Research and data

Is heat the most dangerous extreme weather event? 

hazy hot day golden gate bridge

In the summer of 2020, NOAA’s Weather Prediction Center recorded a temperature of 130 degrees Fahrenheit in Death Valley’s Furnace Creek, located in the deserts of Southern California. The US weather agency announced that if verified, it would be the hottest recorded temperature in the country since 1913 and could be the hottest temperature reliably recorded in the world.

This latest record, while extreme, occurred as temperatures soared across the country this summer. According to NOAA, the average temperature for August 2020 across the contiguous US was 74.7 F, which is 2.6 degrees above the 20th-century average and the third-hottest August on record.

A recent study from Boston University School of Public Health (BUSPH) has found that the US may be underestimating the risks of extreme heat, emphasizing the need for public health measures that will warn citizens before extreme heat events and provide cooling opportunities when summers get oppressively hot. With climate change increasing the likelihood of these events in the future, communities across the country should expect and be prepared for more high-temperature events.

Understanding the True Risk of Rising Temperatures

Rising temperatures are related to many adverse health risks, from poor air quality to increased risk of forest fires. Yet researchers from Boston University School of Public Health have found evidence that the number of deaths directly related to heat in the United States is substantially larger than previously reported.

Their study, published in Environmental Epidemiology, found that, “there are surprisingly few estimates of the number of people who die from heat each year in the United States,” and those greatly underestimate the actual number.

BUSPH researchers found, “these [new] estimates do not depend on anyone recognizing that a given death was due to excess heat, so they are likely closer to the true number than previous estimates,” said Gregory Wellenius, the study’s senior author and director of BUSPH’s Climate and Health Program and professor of environmental health.

Underreporting Underestimates Heat Risk

The study estimates that across 297 counties there were approximately 5,600 deaths per year attributable to heat between 1997 and 2006. This vastly differs from the United States Centers for Disease Control and Prevention (CDC) estimates which put heat-related deaths at an average of 658 people per year between 1999 and 2009.

The difference is due to the way the numbers were reported; CDC estimates are based on death records that listed heat as the cause of death. Because there is no standardized criteria for identifying and recording a death as attributable to heat, and because it can be difficult to identify cases where heat was the driving factor in a death, these numbers were “substantially underestimated,” the study found. For example, a death where heat was a contributing factor may be attributed to another cause, such as cardiovascular disease.

It’s Not Just Heat But Location

The study also confirmed what previous research had found, “that a substantial number of deaths occurred at only moderately hot temperatures.” While extreme temperatures in desert regions or the Southern US pose a threat, there is also risk during hotter than average days in milder US climates, presumably because there are fewer mechanisms available for adapting to the heat. This is especially true for older Americans who are particularly susceptible to heat and may not have the resources or mobility to get to air conditioned spaces when temperatures soar.

“This finding highlights the continued importance of interventions to protect [the health of the public] during hot weather, particularly in light of projected increases in temperature in future decades resulting from continued climate change,” the study concluded. This includes places in the US where extreme heat occurs only a few days a year.

Wellenius said that the threat of an increased number of heat episodes emphasizes the importance of implementing heat action plans that can help residents plan ahead and find places to stay cool.

“Heat is very much a threat to the health of our communities and our families today,” he said. “Public health officials have a responsibility to implement heat action plans—as many communities across the world already have—in order to warn residents ahead of days of extreme heat and to help residents cope with the heat and minimize their health risks.”

The study was co-authored by Keith Spangler, postdoctoral associate in environmental health at BUSPH; Kate R. Weinberger, assistant professor of occupational and environmental health at the University of British Columbia School of Population and Public Health; Daniel Harris of the University of Toronto Dalla Lana School of Public Health; and Antonella Zanobetti of the Harvard T.H. Chan School of Public Health.

For more information about the work BUSPH faculty are doing to understand the threats of increased temperatures and climate, as well as how your company can respond, go to

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