Heat waves are becoming more common, but the number of hospital admissions for heat stroke has declined significantly in the United States in recent years, according to a new study from Harvard T.H. Chan School of Public Health published in the journal Environmental Health. In one of the largest studies of its kind, researchers examined data from more than 23 million Medicare beneficiaries in 1,916 U.S. counties between 1999 and 2010.

Heat stroke is a serious and life-threatening illness that often occurs when patients have a core body temperature over 104 degrees Fahrenheit. In the study, researchers calculated the relative risk of heat stroke among older adults during heat wave days (defined as at least two consecutive days with daily mean temperature greater than the 97th percentile of temperatures in that county) compared to non-heat wave days.

Researchers found that over time, the risk of heat stroke declined, with notable geographic differences. The risk was highest in the Northeast, while it was lower in the South and Southwest. They also found that heat waves early in the summer were more likely to result in heat stroke admissions than those later in the season.

According to Francesca Dominici, professor of biostatistics and senior associate dean for information technology at Harvard Chan School, and senior author of the study, there could be several reasons for the decline, including greater awareness of the risk of heat stroke, expanded use of air conditioning, and the potential that climate change is making people acclimate more easily to higher temperatures.

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