How AC saves lives

During periods of extreme heat, access to AC can make a significant difference in health outcomes — especially for vulnerable populations. According to CDC data, heat-related deaths in the United States doubled from 1999 to 2023.

"The statistics on deaths from extreme heat are shocking, especially for the elderly and immunocompromised," said Jeffrey Siegel, professor of civil engineering at the University of Toronto.

But beyond its cooling effect, AC can also improve indoor air quality.

"A well-working AC system filters the air to reduce intake of dust, pollen, and airborne pathogens," said William Checkley, a pulmonary and critical care physician at Johns Hopkins.

In areas affected by outdoor pollution — including wildfires, vehicle exhaust, or industrial emissions — AC also helps reduce exposure to fine particles known as PM2.5.

"Every time the AC system operates, it's pulling some particles out of the air," said Brent Stephens, professor of civil, architectural, and environmental engineering at the Illinois Institute of Technology.

AC also lowers indoor humidity, which Fuchs says is a major benefit. This reduction helps minimize dampness and limits the growth of mold, bacteria, and dust mites — indoor allergens associated with conditions such as asthma and COPD.

Overuse and dryness create new health risks

While AC plays a protective role in extreme heat, experts caution that excessive or poorly balanced use can lead to new problems — particularly in the form of dry indoor air.

"Multiple studies show that spending too much time in air-conditioned rooms can lead to more respiratory symptoms," Checkley said.

One issue, Checkley says, is that overly dry air can inflame the respiratory tract. "If you're inhaling a lot of dry air, the human body adds water vapor to the inhaled air to maintain your internal humidity level," he said. That moisture leaves the body during exhalation, which over time can lead to dryness, coughing, and nasal congestion.  

According to Fuchs, this condition — also known as "sick building syndrome" — may also impair the lungs' natural defenses, making it easier for viruses to spread indoors.   

"Viruses may be more likely to circulate in drier conditions," Fuchs writes. Think of how often people get sick right after long trips inside a dry airplane. It's the same effect in people's homes or offices."

According to Checkley, "[t]he ideal humidity in a home is between 40% and 60%." Falling below or above this window can either dry out the airways or allow damp microbes to flourish.   

"There are indoor air conditions we can't perceive, which can be good or bad for us—it's not all about comfort," said Stephanie Taylor, a clinician and consultant on healthy indoor environments.

7 ways to reduce risks

Experts interviewed by TIME recommend several steps to ensure AC is protecting — not compromising — your health.

1. Monitor your indoor humidity.

Use a hygrometer or a thermostat with a humidity sensor to keep indoor moisture levels in the recommended 40% to 60% range.    

"If humidity drops below 40%, give the AC a rest," Checkley advises. "If humidity tops 60%, crank it back up." 

2. Change your filters regularly.

Replace AC filters every one to two months during peak use and use filters with a MERV rating of 11 or 13 for better protection against particles.

"The AC coil and ducts could become 'cesspools of mold, bacteria, and allergens,'" warns Taylor.

3. Schedule annual maintenance.

Have a professional inspect the system at least once a year — especially after any leaks or moisture issues — to ensure coils and ducts are clean.

"We only think about servicing it when it's not making cold anymore," Siegel said.

4. Clean window units before use.

Before reinstalling a window AC unit each summer, open it up and clean out any dust or buildup.

"Open it up and clean it first," Taylor advises.

5. Consider installing UV lights.

Some AC systems can be outfitted with UV lights to prevent microbial buildup. Be sure to use models certified by UL or ETL to ensure safety, Taylor notes.

6. Address volatile organic compounds (VOCs).

VOCs from furniture, cleaning supplies, and air fresheners can accumulate in sealed, air-conditioned homes. To minimize exposure:

• Open windows periodically when outdoor air is safe
• Use HEPA air purifiers with carbon filters
• Choose low-VOC or unscented products

"Indoor air has 2 to 5 times more VOCs than outdoor air," Stephens said.

7. Support your indoor microbe.

According to Taylor, managing humidity and reducing pollutants like VOCs can help maintain a healthier mix of indoor microbes — what she refers to as a building's microbiome.

"With optimal humidity and AC maintenance, your indoor microbiome supports healthier, more diverse microbes," Taylor said.

Related Advisory Board heat wave resources

For more Advisory Board resources related to the ongoing heat wave, check out this article on eight ways to stay safe during extreme heat, as well as this piece on how health systems should respond to rising temperatures.