As a cardiologist in the largest city in the nation’s fastest-warming region, Ethan Katznelson has daily, first-hand knowledge of how high temperatures can put stress on the human heart.

Katznelson, who practices at New York Presbyterian Weill Cornell Medical Center, regularly sees the cardiovascular stress suffered by patients who live in homes without air conditioning, or climb steep stairs in multi-story apartment buildings with no elevators, or rely on public assistance to help cope with the heat in a city where residents feel almost 10 degrees hotter than their suburban neighbors because of the urban heat island effect.

He’s long understood the threat—but has wondered whether the same can be said of “the average doctor.” 

So he and a team of research associates set out to make that case as powerfully as possible, examining roughly 500 observational studies of the effects of high temperatures, extreme weather and wildfire smoke—all factors amplified by climate change—on cardiovascular problems.

Their findings, published last week in JAMA Cardiology, noted an increased risk for cardiovascular problems related to high heat that intensified the longer populations were exposed to the heat, particularly in normally cooler locations where buildings often don’t have air conditioning and heart patients aren’t as used to high heat.

The research team also found that extreme weather events such as hurricanes and floods not only increased the risk of cardiovascular health issues, but that the dangers exist long after the event itself.

The research team cited one study of the health impact of Hurricane Sandy in 2012, after which the risk of death from cardiovascular disease remained elevated for up to a year after the storm.

With cardiovascular disease already the leading cause of death worldwide, members of the research team said that they hope their work can help people take steps to address their health as the planet continues to warm.

“It’s really salient that people do think about their cardiovascular health and take cardiovascular events more seriously,” said Dhruv S. Kazi, an associate professor at Harvard Medical School, who was the first author of the review. “And I think we can advance the conversation on climate change. If we point out to people that there are these effects on cardiovascular health, cardiovascular disease is not theoretical or something that is going to happen in the future. This is here and now.”

Kazi said that among the most surprising findings by the research team was how high temperatures can affect people living in communities with older housing stock where central air conditioning is not already in place.

“The places that are at greatest risk are places where air conditioning is not widely available and people are not habituated to heat,” Kazi said. “So, ironically, even though, when you think about high temperatures, you say, ‘Oh, wow, Phoenix is going to be unlivable.’ What we’re going to see is that the Pacific Northwest is going to get into trouble at lower temperatures—starting in the mid-80s even—because people are neither habituated to high temperatures nor have regular access to air conditioning.”

The researchers were also struck by the wide reach of such extreme weather events such as the Canadian wildfires, which blanketed much of the nation with smoke last summer.

“People are going to get exposed, you know, hundreds of miles beyond where the fires are,” said Kazi, who also serves as the associate director of the Richard A. and Susan F. Smith Center for Outcomes Research at Beth Israel Deaconess Medical Center in Boston. 

“People saw that most dramatically last year in New York, where there were days of awful air quality, even though the fires weren’t even in the same country. They were all the way in Quebec,” he said. “So the fact that we’ve traditionally thought of wildfires affecting the American West, and that’s no longer true. There are people in Illinois and New York and Boston and Massachusetts who will get substantial exposure to a lot of fire, smoke and the related risks of cardiovascular disease.”

“The places that are at greatest risk are places where air conditioning is not widely available and people are not habituated to heat.”

While the research team was able to determine that people of color and those with low incomes are disproportionately affected by the threats of cardiovascular problems, Kazi said that it was essential that researchers continued to study how heat affects those communities, as well as those who live in the Global South.

“There’s a really glaring lack of data from low-income countries, in particular,” Kazi said. “There’s very little data coming out of Africa. And this is both a missed opportunity and a real threat because we know that lower wealth communities, particularly in the tropics, are going to have a pretty marked exposure to climate change related events and have limited resources to climate change resilience. And so there really needs to be a systematic effort to understand how that works.”

The review by Kazi, Katznelson and their colleagues is just one component of an expanding body of recently released research that explores the potentially perilous effects of heat.

In May, a team of researchers at the University of California-Irvine announced that they had identified the molecular components that adversely affect the brain, liver and digestive tract during heat waves.

By studying the heat-stressed tissues of mice, the study’s authors were able to identify changes in genes in the brain that are linked to motor dysfunction, cognitive impairment and a weakening of the blood-brain barrier.

“The liver-brain axis is a vital communication network influencing human health, connecting the gut, liver and brain,” the researchers wrote. “The impact of climate change on neurological diseases and gastrointestinal health, including the liver-brain axis, has been highlighted, with potential implications for metabolic liver diseases and associated neuropathologies.”

And in March, another group of researchers announced they had found that short-term exposure to high temperatures could increase inflammation and adversely affect the body’s immune system.

The research, which was presented at a conference hosted by the American Heart Association, found that for every 5 degree increase in the Universal Thermal Climate Index—a measurement of how the human body responds to air temperature and a range of other conditions—there was an increase in the levels of killer T-cells and other markers of inflammation and a decrease in B cells, which help regulate the body’s response to germs and disease.

Daniel W. Riggs, who teaches at the University of Louisville’s School of Medicine and was the lead author of the March study, said the findings could have particularly broad implications for those who regularly work outdoors.

“These findings for our project could be even more extreme for outdoor workers that are just exposed all day, more than the general population,” he said.

Taken together, Riggs said, the expanding study of how temperatures affect the body reinforces the conclusion that heat remains one of the biggest threats when it comes to climate change and human health.

“It’s probably the major environmental risk factor that is happening right now, but also it’ll indirectly affect other things like air pollution,” Riggs added. “So in combination with all these direct and indirect effects, I think it’s the major environmental risk right now for cardiovascular disease.”

Meanwhile, in New York, Katznelson said he is always thinking about how to counsel patients when it comes to heat waves—“which we know are going to be coming and more intense and more frequent.”

He added that he hopes the review will “spread awareness both about the problems, but also about the gaps in our knowledge and what needs to be filled in.”