(CNN) — Extreme heat can take a heavy toll on the heart, and a new report shows how much more deadly the effects of climate change may become over the next few decades in the United States. Black adults, seniors and people living in urban areas are particularly vulnerable.
Between 2008 and 2019, the heat index – which accounts for both temperature and humidity – reached at least 90 degrees on an average of 54 days each summer in the US. Those days of extreme heat were associated with nearly 1,700 excess cardiovascular deaths each year, according to a study published Monday in the journal Circulation.
If fossil-fuel development continues to expand globally and the world only makes minimal efforts to reduce planet-warming pollution, there could be 80 days of extreme heat each summer and the number of heat-related cardiovascular deaths in the US could more than triple – to about 5,500 excess deaths per year, the researchers found.
Even a more plausible scenario, where planned and ongoing climate change mitigation measures are put into place, could lead to a jump to 71 days of extreme heat per year and a 2.6 times increase in heart-related mortality – with more than 4,300 excess cardiovascular deaths related to excess heat by the middle of the century.
Heart disease is the leading cause of death in the US overall and extreme heat causes a small share of total cardiovascular deaths in the US — about 1 in 500 right now, said Dr. Lawrence Fine, a senior adviser at the National Institutes of Health’s National Heart, Lung, and Blood Institute, which partially funded the new research.
But as hot days become more common, risk mitigation becomes more important, he said.
“The thing about heat-related deaths is that they’re concentrated to when it’s very hot, and they’re also concentrated in people who are at greater risk because of their health conditions or other conditions,” he said. A heat wave can strain emergency rooms and the broader health care system and pose a severe threat to certain individuals.
“It’s important to address the root causes of the increase in temperature and heart disease, but we also want people to know if they’re vulnerable and to have a specific plan for what to do when they find themselves in a very hot environment.”
The new study’s findings are based on projections about population growth and migration in the US, as well as trends in greenhouse gas emissions from a report by the United Nation’s Intergovernmental Panel on Climate Change. The analysis captured projections for the years 2036 through 2065, based on county-level data for the 48 states in the contiguous US.
This summer was an alarming example of how dangerous extreme heat can be. Officials in Maricopa County, Arizona, reported last week that 469 people there have died of heat-associated illness this year, with more than 150 deaths still under investigation, making 2023 the deadliest year for heat deaths since the county began tracking them in 2006.
Many of those deaths came as temperatures soared to record-breaking levels in the Southwest this summer, and Phoenix logged 31 consecutive days at or above 110 degrees Fahrenheit from June into July.
Heat-related fatalities have risen dramatically in the US in recent years. In 2022, more than 1,700 deaths were due to heat-related causes, according to an analysis of data from the US Centers for Disease Control and Prevention – more than doubling over the past five years. And that data is likely an underestimate, experts say, because extreme heat exposure isn’t always well documented.
The heart is particularly susceptible to the effects of heat.
The human body can only operate within a narrow temperature range, and the heart plays a critical role in keeping the system regulated, said Dr. Sameed Khatana, an assistant professor of cardiovascular medicine at Penn Medicine and senior author of the new study. When the heart has to work harder than it’s used to, the consequences can be deadly for some.
“When the body senses that its temperature is going up, various things start kicking into gear. A key component of that is that the heart beats faster and harder to get blood away from the core of the body, to transport heat away from the vital organs,” he said. “For people with pre-existing cardiovascular diseases, their heart might not be able to keep up with the increased demands on the cardiovascular system that temperature is causing.”
Longer exposures to heat can also lead to more complex changes such as increased inflammation and blood clotting that can raise the risk for heart attack and stroke, he said.
Black adults in the US are particularly vulnerable to the heart-related harms of extreme heat. Over the next few decades, heat-related cardiovascular deaths could grow six-fold among Black adults in the US, according to the scenarios analyzed in the study – compared with a projected 2.4 times increase among White adults in the worst-case scenario.
Seniors age 65 and older and adults living in metropolitan areas are also projected to be disproportionately affected. Demographic shifts in the US – such as an aging population, diversification and growth in cities – could exacerbate baseline vulnerabilities.
“In some ways, you could argue that no one should be dying due to heat exposure. There is a simple solution: you just get someone to a cool environment,” Khatana said. “But like most public health issues in the United States, the health impacts of climate change – and extreme heat, specifically – are also health equity issues.”
People who are vulnerable to heat exposure are likely vulnerable in many other aspects of their lives – perhaps living in areas with less tree cover or without access to air conditioning. Black people are also more likely to live with conditions that put their heart health at risk, including higher rates of hypertension and diabetes.
“Solutions need to be targeted at people who are the most vulnerable,” Khatana said. “If mitigation efforts are not taken, if efforts to reduce emissions aren’t made, then these inequities that we’ve already seen might continue to widen.”
