Why You Really Need to Change Your Air Conditioner's Filter

Given that the average person in the U.S. spends 90% of their time inside, indoor air quality can have a significant impact on overall health and well-being. But many people overlook one common contributor to indoor air quality: air conditioning, which the vast majority of U.S. homes use in some form.

Air conditioners can enhance air quality by regulating temperature, reducing humidity levels, and improving filtration. On the flip side, a dirty system can degrade air quality.

Studies have shown that bacteria and other pathogens can accumulate in air conditioning units, potentially sickening people exposed to them. Legionnaires’ disease, a form of pneumonia, can be spread by air-cooling systems contaminated by bacteria, and dirty AC can also lead to hypersensitivity pneumonitis, an allergic reaction to irritants like bacteria and fungi that leads to lung inflammation. A 2023 study compared healthy adults in India who spent at least six hours per day in air conditioning with those who spent equivalent time in naturally ventilated buildings and found that those in air-conditioned environments had more health issues, including respiratory symptoms, headaches, and lethargy—possibly because of contaminants accumulating in these buildings, among other factors.

Here’s how to avoid AC-related health risks and improve the quality of the air you breathe.

An air-conditioning filter should be labeled with a minimum efficiency reporting value (MERV), which rates its ability to trap particles in the air, ranging from dust and pet dander to smog and mold. The higher the MERV value, the more the filter catches. If they’re compatible with your air-conditioning system, look for a filter labeled MERV 11 or MERV 13, suggests William Bahnfleth, a professor of architectural engineering at the Pennsylvania State University and a member of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

If your system doesn’t work with these filters—window units, in particular, tend to have lower-quality filters—you may want to purchase a separate HEPA filter to help improve your air quality, says Jeffrey Siegel, a professor of civil engineering at the University of Toronto who researches ventilation and indoor air quality.

More important than which filter you buy is how you use it, Siegel says. Priority number one: replacing or cleaning it regularly. If you put it off for too long, mold and other contaminants can accumulate in your AC system and circulate throughout your home, potentially triggering allergies and other respiratory issues, says Dr. Anthony Gerber, a pulmonologist at National Jewish Health in Colorado.

Exactly how often you need to swap your filter varies depending on the manufacturer’s instructions, how often you use your AC, and the quality of your home’s air. But, as a rule of thumb, you should clean or replace your filter every month or two during seasons of peak use, according to the U.S. Department of Energy. This not only prevents contamination but also helps the system run better. If you have a reusable filter, manufacturers typically recommend vacuuming it, washing it with water and a gentle detergent, and letting it dry fully before reinserting it.

Regardless of which filter you buy, make sure it fits well, Siegel says. “Air will take the easiest path,” he says, so if there are gaps around your filter, air will simply bypass it. If you have central air conditioning, you can likely have an upgraded filter slot installed relatively inexpensively to improve the fit, Siegel says.

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With central air conditioning, the fan comes on when the system is actively working to lower the interior temperature by circulating cool air. That’s about 20% of the time for most residential systems, Siegel’s research has shown. Air only goes through the filter when the fan is on—so if your system allows, Siegel suggests setting the fan to run for around 20 minutes every hour (about 33% of the time) to improve filtration, especially during seasons when there’s lots of pollen or other irritants in the air.

Many window units also have fan settings, but Siegel says extra run time won’t bring meaningful health benefits, since the filters used in window AC systems typically don’t remove most particles from indoor air.

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Most industrial HVAC systems periodically bring in outdoor air to improve ventilation. That’s important, as inadequate ventilation is thought to contribute to “sick building syndrome,” a phenomenon whereby people feel unwell—experiencing symptoms like headaches, coughing, fatigue, and nausea—while inside, even if there’s no obvious cause for those symptoms.

By contrast, most residential air-conditioning units don’t mechanically bring in fresh outdoor air, according to the U.S. Environmental Protection Agency, though some models have a vent that can be left open for fresh-air intake. If yours doesn’t, you can simply turn off your system for a few hours a day and open the windows to get air flowing, Bahnfleth says. It’s also okay to temporarily open the windows while the system is running, he says. (That’s assuming the outdoor air quality is good; if not, you’re better off keeping your windows closed.)

“It’s the most boring thing in the world, but air-conditioner maintenance is really important,” Siegel says. HVAC technicians can make sure your system is working properly and that hard-to-reach parts are clean and free of potentially harmful build-up.

Write to Jamie Ducharme at [email protected].