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Heat Waves Can Be Hard on the Heart

Soaring temperatures increase risks for heart attack and stroke

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It’s not just heat stroke you have to worry about on extremely warm days. Experts say high heat can increase your risk for an actual stroke, as well as a heart attack and other cardiovascular issues. In fact, research suggests the number of deaths from cardiovascular disease (conditions affecting the heart or blood vessels) may double or triple when heat reaches extreme temperatures.

A big reason: The heart has to work harder when it’s hot out, says Arash Harzand, M.D., a cardiologist and assistant professor of medicine at Emory University. It has to pump more blood as the body works to cool itself down. (When you’re hot, the blood in your body gets routed away from the internal organs to just under the skin, where it releases heat — much like a radiator.) The body also demands more oxygen when it gets overheated, and it’s the heart’s job to deliver it.

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Dehydration from heat can put additional stress on the heart, since water helps the heart pump blood through the blood vessels. When you’re dehydrated, “the tank is just emptier. And so with a less full tank, your heart has to work even harder to meet all that metabolic demand,” Harzand says.

Deaths from extreme heat have been increasing in the U.S. There were more than 2,300 heat-related deaths in the summer of 2023, according to federal data, up from 1,722 in 2022. Patricia Best, M.D., an interventional cardiologist and associate professor of internal medicine and cardiovascular diseases at Mayo Clinic, estimates that at least a quarter of these deaths can be attributed to cardiovascular disease.

Hot temperatures + pollution = a dangerous combination 

A study published in July 2023 in the journal Circulation found that the risk of a fatal heart attack among older adults in Jiangsu province, China, was 18 percent higher during two-day heat waves with heat indexes at or above the 90th percentile (ranging from 82.6°F to 97.9°F). Risks were 74 percent higher during four-day heat waves with heat indexes at or above the 97.5th percentile (ranging from 94.8°F to 109.4°F).

The most dangerous days, however, were those of extreme heat and high levels of air pollution — a heart-health risk in and of itself. The researchers estimated that up to 2.8 percent of heart attack deaths may be attributed to the combination of extreme temperatures and high levels of fine particulate pollution, which comes from sources like factories, cars, trucks and wildfires.

“And unfortunately, it’s becoming much more common these days when you have the confluence of wildfires and high temperatures,” says Sanjay Rajagopalan, M.D., a professor at Case Western University School of Medicine and chief of cardiovascular medicine at University Hospitals Harrington Heart & Vascular Institute.

When it comes to stroke, studies suggest that extreme temperatures — both hot and cold — can increase risk. What’s more, research presented at the American Stroke Association’s 2020 International Stroke Conference found that spikes in temperatures may influence the severity of strokes. The researchers found that for every 9°F increase in average temperature range over three days, the severity of strokes increased by 67 percent.

Risks for these heat-related cardiovascular events are not the same for everybody. Precautions are especially important for people with an underlying heart condition, high blood pressure or obesity. Individuals without access to air conditioning and shelter during high-heat days are also at greater risk, Rajagopalan says. 


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Heat-related cardiovascular deaths are expected to rise

Cardiovascular deaths from extreme heat are projected to increase by 162 percent in the U.S. by the middle of the century, new research suggests, even if currently proposed policies to reduce greenhouse gas emissions are implemented. If emission-reducing efforts are ignored or are minimal, heat-related cardiovascular deaths could shoot up by 233 percent over the next several decades, a 2023 study published in Circulation found. Black adults and people 65 and older are most likely to be affected by the impact of heat on the heart, the researchers note.

“The projections of this study focus on cardiovascular disease deaths, and, therefore, they represent conservative estimations of the adverse effects on cardiovascular health due to extreme heat,” American Heart Association volunteer Robert Brook, M.D., a professor of medicine and executive director of cardiovascular prevention at Wayne State University School of Medicine, said in a news release. “Nonfatal heart attacks, strokes and heart failure hospitalizations outnumber fatal events and are also highly likely to be linked with extreme heat days. The full extent of the public health threat, even just due to cardiovascular death, is likely much greater than presented in this study.”

Staying safe in the heat

The takeaway, Rajagopalan says, is to be aware of the risk so you can work to minimize it. “Many people don’t recognize that this could be life threatening,” he says.

Find a cool place to stick out the hot days — whether it’s at home or at a nearby cooling shelter. (Find them listed by state at the National Center for Health Housing.)

“Being in an environment where temperatures are regulated is of the utmost importance,” Rajagopalan says. What’s more, running your air conditioning during unhealthy air quality days can help to filter out pollutants and reduce the number of harmful particles in the air you breathe.

A few other tips:

Time it. If you have to go out in the heat, try to go out in the early or late parts of the day when the sun isn’t as strong. And don’t be afraid to take regular breaks. Find some shade and rest for a bit. In some areas of the country, the temperatures are high even in the evening. On those days, minimize all of your time outside, Best says.

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Stay hydrated. Some people with cardiovascular conditions are used to being told to watch their fluid intake and not overdo it, Harzand says. “But hydration is important,” he adds. People with high blood pressure and heart failure who are fluid-sensitive should talk to their doctors about the best ways to stay hydrated on hot days.

Dress for the heat. Wear lightweight, light-colored clothing, Harzand says. And don’t forget sunglasses, a hat and sunscreen.

Don’t stop taking your medications. “Don’t try and make changes to your medications because you think it’s better or safer to not take a certain medicine when you go outside,” Harzand says. If you have any concerns about your medication regimen and the heat, talk to your doctor.

Be on the lookout for warning signs of heat illness. Older adults are among those at higher risk for heat-related illness. Signs of heat exhaustion include heavy sweating, cold and clammy skin and weakness, among others. Heat stroke symptoms include red, hot and dry skin (no sweating); a fast and strong pulse; confusion, dizziness and nausea; and a throbbing headache. 

Editor's note: This story, originally published July 24, 2023, has been updated.

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