When Outdoor Sports Become Risky

Aug 21, 2019

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  • On extreme heat index days, outdoor sports and heat can become a dangerous mix.
  • 83% of U.S. cities assessed by Climate Central experienced an increase in annual heat index days above 90°F over the past four decades.
  • Climate change’s impact is being felt throughout the world of sports as governing bodies, coaches, and event organizers look to protect athletes and spectators during extreme heat conditions.



  • The National Weather Service’s heat index incorporates relative humidity (a measure of temperature and dew point) with air temperature to measure what temperatures “feel like.” For example, if the air temperature is 88°F and there is 75% relative humidity, the heat index indicates it will “feel like” 103°F degrees. 
  • Climate Central’s analysis of gridMET heat and humidity data shows that in 198 of the 239 cities analyzed around the country, extreme heat index days reaching 90°F or higher have been increasing since 1979 (when the dataset began). 
  • Danger days, when the heat index measures 105°F or higher, occur infrequently, but have been trending upward in 106 cities studied in the contiguous United States.  Analysis of 239 locations shows that 11 cities have experienced an increase of at least 4 days since 1979. Residents of McAllen, Texas, have seen an increase of nearly 22 annual danger days on average, the most across the country. 
  • According to the Center for Disease Control, heat illness is the leading cause of death or disability among high school athletes. 


Is climate change increasing extreme heat days in your area? Check out historic trends and see how your city ranks in extreme heat days, local temperature records being set around the country, and this interactive map for future projections.

How does your state compare when it comes to sport safety policies? The Korey Stringer Institute has developed a ranking of state policies based on best practices for preventing the leading causes of death in secondary school athletics. 

How are your local sports activities being impacted by extreme heat? Contact pro or college teams in your city to find out what precautions they are taking for athletes and spectators. Check in with local Little League softball and baseball teams or other local amateur organized sports leagues to find out if they are working to prevent heat related illnesses. 


Check in with your state’s high school sports association to find out what state-level policies are in place to ensure the safety of students who are playing sports during extreme heat conditions.

Talk to a local race director about challenges for events near you during extreme heat. The Road Runners Club of America has a list of certified race directors who have completed coursework in best practices for running events.

Contact the Korey Stringer Institute to speak with “ambassadors” from around the country who have personal stories regarding heat exertional illness. 

The SciLine service, 500 Women Scientists or the press offices of local universities may be able to connect you with local scientists with expertise in heat and sports. 


Douglas J. Casa, PhD, ATC, FNAK, FACSM, FNATA
Chief Executive Officer, Korey Stringer Institute; Professor, Department of Kinesiology; Director, Athletic Training Education, University of Connecticut

Andrew Grundstein, PhD
Department of Geography, University of Georgia
Research interests: Applied climatology, climate and human health, hydroclimatology

Jennifer Vanos, PhD
Assistant Professor Department of Sustainability, Arizona State University
Research interests: Climate change, public health

Kristina Dahl
Senior Climate ScientistUnion of Concerned Scientists, Co-Author, Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days
Media contact: Ashley Siefert Nunes, Climate Communications Officer, ASiefert@ucsusa.org


Climate Central analyzed daily maximum temperature and minimum relative humidity using the gridMET modeled dataset and based on the findings of Dahl et al. 2019. Heat index temperatures were calculated using the National Weather Service heat index algorithms. The change in the number of 90oF+ and 105oF+ days are based on linear regression. The full methodology can be found in our report. 

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