When the human body is exposed to extreme heat, it tries to fight back. To keep us from cooking, our hearts pump faster and harder to distribute the hot blood out to our fingers and toes, away from precious internal organs. We produce more sweat, and when it evaporates, the blood beneath the skin’s surface cools down, helping to lower our body temperature.
Heat stroke occurs when the body can’t cool itself down enough, causing organ damage. But other deadly cardiovascular problems can arise on the road to heat stroke, especially for people already at increased risk for conditions like heart disease. If someone’s blood pressure is too low or if their heart is beating too fast in higher temperatures, a heart attack, stroke, or heart failure could follow.
Between 2008-2019, extreme heat was associated with 1,651 excess cardiovascular deaths like this each year in the U.S., according to a new study published in Circulation. The study predicts that, as a result of climate change over the coming decades, these deaths could increase by as much as 233% annually.
Just how big of an increase in excess cardiovascular deaths the U.S. will face depends on how much greenhouse gas emissions continue to rise. For the study, researchers used two potentially plausible projections for what extreme heat in the U.S. will look like in the mid-century, between 2036-2065.
First, there’s the more ambitious, optimistic possibility, in which the U.S. sees “intermediate” increases in greenhouse gas emissions, having been curbed by global climate efforts. In this scenario, excess cardiovascular deaths in the U.S. may increase by 162%, to 4,320 excess deaths annually. The even more dire prediction, based on a large increase in emissions, showed a 233% increase, to 5,491 excess deaths annually. Researchers also accounted for the different potential socioeconomic trajectories and population changes based on each scenario in their calculations.
“The lower range of that mid-century period is just over a decade away,” said Sameed Khatana, an assistant professor of cardiovascular medicine at Penn Medicine and senior author of the study. “The rapid increase was somewhat shocking to me, especially in certain subgroups.”
Non-Hispanic Black adults in the U.S. are projected to have more than four times the increase in heat-related cardiovascular deaths than non-Hispanic white adults, according to the study. Elderly adults will also see greater increases in deaths compared to younger adults.
“If we as a society don’t want these preexisting disparities to continue to widen, I think trying to mitigate the impact of climate change on people’s health is something that we need to really pay a lot of attention to,” Khatana said.
The researchers did not break down different types of cardiovascular deaths in their calculations, but previous research links heart failure to the highest number of excess deaths in both extreme heat and extreme cold.
But extreme temperatures aren’t the only factors related to climate change that affect cardiovascular disease and deaths. Air pollution, especially from tiny particulate matter, can also have deadly consequences. The World Health Organization estimates that outdoor air pollution caused 4.2 million premature deaths worldwide in 2019.
“So this is new. It’s important. It is a comparatively smaller direct impact from the heat than our other environmental factors,” said Robert Brook, the executive director of cardiovascular prevention at Wayne State University School of Medicine and an American Heart Association volunteer who was not involved with the research.
The increase in excess deaths will likely be even greater in countries and regions with even more extreme heat, more population density, and fewer resources than the U.S.
“This is a bad harbinger for the rest of the world that is less climate-resistant generally than we are,” Brook said.
In order to prevent the study’s predicted future, Khatana said that heat needs to be recognized as a public health issue. Hospitals could fill a gap, working to become cooling centers for their communities.
It’s also important for providers to recognize who might be at the highest risk. In his own clinic, Khatana sees patients who report difficulty trying to exercise in high temperatures. They have a harder time breathing and can feel their heart beating faster.
On the individual level, it can be tricky to pinpoint heat as a factor in a cardiovascular death, which is why many people call heat the “silent killer,” Khatana said. It’s possible that his calculations are under-estimates. No matter what, he said, large-scale changes are required to curb the heat’s deadly effects.