Should you run the AC when air quality is bad or weather is bad?

When the air quality is poor outside, is it safe to use your air conditioner? Here’s what doctors say.

With millions of Americans facing poor air quality alerts for the third day in a row, you may be wondering if you can run your AC while also keeping the air inside your home as clean as possible.

Even with the windows and doors closed and sealed, it’s still possible for some pollution to make its way into your home. And certain activities around the house, such as vacuuming or burning essential oils, can actually make indoor air quality worse. And turning on certain devices, like your kitchen vent, may even bring the outdoor air inside.

Here’s what to know about running your AC when air quality is bad and how to stay safe.

Should you run your AC when air quality is bad or when it’s smoky outside?

Yes, you can run your AC when air quality is bad, as it is currently in much of the United States due to the wildfire smoke from Canada — but only do so if it’s recirculating inside air, which means that it’s not pulling air in from the outdoors, Dr. Roshini Rajapaksa of NYU Langone said in a June 8 segment on TODAY.

When it’s smoky outside, air conditioners that pull air from outside can bring fine particulate matter, or tiny airborne particles, from the smoke into your home, which can have negative health effects.

Many window units pull in air from outside into the home and do not have strong filtration systems, whereas centralized AC and AC units attached to walls tend not to pull in air from the outside and usually filter well the little bit that might get inside, according to Good Housekeeping’s experts.

The most effective type of air filter for fine particulate matter, used in many air conditioners and HVAC systems, are HEPA filters, which “theoretically remove remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns,” according to the United States Environmental Protection Agency.

When running AC in poor air quality, it’s important that its filter is up to date, so consider replacing it.

You can also buy air purifiers with HEPA filters, which are not only safe to run when the air quality is poor — it’s recommended, if you have one.

Azar stressed the importance of air circulation when spending time indoors (which is highly recommended) when air quality is poor outside. Air conditioners that are not pulling air from outside are one way to circulate air indoors, but you can also use box fans.

If you’re going to use a built-in fan or vent in your home to circulate air, such as in the kitchen or bathroom, make sure it’s not pulling in air from outside, Azar added.

What happens if you go out in unhealthy air quality?

If you go outside in unhealthy air quality, especially from wildfire smoke, you inhale fine particulate matter, or small particles found in dust, smoke, vehicle exhaust, etc., which can cause airways to constrict.

“When we inhale, the particles (from smoke) stimulate our airways to become inflamed to not function normally,” Dr. Ronald Crystal, a pulmonologist and the chair of genetic medicine at Weill Cornell Medicine, previously told

For healthy individuals, i.e. people without respiratory conditions such as asthma, COPD or post-COVID breathing problems and people without underlying heart problems, the effects of breathing in poor quality air are likely to be short term, such as irritation, like a tickle in your throat, Crystal said.

Examples of short-term symptoms due to unhealthy air quality include light coughing, eye burning or irritation, headaches, fatigue and chest tightness. (If you’re concerned by any of your symptoms or you start to have trouble breathing, wheezing, heavy coughing or dizziness, seek medical attention.)

Inhaling fine particulate matter can also increase risk of infection and worsen allergies, Azar said.

How to keep indoor air safe when air quality is poor

When air quality is poor, it’s important to spend as much time inside as possible, especially for pets, children, pregnant people and those with underlying health conditions.

To keep your indoor air as safe as possible, experts recommend:

  • Use an air purifier, or an air conditioner that pulls in inside air and has an up-to-date, effective air filter. If your air purifier is small, run it in a smaller room and say in there as much as possible.
  • Keep doors and windows closed and sealed.
  • Run a box fan.
  • Avoid running any fans that pull in outside air, such as hood vents in the kitchen or vents in the bathroom.
  • Avoid burning essential oils or candles.
  • Avoid smoking or vaping inside.
  • Avoid vacuuming.
  • Avoid cooking foods indoors that will produce a lot of smoke or fumes, such as frying or grilling.

Crystal speculated that the unhealthy air quality will dissipate in the next few days. So it’s important to continue to check the air quality in your area before going outside and take appropriate precautions, such as wearing a N95 or KN95 mask, avoiding exercising or other strenuous activities outside and limiting time outdoors.

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