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Around the world, cardiovascular deaths associated with cold temperatures vastly outnumbered those from the heat, though high temperature-related deaths gained steam in recent years.

From 2000 to 2019, based on daily cardiovascular deaths and temperature data from 32 countries, researchers estimated that over 1.8 million annual cardiovascular deaths (corresponding to 26 deaths per 100,000 population) were associated with local nonoptimal temperatures, representing 8.86% of total cardiovascular mortality during that period.

Cardiovascular deaths were deemed cold-related in 8.20% of cases, and heat-related in 0.66%.

However, the heat-related mortality burden has been on the rise through 2019.

"Although the current study noted a slightly lower burden of heat-related cardiovascular mortality compared with previous findings, it exhibited a slight upward trend, which aligns with reports of increasing heat-related cardiovascular deaths," according to Yuming Guo, PhD, of Monash University in Melbourne, Australia, and colleagues.

"These findings highlight the need for global communities and governments to address both cold- and heat-related mortality burdens when planning for climate change adaptation," they stated in the .

According to the CDC, can result in conditions ranging from mild heat exhaustion to heat stroke.

As a risk factor for coronary events, heat exposure is thought to induce vasodilation, disrupt body core temperature regulation, and increase metabolic activity and oxygen consumption. In addition, heat-induced fluid shifts can disrupt electrolyte balance, exacerbating arrhythmias, according to Guo and colleagues.

For the present study, they relied on the Multi-Country Multi-City Collaborative Research Network for daily temperature data plotted in 55 km x 55 km (~34.2 mile x 34.2 mile) plots.

Between 2000-2003 and 2016-2019, the cold-related global excess death ratio declined by a 0.53 percentage point while the hot-related excess death ratio increased by a 0.20 percentage point -- the result being an overall decline in temperature-related deaths.

"Even if there has been a decline in the number of cold-related deaths, other studies suggest that, depending on the particular trajectory of greenhouse gas emissions the world follows, temperature-related deaths may eventually start increasing as the rise in the number of heat-related deaths outnumbers the fall in cold-related deaths," commented Sameed Khatana, MD, MPH, of University of Pennsylvania in Philadelphia.

"In addition, an analysis focusing specifically on temperature-related deaths is unable to account for the impact of rising temperatures on food security, vector-borne illnesses, and socioeconomic and health system disruptions that are likely to also increase the adverse health effects of climate change," Khatana continued in an .

He also questioned whether the reported links between deaths and ambient temperature were indeed causal, as the deaths generally occurred in relatively mild -- not extreme -- hot and cold.

"For example, in the United States, the minimum mortality temperature was found to be approximately 24°C (75.2°F) with a slow gradual increase in the relative risk of mortality at colder temperatures. Despite only a small increase in the risk of death at mild temperatures, due to the large number of mildly cold days (approximately 65% of all days per year), the majority of deaths are attributed to these temperatures," Khatana wrote.

"Given the large number of mildly cold days that occur, it may be challenging for policy makers or providers to know what the implications of this may be," he said. "The primary way to mitigate such deaths is through efforts to improve the general cardiovascular health of populations, immunization against respiratory viruses, and striving to ensure adequate housing for all individuals."

Guo and colleagues acknowledged that their study relied on country-level death data lacking the high-resolution mortality information to match the temperature plots.

The study covered 1,847 locations spanning 32 countries across five continents. Approximately 6.9 billion individuals lived on the plotted grids during the study period, representing 98.5% of the global population.

Study authors reported that the most temperature-related deaths were observed in Central Asia and Eastern Europe. Excess cold-related deaths were more concentrated in Asia and Europe, whereas heat-related deaths disproportionately affected Asia and Africa.

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