Discovering a relationship between extreme temperatures and a fatal condition

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Extreme heat and cold are associated with an increased risk of death from ischemic and hemorrhagic stroke, especially in low-income countries, according to a new study.

Researchers at Harvard's T. H. Chan School of Public Health found that the relationship between extreme temperatures and deaths from strokes was stronger in low-income countries than in high-income countries.

Lead author Barak Al-Ahmad, a researcher in the Department of Environmental Health , said : “Our findings are another step towards understanding the effects of climate change on stroke. As temperatures rise, we expect an increase in fatal strokes and a widening of disparities in stroke deaths between high-altitude countries.” and low-income groups, as the latter are likely to bear the brunt of climate change.”

Previous research on the relationship between extreme temperatures and stroke deaths has produced mixed or inconclusive results. Most studies were limited to single cities or countries, mostly high-income countries, and only a few differentiated between stroke subtypes.

To address these gaps, researchers leveraged data from the Multi-Country Multi-City Network, a global environmental health consortium, to build a multinational and multi-regional database of ischemic and hemorrhagic stroke deaths.

The database consists of more than 3.4 million ischemic stroke deaths and more than 2.4 million hemorrhagic stroke deaths, reported between 1979 and 2019 in 522 cities in 25 countries.

The study found that for every 1,000 deaths due to ischemic stroke and hemorrhagic stroke, about 11 were attributed to very cold or very hot days. Of those 11 deaths, colder and hotter days contributed 2.5% to 9.1 and 2.2 excess deaths, respectively. For every 1,000 hemorrhagic strokes, colder and hotter days contributed 2.5% to 11.2 and 0.7 excess deaths.

The study also found that low-income countries bear a greater burden of deaths from heat-related hemorrhagic strokes than high-income countries. It may bear a greater burden of deaths from cold-related hemorrhagic strokes as well (although the evidence has not been conclusive). The researchers found no relationship between countries' GDP and the risk of temperature-related stroke deaths.

The researchers suggest that the disparities can be explained by factors such as better indoor temperature control systems and lower rates of outdoor working in high-income countries, as well as worse quality health care in low-income countries.

Further studies will be necessary to uncover the drivers behind low-income countries' higher burden of deaths from temperature-related hemorrhagic strokes and to identify useful interventions.