Extreme temperature fluctuations linked to climate change could be a factor in roughly a half-million deaths and additional cases of disability from stroke around the world, according to a new study published in Neurology, the journal of the American Academy of Neurology.

While the study, which looked at data spanning three decades, doesn’t prove climate change directly causes stroke, it showed an association between “non-optimal temperatures, those above or below temperatures associated with the lowest death rates, were increasingly linked to death and disability due to stroke,” according to a news release from the academy.

“Dramatic temperature changes in recent years have affected human health and caused widespread concern,” study author Quan Cheng, of Xiangya Hospital Central South University in Changsha, China, said. “Our study found that these changing temperatures may increase the burden of stroke worldwide, especially in older populations and areas with more health care disparities.”

While the greatest number of stroke deaths were associated with cold temperatures, heat was associated with stroke, as well. And the academy said that stroke-related deaths are expected to increase in regions that have high temperatures.

The term global warming focused on rising temperatures, but climate change is now favored in part because the temperature extremes occur at both ends of the thermometer. While the world is reportedly getting hotter overall, as CNN explained in January, many scientists believe that the wetter-than-normal, colder-than-normal weather in different places are also a byproduct, creating “icy blasts, as warming in the arctic increases the likelihood that frigid, polar air can sweep southwards.”

The mechanisms for how hot and cold weather may lead to stroke are, not surprisingly, very different. “With lower temperatures, a person’s blood vessels can constrict, increasing blood pressure. High blood pressure is a risk factor for stroke,” per the academy. “Higher temperatures may cause dehydration, affecting cholesterol levels and resulting in slower blood flow, factors that can also lead to stroke.”

Study nuts and bolts

The study looked at health records spanning three decades in 204 countries and territories to see the number of stroke deaths and stroke-related disability because of temperatures that were outside the optimal range. They also “divided the data to look at different regions, countries and territories,” then considered age and gender.

In 2019, of 521,031 stroke deaths linked to temperature, 474,002 were associated with low temperatures, the rest with high temperatures. More men than women died from strokes associated with temperature change, according to the study.

The study in addition noted 9.4 million “disability-adjusted life years” due to strokes that might be temperature related. It defined that as the number of years stolen by “premature death and years lived with illness.”

Dangers of extreme weather

Last summer, an estimated 45 million people in the United States faced extremely hot temperatures, as Deseret News reported. The list included Arkansas, Florida, Kansas, Louisiana, Missouri, Oklahoma and Texas. USA Today at the time described it as a “heat dome.”

In 2021, shortly after devastating cold temperatures shrouded a large swath of the country, Deseret News looked at why weather is especially hard on older adults. Both hot and cold temperatures can be deadly.

“Extreme heat or cold is no joke for older people, including older workers. And they may be in medical crisis before they even recognize it,” said Dr. Ronda McCarthy, an environmental and occupational exposure specialist near Waco, who’s on the steering committee for the Medical Society Consortium on Climate and Health, in the article.

How extreme weather kills older adults and how to keep loved ones safe

According to the National Institute on Aging, older adults lose heat faster than younger people and yet may not realize they are cold. And being too hot can kill, too. Both can be dangerous for anyone. The difference, based on age, is what counts as extreme. Older adults can be dangerously hot in 80 degrees, or too cold below 70 degrees. That varies with the individual.

A study in the New England Journal of Medicine said in the U.S., at least 1,500 deaths are related to cold temperatures.

While stroke is sometimes the result, many factors can be at play. The internal thermostat of older people may not work as well as in the past, for example. And older adults may take medicine that interferes with temperature regulation. Plus, older adults are more likely than younger adults to have existing heart disease, which extreme weather can impact, including by triggering stroke.

For those wanting to know more about stroke, the academy has a website with a free patient and caregiver magazine at BrainandLife.org.