Extreme temperatures, both hot and cold, are linked to an increased risk of death from ischemic and hemorrhagic strokes, according to a new study by the Harvard T.H. Chan School of Public Health, published in the journal Stroke. Researchers discovered that the association between extreme temperatures and stroke mortality is more pronounced in low-income countries compared to high-income ones.

“Our findings are a crucial step toward understanding the impact of climate change on stroke,” said lead author Barrak Alahmad, a researcher in the Department of Environmental Health. “As temperatures become more extreme, we expect a rise in fatal strokes and a widening gap in stroke mortality between high- and low-income countries, with the latter likely bearing the heavier burden of climate change.”

Previous research on the relationship between extreme temperatures and stroke mortality has produced mixed or inconclusive results. Most studies were confined to individual cities or countries, primarily high-income ones, and few distinguished between stroke subtypes. To address these gaps, the researchers used data from the Multi-Country Multi-City Network, a global environmental health consortium, creating a multinational and multiregional database on ischemic and hemorrhagic stroke mortality. This database included over 3.4 million ischemic stroke deaths and more than 2.4 million hemorrhagic stroke deaths reported between 1979 and 2019 in 522 cities across 25 countries.

The study found that for every 1,000 ischemic or hemorrhagic stroke deaths, about 11 could be attributed to extremely cold or hot days. Specifically, the coldest and hottest 2.5% of days accounted for 9.1 and 2.2 excess deaths, respectively, per 1,000 ischemic strokes. For 1,000 hemorrhagic strokes, these extreme days contributed to 11.2 and 0.7 excess deaths, respectively.

Additionally, the study revealed that low-income countries experience a greater burden of heat-related hemorrhagic stroke mortality compared to high-income countries and might also face a higher burden of cold-related hemorrhagic stroke mortality, though the evidence was suggestive rather than conclusive. No correlation was found between a country’s gross domestic product and the risk of temperature-related ischemic stroke mortality.

The researchers suggested that better indoor temperature control systems and lower rates of outdoor work in high-income countries, along with poorer healthcare quality in low-income countries, might explain these disparities. They emphasized the need for further research to identify the factors driving the higher burden of temperature-related hemorrhagic stroke mortality in low-income countries and to develop effective interventions.

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