Heart attack: Risk of death soars on days of heat and pollution

June 2023 was Earth’s hottest month in the 174 years that scientists have been tracking global temperatures, and July appears to be every bit as torrid. More than 1,000 high-temperature records have been broken in the United States so far this summer.

The heat has also dried out large swaths of vegetation, creating a massive source of wildfire fuel.

About 1,090 active fires are currently burning throughout Canada as of this writing, and wildfires have been burning there for weeks. The wildfires have sent continuous smoke clouds of fine particulate matter aloft, affecting a wide geographic area and potentially threatening the health of millions of North Americans.

A new study from researchers in China describes how dangerous the combination of extreme hot weather — and to a lesser extent, extreme cold weather — and airborne particulate matter can be.

The study found that extreme high temperatures combined with fine particulate matter in the air — such as the sort sent aloft by wildfires — can double one’s risk of a myocardial infarction, or heart attack.

The fine particles discussed in the article are called PM 2.5, which is short for “particulate matter, 2.5 micrometers or smaller.” They are tiny pieces of solids or liquids floating in the air, and may or may not be visible.

According to the National Institutes of Environmental Health Sciences, PM 2.5 consists of an ever-changing slurry of sulfates, nitrates, carbon, or mineral dusts.

While PM 2.5 may come from construction sites, cars, and other gas-burning vehicles, factory smokestacks, and unpaved roads, their main sources, says the Center for Disease Control (CDC), are fires and wildfires.

The new study analyzed 202, 678 myocardial infarction deaths in Jiangsu province, China, from 2015 to 2020. This is an area with four seasons, and thus a wide range of high and low temperatures. The researchers aligned weather patterns with heart attacks to derive their insights.

The study is published in Circulation.

Cardiologist Dr. Rigved Tadwalkar, from the Pacific Heart Institute in Santa Monica, CA, not involved in this study, explained to Medical News Today that the tiny size of these particles allows them to reach places in the human body that larger particles cannot.

Dr. Tadwalkar said “[t]hey’re so small, and can sort of integrate themselves within the bloodstream, and that can lead to inflammatory responses.”

This can result in oxidative stress, “[a]nd we know that this is sort of like the central mechanism behind how damage occurs to the blood vessels and also the heart,” he added.

Dr. Tadwalkar described “a cascade of events that leads to a common problem that we see in cardiology, which is the creation of atherosclerosis or plaque that can lead to lack of blood flow risk for heart attack, cardiovascular events, et cetera.”

It is difficult to determine an ideal temperature for humans, said epidemiologist Dr. Rakesh Ghosh because we are resilient and can acclimatize to different temperatures.

Dr. Ghosh, who was not involved in the current study, is a specialist at the Institute for Health & Aging in the School of Nursing at the University of California, San Francisco.

“In other words,” said Dr. Ghosh, “what is a heat wave for Europeans will not be so unbearable for South Asians because for most of the year, they encounter hot weather and winters are not as cold in South Asia as they are in Europe.”

“It’s amazing what the human body can sort of get used to,” said Dr. Tadwalkar.

It is this acclimatizing that led the authors of the new study to assess the effects of heat and PM 2.5 based on typical temperatures in Jiangsu province.

The study found that during two-day heat waves with temperatures at or above the 90th percentile for normal weather — 82.6 degrees to 97.9 degrees Fahrenheit — the risk of dying from a heart attack increased by 18%.

During 4-day heat waves above the 97.5th percentile, the risk was increased by 74%, and on such hot days with high PM 2.5, double that.

“The idea,” Dr. Tadwalkar said, is we require a climate “where we can have a bit of a balance between the production of body heat to keep ourselves warm, and heat loss, without it really causing strain on the body as well as the cardiovascular system.

Dr. Ghosh raised the issue of temperatures that are so extreme that they affect our core body temperature: “The interesting part is core body temperature in humans is maintained within a very narrow range, around 37 degrees Celsius [98.6 degrees Fahrenheit]. What happens to that core body temperature when exposed to extreme temperature is unknown.”

The new study also found an effect, though smaller, in the combination of extreme cold and high PM 2.5 counts. During 2-day cold snaps with poor air quality, the risk of myocardial infarction increased by .04%, increasing with lower temperatures and duration, and during a 3-day bout, 12%.

Dr. Tadwalkar speculated that extreme cold’s lesser effects may have to do with the idea that people tend to move inside when it is that cold, thus reducing their exposure to the weather and risk.

He also suggested that it could be that the dilation of blood vessels caused by heat encourages the distribution of PM 2.5 throughout the body, whereas cold may have the opposite effect, slowing down their incursion.

On days when the local air quality — a reflection of current PM 2.5 air content — is high, try to stay indoors as much as possible. Use air conditioning if you can, blowing out, with no outside air coming in. If you lack air conditioning and have bathroom fans or stove fans that vent to the outside, keep them turned on.

Wearing an N95 face mask can also limit the ability of PM 2.5 to enter your body.

For real-time information regarding the amount of PM 2.5 in the air where you are, visit the United States government’s AirNow website. There are also free AirNow apps for iOS and Android phones.

To deal with extreme heat, said Dr. Tadwalkar, “Staying hydrated is key, since the body is losing water through sweating to cool the body.”

“Break the prolonged period with intermittent short gaps,” said Dr. Ghosh, in “air-conditioned areas so that you’re just exposed to cooler temperatures.” Dr. Tadwalkar described the value of cool-air breaks “that help your body regulate things throughout the day as opposed to constantly being exposed to the heat.”

“Wear light and loose clothes,” said Dr. Ghosh, “including wide-rim hats.” He also recommended remaining aware of “heat-related illnesses such as heat cramps, heat exhaustion, and heat stroke, so that [you] can seek immediate help in [an] emergency.”