In 2019 alone, more than half a million people died due to a stroke linked to high and low temperatures, a new study found. With the world getting warmer due to human-made climate change, that number is expected to rise.
The study, published Wednesday in the medical journal Neurology, found that since 1990, the number of strokes attributable to high and low temperatures have been increasing all around the globe. Men had more strokes related to extreme temperatures than women, but it affected people across all age groups.
For this study, researchers looked at temperatures and strokes in 204 countries and territories. Researchers from Xiangya Hospital Central South University in China created a model using global data on disease, deaths and disability and climate data that captures temperatures, cloud cover and weather variables.
The study authors noted that the number of people having strokes has risen as the population ages and grows, but this doesn’t account for everything. “Nonoptimal temperatures” made a difference: The number of people who had a stroke due to hot and cold temperatures grew and was significantly larger in 2019 than in 1990.
In 2019, it was low temperatures that led to the higher number of strokes. While that may sound counterintuitive for global warming, cold temperatures also come along with climate change. Warmer temperatures on land interfere with the polar vortex — the dense cold air mass around the poles — and when it is weakened, it can lead to cooler temperatures.
Right now, stroke deaths connected to extreme temperatures are disproportionately concentrated in parts of the world with with a higher levels of people living in poverty and where health care systems are fragile, like in Africa. The study said the rapid increase in stroke burden due to high temperature in Central Asia “also requires special attention.”
As the planet gets warmer, the study said, the burden of strokes due to high temperature “has increased rapidly” and that number will grow “sharply” in the future.
Higher temperatures are already here. Last year was the warmest since scientists started recording global temperatures in 1850 and temperatures are expected to break more records in the near future. This March was the hottest one on record.
Dr. Mary Rice, an associate professor of medicine at Harvard Medical School who did not work on the new study, said its findings are significant.
“I really think that group did a very nice job of taking a global approach looking at historic data and to draw attention to a health issue that I think it’s not really getting a lot of attention,” said Rice, a pulmonologist at Beth Israel Deaconess Medical Center in Boston. “The total burden of people who died from strokes from temperature is actually a very large number.”
Rice recently published a study in Frontiers in Science that found climate change is also driving an increasing number of immune-mediated diseases such as allergies, asthma, autoimmune diseases and cancers. Rice’s study suggests that multilevel mitigation actions are urgently needed to reduce emissions and to improve air quality while addressing the climate crisis.
Without immediate global action, the world is going to see a much greater burden of disease, she said.
‘It’s happening across the board’
Stroke is already a significant health problem. It is the third leading cause of disability worldwide, and one of the leading causes of death, earlier studies showed.
The new study wasn’t designed to show why extreme temperatures that come with the climate crisis seem to be causing so many strokes. Other research has shown that when temperatures are excessively hot, it’s difficult for the body to regulate and cool itself by sweating. This can lead to what doctors call a hypercoagulable state of the blood, when the blood clots easier and increases the risk of stroke. People may also become dehydrated, which can force the heart to work too hard, also increasing a person’s chance of having a stroke or heart attack.
Extreme cold temperatures also can lead someone to have a stroke. When the body is exposed to cold, it stimulates the skin’s cold receptors which triggers what’s known as the sympathetic nervous system, the network of nerves that control the body’s fight or flight response. That can cause vasoconstriction, the constriction of blood vessels in the skin, arms, and legs, leading blood pressure to spike and potentially leading to a stroke.
Dr. Ali Saad, a neurologist who’s affiliated with the Climate and Health Program at the University of Colorado, said when he is already talking about this phenomenon with stroke patients and reminding them how dangerous extreme temperatures, particularly heat, can be. He said he will take their phone and add weather alerts so they are mindful of when temperatures will spike.
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“I tell them ‘I’m worried that you’re going to overheat and there are things we can do to prevent both stroke and worsening climate change,’ ” Saad said.
Saad did not work on the new study, but said he hopes this latest research will grab the attention of global leaders and influence public policy.
“The fact that extreme weather, or extreme temperatures more specifically, is a risk factor for stroke is known, but what this study adds is that it’s the first to examine things on a global scale,” Saad said. “People, when they think of pollution or heat when it’s related to health outcomes, stroke or otherwise, tend to think of low- to middle-income countries, but it’s happening across the board and it’s expected to get worse.”