Your air conditioning questions, answered

HAir conditioners are the soundtrack of summer in the US, as almost 90% of families use them for cooling. Although air conditioning is everywhere, many people still have questions about how exactly it works, how to best use it 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 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 to an indoor coil, where it evaporates and extracts heat from the surrounding air. The gaseous refrigerant then flows to the system’s outdoor coil, removing heat from your home. The gas condenses back into a liquid on the system’s external coil and the process begins again.

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

How do air conditioners affect air quality?

It depends on what type of system you have. Window units and mini-splits typically don’t have high-quality filters, so they don’t contribute much to air quality beyond temperature regulation, says Siegel. Central air conditioning systems, however, can provide valuable filtration, capturing particles such as dust, pet dander, mold and smog.

The higher the minimum efficiency reporting value of a filter, or MERV, the more he can capture. High MERV filters can even help remove viruses from indoor air, according to the US Environmental Protection Agency, but not effectively enough to rely solely on your AC system for disease prevention. If you have central AC, be sure to run the fan for at least 20 minutes per hour, as air only passes through the filter when the fan is actively running, says Siegel.

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The downside is that rooms sealed for cooling do not receive much, if any, fresh air ventilation. “Dilution with outside air helps get rid of a lot of things,” says Siegel, so it’s a good idea to turn off your air conditioning and open your windows for at least part of the day, as long as outdoor conditions allow. “Sick building syndrome”, which makes people feel unwell when they spend a lot of time indoors, is associated with inadequate ventilation.

Do you really need to change your air conditioning filter?

In a word: yes. A dirty filter won’t do your health any favors, especially if it harbors bacteria or other dirt that causes illnesses or allergies.

But the health of your cooling system may be an even more pressing concern. “If you let that gunk build up in your filter, you can destroy your system,” says Francis Dietz, vice president of public relations at the Institute of Air Conditioning, Heating and Refrigeration, a manufacturers trade association. “The air can no longer pass through this filter, so it goes back into the system and freezes.” (Without a total system breakdown, Siegel says, you might just 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 the system, says Siegel, but roughly every three months is a good rough estimate.

Is it normal to run the air conditioning all the time?

From a mechanical standpoint, a properly maintained system should run 24/7, says Dietz. But an old or neglected unit can die under the pressure of constant use—hence the reason many people’s systems “shut down” during a heat wave, he says. Save on emergency repair costs and schedule a routine maintenance appointment every spring, suggests Dietz. And keep in mind that the central air conditioning system has an average lifespan of about 15 years. Especially if yours is getting close to that point, says Dietz, it’s worth getting it inspected before a problem arises.

Even though it’s technically possible, running your AC constantly has drawbacks, says Stan Cox, author of Losing Your Cool: Uncomfortable Truths About Our Air-Conditioned World. Your utility bills will increase, as will your energy usage. Air conditioners use about 6% of all electricity produced in the US and are responsible for releasing 117 million metric tons of carbon dioxide into the atmosphere every year. according to the US Department of Energy. This means that the systems we rely on for cooling are actively contributing to the warming of our planet.

Studies also suggest that the human body can develop a tolerance to heat through exposure – so avoiding the heat altogether by locking yourself in a cool room at the first sign of spring thaw means you could find yourself even more miserable come August. “Our comfort range is not fixed,” says Cox. “It moves up and down depending on the temperatures you are exposed to.”

Why does air conditioning make you feel bad?

Some people experience respiratory symptoms, such as a runny nose or sore throat, after sleeping in an air-conditioned room. This could be the result of a dirty system, says Siegel, or it could be due to a lack of ventilation. Air conditioning also removes moisture from the air, says Cox, and this dryness can contribute to symptoms like sore throats and headaches.

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Most of the time, these symptoms are not a cause for major concern. But contaminated refrigeration systems have been linked to serious illnesses like Legionnaires’ disease and can also cause an allergic reaction which leads to lung inflammation.

Which type of air conditioning is most efficient?

In general, central AC is more efficient if you’re trying to cool an entire house because it offers “economies of scale,” says Dietz. It’s designed to reach every corner of your home using a relatively compact system, whereas you’d need many window units or mini-splits if you cooled each room individually.

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

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

Many people have rules about when they’ll first turn on the air conditioning—not before June 1, say, or not until temperatures reach 70 degrees Fahrenheit—but those are just personal preferences, says Dietz. There’s no reason you can’t use your air conditioner on a mild day, other than concerns about energy use.

If you’re unsure whether it’s hot enough to turn on the air conditioning, Cox recommends trying a fan first. You might be surprised at how well fans work, whether on their own or as a way to reduce air conditioning use. The research even showed that people don’t feel any difference between AC on and a fan paired with AC on a lower setting.

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

That said, fans don’t work on scorching days when the air around you is hotter than your body temperature. In these cases, you must use air conditioning, whether in your home or in a community refrigeration 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 inside your home to the air outside, says Siegel. Still, as long as your unit is properly maintained, it should work well enough to keep you cool and safe on even the hottest summer days.