Most of us associate air pollution with the outdoors—from car or bus exhaust in a city to industrial waste or wildfire smoke. But you may not realize the air inside your home can be even more contaminated, especially in winter. Since home is where we spend some 70 percent of our time—more if you’re one of the 35 percent of workers who do some or all business from there—these toxins can have an outsized impact on your health.

Outdoor pollutants are regulated by the Clean Air Act, but no comparable federal law governs what lurks inside your dwelling. Yet exposure to common winter indoor toxins can cause myriad health conditions, including respiratory and cardiovascular diseases, chronic inflammation, and cancer. Some research has even linked them to premature death, says Nicholas Nassikas, a pulmonology and critical care doctor at Beth Israel Deaconess Medical Center. He coauthored a March 2024 report on indoor pollution for the American Thoracic Society that explored ways this pollution also seeps outside and causes health and environmental problems there.  

(Learn how outdoor air pollution kills millions every year.)

Indoor air pollutants range from aerosolized chemicals and metals to tiny particles that can sneak into the bloodstream through the lungs. Common sources of indoor pollutants include many of the things we use to make a winter home cozy, such as wood-burning stoves, fireplaces, candles, and cooking comfort foods.

While gas and wood burning stoves have made headlines for indoor air pollution, ultrasonic humidifiers are a lesser-known culprit. These devices use high-frequency vibrations to push out cool mist and moisten heated environments, sometimes releasing toxic heavy metals that linger in the air.

“In winter when air is dry, the water evaporates quickly but whatever particles were in the water remains,” says Andrea Dietrich, as an environmental and water engineer at Virginia Tech in Blacksburg, who has extensively studied the issue.

How dangerous your ultrasonic humidifier may be depends on a number of factors, including how often you use it and, especially, what’s in the water you put in the tank. Many package inserts recommend placing only distilled or reverse-osmosis water in devices. Yet nearly a quarter of respondents told researchers from the U.S. Centers for Disease Control and Prevention they generally fill humidifiers or other respiratory devices with water from the tap.

Dietrich’s studies found that certain tap water can emit neurotoxins including arsenic and lead that can linger in the air. These chemicals are safe to drink in the amounts present, but that may not be the case when you breathe them in.

“We looked at how much you’d get exposed to if you drank the water or if you breathed the air from a humidifier for eight hours a day,” Dietrich says. “You not only inhale more of it, the risks are greater because they’re more toxic when inhaled.”

Even minerals like calcium and magnesium that are healthful when ingested can be dangerous to airways. “The lungs weren’t designed for these particles, which can clog the lung passages and make it more difficult to breathe,” Dietrich says. 

Most of an ultrasonic humidifier’s chemicals linger close to the device, but some travel across the room, near where your bed may be located. Children are particularly sensitive, since they inhale more air per body weight than adults do. One of Dietrich’s studies modeled common humidifier exposures and found it can lead to unsafe levels of the metal manganese in children’s brain and lungs when used in small rooms with poor ventilation.

Combustion pollutants—released from burning oil, kerosene, gas, coal, or wood—are another prime source of winter’s indoor toxins.

Nitric oxide and volatile organic compounds irritate the eyes, nose, and throat and, in extreme cases, damage the central nervous system. Burning also produces fine particulate matter or PM2.5s—particles with a diameter of 2.5 micrometers or less. If these particles make it to the bloodstream or deep in the lungs, they've been linked to respiratory and heart disease, as well as complications from respiratory viruses.

Half of the PM2.5s in an average home come from outdoor pollutants that make their way inside, but the other half originate internally from burning.

Wood-burning stoves and fireplaces used to heat homes are some of the worst offenders. Propane fireplaces generate PM2.5s as well, though in lower quantities because more are burned off with the gas. 

Of course, these devices vent to a home’s exterior, but some particles nonetheless remain in the room. And in older homes where air regularly moves through cracks and joints and around windows and doors, “some of the particles that are vented outside can come back in,” Nassikas says.

Even candles release combustion pollutants. One study of several dozen Danish homes in winter found that extensive candle use for hours a day caused almost 60 percent of the particle exposure in those homes.

Most of us do much of our burning in the kitchen, and not only when we leave bread too long in the toaster. All cooking involves heat that generates a large amount of indoor pollutants. Gas stoves are especially problematic, because they release nitrogen dioxide and formaldehyde that affect a person’s airways. Nearly 13 percent of childhood asthma cases in the U.S. have been linked to gas stoves; in some states, like Illinois and California, the figure is closer to 20 percent. 

But electric stoves aren’t off the hook. If you smell something burning when you turn on the stove, “that’s the crud that’s left over and becomes aerosolized when heated to high temperatures,” releasing particulate matter, Nassikas says.

The other factor is what’s for dinner. Stews, casseroles, fried chicken, and other fatty fare may warm your insides on a cold winter evening, but they also generate numerous PM2.5s. Frying on the stove is the most polluting, but baking in the oven carries risks, too.

“If you’re bubbling a lasagna, it’s generating a lot of particles through the high temperature and percolating action, and they don’t stay locked in the oven box,” says William Nazaroff, an environmental engineering expert at the University of California, Berkeley.

All particles peak soon after cooking, but some have been found in kitchen and living room air up to 10 hours later, according to a study in the journal Building and Environment.

Unlike outdoor pollution which is harder for individuals to control, people can reduce at least some of their indoor exposure. When cooking, turn on the range hood, and place items on the back burner of the stove, where more will be sucked into the vent. If it’s not too cold outside, crack open a window nearby. 

Avoiding tap water (even if it’s filtered) can make ultrasonic humidifiers safer. Another option: switch to an old-fashioned thermal humidifier, which traps the minerals inside the device. And when cleaning any type of humidifier, never use chemicals, which leave residues that add to the air burden; simply rinse it with water and air dry.

Be sure to regularly maintain all wood-burning heaters, fireplaces, and gas stoves according to the manufacturers’ recommendations, as well as the mechanical ventilation system used in tighter, newer-construction homes to exchange air.

Free-standing air purifiers help remove many pollutants. They’re most effective when you place them close to your home’s main combustion sources. Putting it in the kitchen, rather than the living room or bedroom, for example, resulted in many fewer PM2.5 particles in the home, the Building and Environment study found. 

(Find the best air purifiers for your home.)

Nazaroff recommends making your own do-it-yourself device, the so-called Corsi-Rosenthal box, which provides an extremely high filtration rate. Start with a 20-inch box fan and four 20-by-20-inch MERV 13 filters. Tape the filters together to form the sides of a square (with filter arrows pointing inward). Cut two pieces of cardboard (use the box the fan came in) the size of the openings; tape one securely to create the square’s bottom. Now tape the fan to the top so it blows upwards, taking care to seal all edges and corners (but ensure the plug remains outside). Finally, cut a 15-inch-diameter circle in the second piece of cardboard and tape it on all sides over the top of the fan. Leave the fan positioned on top as you plug it in.

Better than ventilating and filtering, though, is preventing toxins from forming in the first place. When wood and gas appliances break down, replace them with electric versions. And do your best to limit other sources generating unwanted particles inside the home.

“My kids joke that they couldn’t have candles on their birthday cake, and while that’s not true—although now we do use electric candles—we don’t use a fireplace or ultrasonic humidifiers,” Nazaroff says. When he does occasionally need to stir-fry vegetables to make his beloved ratatouille, “I always cook on the back burner and use the range hood, even though it’s loud.”