Extreme Heat: When Outdoor Sports Become Risky

A 2019 Report by Climate Central 

Format slightly changed for easier online reading

Extreme heat index days—when heat and humidity make it difficult for the body to cool itself off—have been increasing in cities across the country.

Download this report as a PDF.

Summary

The National Weather Service heat index includes a combination of air temperature and the relative humidity to capture what it actually feels like outside (which is usually warmer than what the thermometer is reading). When the heat index reaches 90 °F, the NWS advises individuals to use “extreme caution” if exercising or working in the outdoors (and that’s for a heat index calculation that assumes a shady location with a slight breeze). 

 A Climate Central analysis of 239 locations in the United States shows that 198 cities have experienced an increase in the annual average number of days with heat index temperatures of 90 °F or higher over the last four decades, based on a linear regression analysis. These extreme heat days are now comprising much of the summer for many cities in the South and Southwest, while areas of the country that had relatively few summer days reach the 90 °F heat index in the past are now experiencing weeks of them. 

A “danger” day occurs when the combination of heat and humidity makes it feel like it's 105 °F or hotter. Nearly a dozen U.S. cities experienced an increase of at least 4 danger days on average since 1979. 

 Climate change’s impact is being felt throughout the world of sports as these extreme heat events become more common. On high heat index days, sports and heat become a dangerous mix. 

According to the Center for Disease Control, heat-related illnesses are the leading cause of death or disability among high school athletes. During hot, humid weather, sweat cannot evaporate as easily from the skin, so athletes are at greater risk of developing  illnesses such as heat exhaustion and heat stroke - the latter being potentially fatal.

 With an estimated 8 million high school athletes across the U.S., late summer is the time when many head back to football, soccer, field hockey, or track and field practice—and when parents, guardians and coaches need to be vigilant about the potential risk for exertional heat illnesses. 

 The increased intensity and frequency of high heat index days are also complicating professional and amateur sports events around the country. 

The July heatwave that affected Midwestern and Eastern states led to the cancellation of the New York City Triathlon, a number of running races, as well as horse races in New York, Maryland, Pennsylvania, and Kentucky.

What is the Heat Index?

The National Weather Service’s heat index incorporates relative humidity 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. 

Relative humidity is a calculation of temperature and dew point; the higher the dew point, the more moisture in the air, and the more uncomfortable it is to breathe and be active outdoors. Humidity and dew point levels are critical to consider for outdoor sports events and practices because the higher the moisture in the air, the less efficient the human body is at cooling down. 

During exercise, the body heats up and perspires to release that heat. But the body can only get cooler if this perspiration or sweat evaporates. When the humidity is high, sweat can’t evaporate as well and the body can’t cool itself down as efficiently. 

To understand how “humid” or “muggy” it will feel outside, the dew point is an excellent indicator. A dew point in the 50s or lower  indicates that the air will be drier and more comfortable; when it gets in the 60s, it really starts to raise the heat index; and when the dew point is in the 70s or higher, there is a lot of moisture in the air and it will feel oppressive. 

 The NWS heat index categories are based on how the heat and humidity will affect a healthy individual:

• Heat index of 80 °F-90 °F: Prolonged exposure or physical activity could lead to fatigue and “caution” is advised.
• Heat index of 90 °F-103 °F: A person can experience heat stroke, heat cramps, or heat exhaustion and “extreme caution” is recommended.
• Heat index of 103 °F/105 °F-124 °F (depending on location): On “danger” days, a person is likely to experience heat cramps or heat exhaustion, and heat stroke becomes possible.
• Heat index of 125 °F: On “extreme danger” days, heat stroke is highly likely.

Source: Climate Central- 2019HeatIndex Report