Each extreme heat and extreme cold are related to an increased risk of death from ischemic and hemorrhagic stroke, based on a latest study led by Harvard TH Chan School of Public Health. The researchers found that the link between extreme temperatures and stroke mortality was stronger in low-income countries than in high-income countries.

The study will likely be published on May 22, 2024, within the journal Stroke.

Our findings represent one other step toward understanding the impact of climate change on stroke. As temperatures change into more extreme, we anticipate a rise in fatal strokes and widening gaps in stroke mortality between high- and low-income countries, because the latter are prone to bear the brunt of climate change.”

Barrak Alahmad, lead creator, research fellow on the Department of Environmental Health

Previous research on the link between extreme temperatures and stroke mortality has produced mixed or inconclusive results. Most studies were limited to single cities or countries, most of them high-income, and few of them distinguished between stroke subtypes. To fill these gaps, researchers used the Multi-Country Multi-City Network, a worldwide environmental health consortium, to construct a world, multiregional database on ischemic and hemorrhagic stroke mortality. The database included over 3.4 million ischemic stroke deaths and over 2.4 million hemorrhagic stroke deaths reported between 1979 and 2019 in 522 cities in 25 countries.

The study found that of each 1,000 deaths from ischemic or hemorrhagic stroke, about 11 occurred on extremely cold or hot days. Of those 11 deaths, the coldest and hottest 2.5% of days contributed to 9.1 and a couple of.2 additional deaths, respectively. For each 1,000 hemorrhagic strokes, the coldest and hottest 2.5% of days contributed to 11.2 and 0.7 additional deaths. The study also found that low-income countries have the next risk of heat-related hemorrhagic stroke mortality than high-income countries, and may additionally have the next risk of cold-related hemorrhagic stroke mortality (although the evidence was suggestive, and never conclusive). There was no association between a rustic’s gross domestic product and the danger of temperature-related ischemic stroke mortality.

The researchers hypothesized that higher indoor temperature control systems and lower rates of outside work in high-income countries, in addition to poorer quality of health care in low-income countries, could explain these disparities. They noted that further research is required to uncover aspects contributing to higher temperature-related hemorrhagic stroke mortality in low-income countries and to discover helpful interventions.

Limitations of the study included a limited geographic scope – rural areas and countries in South Asia, Africa and the Middle East were underrepresented – and an absence of collection and examination of demographic data at the person level. Moreover, the study focused only on stroke deaths; additional research on the incidence of non-fatal stroke would supply a greater understanding of the true burden of temperature and stroke.

“We call on skilled stroke societies to take a position in additional of this research, especially as climate change continues to escalate, and to shine a brighter light on the emerging environmental risk aspects that may cause strokes – already a major think about mortality world wide – much more deadly,” Alahmad said.

Other Harvard Chan contributors include Antonella Zanobetti, Joel Schwartz and Petros Koutrakis.

The study was supported by the Kuwait Foundation for the Advancement of Science (grant CB21-63BO-01), the UK Medical Research Council (grant MR/V034162/1), the EU’s Horizon 2020 Project Exhaustion (grant 820655), the Swiss National Foundation Sciences (grant TMSGI3_211626) and National Institutes of Health (grant R01ES034038).