Two-Thirds of America’s Population Now Threatened by More Extreme Heat Days

New Interactive Map Shows Nearly 210 Million Americans Exposed to a More Than Expected Number of Extreme Heat Days

Nearly two-thirds of Americans are now facing increased extreme heat days, according to a new analysis by the Natural Resources Defense Council (NRDC). An interactive map of counties across the U.S. included in NRDC’s 2007-2016 analysis shows a greater than expected number of summer days with extreme heat compared to a period from 1961 to 1990, which can intensify a range of serious public health risks.

Severe heat is the number one cause of U.S. weather fatalities. High summer temperatures can cause heat exhaustion and heatstroke or worsen preexisting cardiovascular and respiratory conditions. Research has shown that more than 65,000 people end up in U.S. emergency rooms each summer with heat-related illnesses and an estimated 1,300 additional deaths occurred annually during extreme summer heat across 40 major US cities from 1975 to 2004.

“Extreme heat isn’t just an inconvenience – it can kill,” said Dr. Kim Knowlton, senior scientist and deputy director of NRDC’s Science Center. “This analysis gives a sense of the degree to which the present is really not like the past. Climate change is fueling more extremely hot days that pose a clear and present threat to public health.”

City residents face a heightened risk because of the urban heat island effect, caused by the mostly paved urban surfaces that absorb and re-radiate heat. A lack of green space and tree cover in urban areas can substantially raise temperatures compared to surrounding regions. An NRDC study from earlier this year found that large cities like New York, Philadelphia, Chicago, and Boston could each experience at least six times as many dangerously hot summer days by 2100 as they did, on average, from 1975 to 2010. The study also found that, collectively, 45 major urban areas in the United States could see about 28,000 more deaths each year due to extremely hot summer days by the 2090s.

“Peoples’ lives depend on acknowledging that climate change is happening in every community, and developing policies and programs to reduce climate change impacts on the health of our communities,” said Dr. Linda Rudolph, director of the Center for Climate Change and Health at Public Health Institute. “We need to act now to protect and promote health in the era of climate change."

“Extreme heat days” are essentially the hottest summer days. The analysis defines “extreme heat days” as June to August days in 2007 to 2016 on which the daily maximum temperature exceeded the 90th percentile calculated from 1961 to 1990 at each weather monitoring station. In total, 210 million Americans are experiencing a greater number of extreme heat days in the past 10 years, compared to prior decades. NRDC’s map explores down to the county level which parts of the country are most affected by the increasing frequency of extreme heat.

“Climate action is more urgent than ever as cities and states across the U.S. continue to break records for summer heat,” Dr. Samantha Ahdoot, Assistant Professor of Pediatrics at Virginia Commonwealth University School of Medicine, and pediatrician with Pediatric Associates of Alexandria. “Vulnerable populations like children and the elderly face the biggest health risks during extreme heat days.”

NRDC recommends that local communities take steps to limit the worst effects of climate warming and reduce climate-health threats by:

  • Calling on companies and decisionmakers at all levels of government to help reduce heat-trapping carbon pollution from power plants, vehicles, and other sources.
  • Ensuring our state and local governments are prepared for the health threats of climate change.
  • Taking steps to protect ourselves on dangerously hot days. For example, avoid direct sun and use the shade, stay hydrated, check in with neighbors who may need assistance in the heat, and listen for local warnings and tips to avoid heat stress.

NRDC’s new extreme heat map analysis can be found here:

NRDC’s 2011 extreme heat analysis can be found here:

States where over 75 percent of the population face at least nine more than expected extreme summer heat days annually:

D.C.                                 100%

Hawaii                             100%

Nevada                           100%

Wyoming                         97%

Florida                             94%

Arizona                           93%

Massachusetts                92%

Alaska                             91%

Connecticut                     88%

Texas                               88%

Rhode Island                   88%

Utah                                86%

Oregon                            85%

Colorado                         84%

California                        83%

Washington                    82%

New Hampshire              82%

Kansas                           80%

North Carolina                80%

Delaware                        78%

Montana                         77%

Tennessee                      76%


The Natural Resources Defense Council (NRDC) is an international nonprofit environmental organization with more than 3 million members and online activists. Since 1970, our lawyers, scientists, and other environmental specialists have worked to protect the world's natural resources, public health, and the environment. NRDC has offices in New York City, Washington, D.C., Los Angeles, San Francisco, Chicago, Bozeman, MT, and Beijing. Visit us at and follow us on Twitter @NRDC.​ 

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