Record-breaking heat and severe cold spells are having a significant impact on health and mortality in the United States, say researchers at the Yale School of Public Health (YSPH).
In a new study, YSPH researchers found that while cold weather continues to be a leading contributor to U.S. deaths annually, deaths linked to heat exposure surged more than 50% over the past two decades. Cold-related deaths rose from about 44,000 between 2000 and 2009 to more than 47,500 between 2010 and 2020, a 7% increase. Meanwhile, deaths associated with high temperatures climbed by 53%, from an annual average of 2,670 between 2000 and 2009 to more than 4,000 between 2010 and 2020.
“These findings underscore that extreme temperatures are significant threats to human health,” said Dr. Kai Chen, PhD, senior author of the study and associate professor of epidemiology (environmental health sciences) at YSPH. “Cold remains a dominant risk, yet heat is becoming increasingly dangerous as extreme heat events grow more frequent and intense.”
The study was published Nov. 7 in the peer-reviewed science journal JAMA Network Open.
For the study, researchers analyzed more than 54 million death records from every county in the 48 contiguous U.S. states between 2000 and 2020. It is believed to be one of the most comprehensive examinations to date of how heat and cold temperatures affect public health in the U.S.
The Yale team employed advanced statistical models to measure how temperature influenced mortality risk after accounting for local conditions, such as humidity, and demographic factors.
They found that both hot and cold days increased the likelihood of death within a week of exposure and cold temperatures were responsible for a larger share of excess deaths. Mortality risks rose by 5.7% on cold days (defined as low temperatures in the 5th percentile) and 1.1% on hot days (defined as high temperatures in the 95th percentile).
Specific impacts of heat and cold temperatures varied in different parts of the country. The western U.S. had higher proportions of heat-related deaths, while the southwestern U.S. recorded higher proportions of deaths linked to cold. Temperature vulnerability also differed by age, sex, and marital status. Older adults, women, and widowed or divorced individuals were found to be particularly susceptible to cold exposure, while younger, single adults were more vulnerable to heat.
“By examining every U.S. county over two decades, we were able to show how climate change is reshaping temperature-attributable mortality patterns, especially in places that historically may not have considered themselves as high-risk,” said Dr. Lingzhi Chu, PhD, the study’s lead author and a postdoctoral associate at YSPH’s Center on Climate Change and Health.
The study also identified links between heat and cold temperatures and specific causes of death. Cold exposure was associated with excess deaths from cardiovascular, respiratory, and metabolic diseases, while heat exposure was strongly linked to increased deaths from circulatory diseases as well as “external” causes of death associated with injuries and accidents, such as transport accidents, falls, and drowning.
The authors emphasized that the increase in temperature-related deaths is likely not due to climate warming alone but also reflects demographic changes — such as population growth and aging — that have elevated human vulnerability to environmental stressors.
Extreme temperatures have become increasingly common over the past two decades. Since 2012, the U.S. has experienced 10 of its warmest years on record. And 2024 was officially the hottest year ever recorded globally, according to the National Oceanic and Atmospheric Administration.
The research was funded by the National Heart, Lung, and Blood Institute of the National Institutes of Health. Dr. Robert Dubrow, MD, PhD professor emeritus and a senior research scientist at YSPH, is a co-author of the study.