Every Question You’ve Ever Had About Air Conditioning, Answered

Humming air conditioners are the soundtrack to summer in the U.S., given that almost 90% of households use them for cooling. Even though air conditioning is everywhere, however, many people still have questions about exactly how it works, how to use it best, and how it affects air quality, human health, and the environment. TIME spoke to experts to get answers.

How does air conditioning work?

In the most basic terms, “an air conditioner absorbs heat from the building and dumps it to the outside,” explains Jeffrey Siegel, a professor of civil engineering at the University of Toronto who researches ventilation and indoor air quality. More specifically, a compressor moves liquid refrigerant into an indoor coil, where it evaporates and pulls heat from the surrounding air. The gaseous refrigerant then flows to the system's outdoor coil, moving heat out of your home. The gas condenses back into a liquid in the system’s outdoor coil, and the process starts again.

Central air-conditioning systems, window units, and mini-splits (which usually sit high on the wall) work in conceptually the same way, but there are differences in how they’re installed and exactly how they transfer heat. Functionally, the biggest difference for consumers is that central systems cool an entire home, whereas window units and mini-splits are designed to work in only one room.

How do air conditioners affect air quality?

It depends on what kind of system you have. Window units and mini-splits typically don’t have high-quality filters, so they don’t do much for air quality beyond temperature regulation, Siegel says. Central air-conditioning systems, however, can offer valuable filtration, catching particulate matter like dust, pet dander, mold, and smog. 

The higher a filter’s minimum efficiency reporting value, or MERV, the more it can trap. High-MERV filters can even help remove viruses from indoor air, according to the U.S. Environmental Protection Agency, but not effectively enough to rely on your AC system alone for disease prevention. If you have central AC, make sure to run the fan for at least 20 minutes an hour, as air only passes through the filter when the fan is actively running, Siegel says.

Read More: What to Wear When It's Really Hot Outside

The tradeoff is that rooms that are sealed for cooling don’t get much, if any, ventilation from fresh air. “Dilution with outdoor air helps get rid of a lot of stuff,” Siegel says, so it’s a good idea to shut off your AC and open the windows for at least a chunk of every day, as long as outdoor conditions allow. “Sick building syndrome,” which causes people to feel unwell when they spend lots of time indoors, is thought to be linked to inadequate ventilation.

Do you really need to change your air conditioner’s filter?

In a word: yes. A dirty filter won’t do your health any favors, particularly if it’s harboring bacteria or other disease- or allergy-causing grime.

But the health of your cooling system may be an even more pressing concern. “If you let that gunk build up on your filter, you can destroy your system,” says Francis Dietz, vice president of public affairs at the Air-Conditioning, Heating, and Refrigeration Institute, a manufacturer’s trade association. “The air can no longer get through that filter, so it backs up into the system and it freezes up.” (Short of a total system meltdown, Siegel says, you may simply notice that it’s not working as well as usual.)

Exactly how often you need to change or clean your filter depends on the manufacturer’s instructions and how heavily you use your system, Siegel says, but roughly every three months is a good ballpark estimate.

Is it okay to run air conditioning all the time?

From a mechanical perspective, a properly maintained system should be fine to run 24/7, Dietz says. But an old or neglected unit may die under the strain of constant use—hence why many people’s systems “conk out” during a heatwave, he says. Save yourself the emergency repair costs and schedule a routine service appointment every spring, Dietz suggests. And keep in mind that the average central air-conditioning system has a lifespan of about 15 years. Especially if yours is getting close to that point, Dietz says, it’s worth having it inspected before a problem arises.

Even if it’s technically possible, though, constantly running your AC does have downsides, says Stan Cox, author of Losing Our Cool: Uncomfortable Truths About Our Air-Conditioned World. Your utility bills will soar, as will your energy usage. Air conditioners use about 6% of all electricity produced in the U.S. and account for the release of 117 million metric tons of carbon dioxide into the atmosphere each year, according to the U.S. Department of Energy. That means the systems we rely on for cooling are actively contributing to our warming planet.

Studies also suggest that the human body can build tolerance to heat through exposure—so completely avoiding warmth by shutting yourself in a refrigerated room at the first sign of spring thaw means you may be extra miserable come August. “Our comfort range is not fixed,” Cox says. “It moves up and down depending on the temperatures you’re exposed to.”

Why does air conditioning make you feel sick?

Some people experience respiratory symptoms, like a runny nose or scratchy throat, after sleeping in a heavily air-conditioned room. That may be the result of a dirty system, Siegel says, or it could be due to a lack of ventilation. Air conditioning also removes humidity from the air, Cox says, and that dryness can contribute to symptoms like sore throats and headaches.

Read More: How to Properly Cool Your Home With a Fan

Most of the time, these symptoms aren’t cause for great concern. But contaminated cooling systems have been linked to serious illnesses like Legionnaires’ disease and can also cause an allergic reaction that leads to lung inflammation.

Which type of air conditioner is most efficient?

In general, central AC is most efficient if you’re trying to cool an entire house because it has “economies of scale,” Dietz says. It’s designed to reach every nook and cranny of your house using one relatively compact system, whereas you’d need lots of window units or mini-splits if you cooled each room individually.

But, Cox says, it’s worth considering how much of your home actually needs to be cooled. You could, for example, place window units or mini-splits in your bedrooms for overnight use and simply use fans in living spaces. That may not be a viable option depending on your family’s schedule, living space, and local climate, but such a system “uses a lot less electricity than keeping the whole house cooled off continuously,” Cox says.

Is there a minimum or maximum temperature at which you should use AC?

Lots of people have rules about when they’ll turn on their AC for the first time—not before June 1, say, or not before the temperatures top 80°F—but those are just personal preferences, Dietz says. There’s no reason you can’t use your air conditioner on a mild day, aside from concerns about energy usage.

If you’re on the fence about whether it’s hot enough to turn on your AC, however, Cox recommends trying a fan first. You may be surprised by how well fans work, either on their own or as a way of reducing your air-conditioning usage. Research has even shown that people don’t feel any difference between cranked-up AC versus a fan paired with AC on a lower setting.

Read More: 8 Ways to Stay Hydrated If You Hate Drinking Water

“If the ambient air is cooler than your body, the fan is flooding your body with air that’s lower than your skin temperature and blowing away the warm air that accumulates around the surface of your skin,” Cox explains. Circulating the air around you also helps sweat evaporate from your skin, a key part of the cooling process.

That said, fans don’t work on scorching-hot days, when the air around you is hotter than your body temperature. In those cases, you should resort to air conditioning, either in your home or at a community cooling center.

You may notice that even your AC isn’t as effective as usual on an extremely hot day, because it has to work harder to transfer heat from your home’s interior to the outdoor air, Siegel says. Still, as long as your unit is properly maintained, it should work well enough to keep you cool and safe on even the hottest of summer days.