Extreme heat and pollution can double the risk of a fatal heart attack

Exposure to extreme temperatures combined with suffocating air pollution can double the risk of dying from a heart attack, according to Chinese researchers who analyzed more than 200,000 cardiac deaths in China between 2015 and 2020.

Experts, who already believe that prolonged heat waves, cold snaps and polluted air are bad for the heart, said the study, published Monday in the journal Circulation, further strengthens the relationship by connecting it to the risk of cardiac death.

“As far as I am aware, this is one of the first studies that looks at death from heart attack as the end result of this type of exposure, and it does not surprise me,” said Catharina Giudice, an emergency medicine physician and a fellow at the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health, who was not involved in the study.

“We’re seeing record-breaking heat after record-breaking heat year after year,” she said. “It’s getting hotter, lasting longer and happening more frequently. Heat makes cardiovascular disease worse. Pollution makes it worse. The two together are worse than each one independently.”

Heart disease is the leading cause of death in the United States, killing about 695,000 Americans in 2021, according to the Centers for Disease Control and Prevention.

The study, funded by China’s Ministry of Science and Technology, comes against a backdrop of prolonged and sweltering record-setting heat that has been smothering the United States and other countries, combined with industrial and wildfire air pollution, all fueled by a warming planet.

The dangerous health effects of climate change are well documented, including threats from the spread of infectious diseases, food insecurity, the worsening of seasonal allergies, the risk of dementia and the deadly effects of heat waves, floods, drought and wildfire smoke. Existing evidence also suggests that climate change-induced stress is tied to heart problems. Several studies by cardiologists in New Orleans, for example, found a dramatic jump in the number of heart attacks in the years following the devastation wrought by Hurricane Katrina.

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(China and the United States are the world’s largest emitters of greenhouse gases, the pollutants that drive climate change.)

Greater risk for women and older adults

The researchers examined the effects of extreme temperatures with and without high levels of fine particulate pollution on 202,678 heart attack deaths between 2015 and 2020 that occurred in Jiangsu province, a region with four distinct seasons and a wide range of temperatures and fine particulate pollution levels.

They found that days with extreme heat, extreme cold or high levels of fine particulate matter air pollution were significantly associated with the risk of death from a heart attack, especially for women and older adults, those with an average age of about 77.

The greatest increases occurred on days with both extreme heat and high levels of PM2.5 (Particulate Matter), which are particles less than 2.5 microns in width, (for comparison, the width of a human hair is 50 to 100 microns) estimating that up to 2.8 percent of the heart attack deaths were related to the combination of extreme temperatures and high levels of fine particulate pollution of more than 37.5 micrograms per cubic meter. (A microgram is a unit of mass equal to one-millionth of a gram.) They also found that heat waves interact synergistically with the fine particles, while cold spells do not.

Inhaling these microscopic particles — the result of fuel combustion from cars and factories, and wildfire smoke — deep into the lungs can irritate them and the blood vessels around the heart. Research has linked their exposure to heart disease, stroke and other health issues, including dementia. The World Health Organization’s target for average annual exposure to fine particulate pollution level is no more than 5 micrograms per cubic meter and no more than 15 micrograms per cubic meter for more than three to four days per year.

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Compared with control days, the risk of a fatal heart attack was 18 percent higher during two-day heat waves with heat ranging from 82.6 to 97.9 degrees Fahrenheit, and 74 percent higher during four-day heat waves with temperatures between 94.8 and 109.4 degrees Fahrenheit. The risk was 4 percent higher during two-day cold snaps with temperatures between 33.3 to 40.5 degrees Fahrenheit, and 12 percent higher during three-day cold snaps of 27 to 37.2 degrees Fahrenheit.

The researchers measured extreme temperatures according to the daily heat index for an area, which records the combined effect of both heat and humidity, and also evaluated the length and severity of heat waves and cold snaps.

They compared heart attack deaths or “case days” with control days on the same day of the week in the same month — if a death occurred on a Wednesday, all other Wednesdays in the same month were regarded as control days. Days with an average level of fine particulate matter above 37.5 micrograms per cubic meter were considered high air pollution days.

“It’s pretty amazing that they started seeing these increases with temperatures over 90 degrees,” said Mark Link, a professor of internal medicine at UT Southwestern Medical Center in Dallas, and a member of its cardiology division, who was not part of the study. “In Dallas, we would call 90 degrees a cool day, but that’s when they saw mortality start to increase — and the combination of high heat and pollution was the deadliest. It’s pretty remarkable when you think about what’s happening down here now, where the average high is 102 or 103 degrees.”

On such days, “emergency visits for cardiac issues and everything else are up,” he said.

Lifestyle changes may not be enough to prevent heart disease

Yuewei Liu, associate professor of epidemiology in the School of Public Health at Sun Yat-sen University in Guangzhou, China, and the study’s senior author, said it was still unknown how these exposures trigger a greater risk of dying of a heart attack. Such questions remain “a great public health challenge due to its substantial disease burden worldwide,” he said.

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Health-care professionals typically urge lifestyle modifications to prevent heart disease, such as avoiding or quitting smoking; controlling hypertension, high cholesterol and diabetes; losing weight; and exercising. Now, these may not be enough, experts said.

This study demonstrates “we cannot ignore the environment around us,” said Hitinder Gurm, an interventional cardiologist and chief medical officer at the University of Michigan, who was not part of the study but whose research has focused on temperature deviations and the risk of heart attacks. “Air pollution and extreme weather are emerging as important cardiac risk factors and require both individual and community level interventions.”

What to do during heat waves and high pollution days

During heat waves and high pollution days, experts recommend that people:

  • Wear an N95 mask outdoors in areas of high pollution or fire.
  • Stay inside when it is excessively hot.
  • Drink lots of fluids.
  • Follow the weather forecasts and monitor air quality levels.
  • Use fans and air conditioners during hot weather.
  • Install window blinds to reduce indoor temperatures.
  • Use air purifiers to reduce indoor pollutants.
  • Avoid busy highways when walking.
  • Choose less strenuous or indoor exercises.

Gurm noted that most heart attacks still occur in people with risk factors, but said additional measures were necessary to “safeguard the most vulnerable from exposure to severe weather conditions and poor air quality.”

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