside their living quarters. And not surprisingly, homes that reported indoor smoking had close to double the average particle level than those that didn’t.

More unexpected was the fact that, overall, marijuana smoke contributed to in-home air pollution just about as much as tobacco smoke—the first time such a finding has been reported in a published study.

Burning candles and incense, frying food in oil (regardless of whether or not the food was burned), and cleaning the house (by vacuuming, dusting, sweeping, or spraying aerosol products) also increased the level of fine particles in the air.

“People might think that smoking a little bit of pot or cooking can’t be that bad, but we saw that in some homes, it really is palpable,” says Klepeis. “If done regularly over time, those effects could add up.”

The researchers did ask about whether fans was running or windows were open while these activities took place. They didn’t see much of a reduction in particle levels when these ventilation methods were used, Klepeis says, but he points out that they have been shown to be effective in other studies.

“Some kitchen fans don’t vent outside, or they don’t extend far enough over the stove and low enough to really suck up those fumes,” he says. “But if you can get a strong ventilation fan, one that works well and sends those fumes outside, it’s a good idea.”

Refraining from burning candles or incense indoors, or choosing cleaner-burning formulas, can also help reduce particulate matter in your house, Klepeis says. “You get the most smoke when you extinguish a candle, so you might want to pay attention to where you blow them out,” he adds—near an open window, for example, or at least away from a child’s bedroom.

People who are really curious about the air in their homes can purchase a commercial air-quality monitor; the ones used in the study retail for $465, but less expensive ones are available as well. “It can be a good way to experiment and learn what really does affect the air in your own home, and to see how effective it is to open windows and use exhaust fans,” says Klepeis.

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Although this study focused on the potential impact on children, Klepeis says these types of pollutants can affect adults as well—especially the elderly and anyone with asthma, COPD, or other breathing issues. Even healthy adults can suffer from watery eyes, sneezing, coughing, and respiratory irritation when these chemicals are present.

“We want people to be aware that the things they do in their homes—especially combustion-related things, like smoking and cooking, can degrade indoor air quality,” he says. (Cigars, pipes, hookahs, and e-cigarettes can also release harmful chemicals into the air, he adds.)

“If you can’t ban the source from the home entirely,” he adds, “at least try to contain it within a room, and make sure you have ventilation to help get those fumes out of the house.”