A combination of extreme heat and high humidity levels — commonly referred to as the “heat index” or the “feels like” temperature you see on your weather app — is increasingly placing athletes at risk as global average temperatures rise, according to a report released on August 21st by environmental science and news organization Climate Central. It studied data from 239 US cities and found that 83 percent of them have experienced an increase in the average number of days with heat indices above 90 degrees Fahrenheit, which the report characterizes as “extreme heat days.”

The city that has seen the biggest jump is McAllen, Texas, where the number of annual extreme heat days has increased by 31.6 since 1979. It’s followed by Baton Rouge, New Orleans, and Miami, which have seen between 23 to 24 more extreme heat days since ‘79.

It gets worse. The number of “danger” days when the heat index exceeds 105 degrees Fahrenheit have also been trending upward in more than 40 percent of the cities it studied. The four cities that have seen the biggest increase in such days are all in Texas: McAllen, Laredo, Victoria, and Houston.

People exposed to “feels like” temperatures above 90 degrees run the risk of heatstroke, heat cramps, and heat exhaustion. The National Weather Society recommends “extreme caution” under such conditions. The reason why humidity can make already hot days even riskier is because it impedes the body’s ability to cool down. Sweat helps the body cool down by evaporating on the skin. On humid days when the air is already carrying a lot of moisture, our bodies lose much of the cooling effect of evaporation. That’s why NWS warnings are especially targeted toward people who spend lots of time outdoors, including construction workers and athletes.

64 football players have perished from heatstroke since 1995, including 47 high school students

The report points to the effects that can have on sports in schools and in professional leagues and sporting events. Sixty-four football players have died from heatstroke since 1995, including 47 high school students, making it a leading cause of death in high school sports, according to the Centers for Disease Control and Prevention. Last month, the New York City Triathlon was canceled because of severe heat warnings and “extreme” levels of humidity.

To keep athletes safe, some are advocating for a simple piece of technology and improved guidance in schools and athletic organizations. In the 1950s, the US military began using an environmental monitoring system called WetBulb Globe Temperature monitoring (WBGT), which goes a step beyond measuring temperature and humidity to also include wind and solar radiation. Taking more specific measurements can be key to preventing deaths because microclimates can exist on school blacktops, astroturf, and playing fields with little shade and a lot of dark surfaces that absorb heat.

Douglas Casa, CEO of the University of Connecticut’s Korey Stringer Institute, has pushed for WBGT monitoring and guidelines in schools. “Isn’t that amazing that you could dramatically reduce the likelihood of these dangerous things happening to kids for something that would be a few dollars per year for each high school?” Casa tells The Verge. One of the monitoring devices that Casa’s institute recommends retails for $479 online. Coaches and trainers can use the tool to more accurately decide when to take rest breaks or switch up outdoor practices based on environmental conditions.

For Casa, the issue is personal. This month 34 years ago, when he was just 16, he suffered a severe exertional heatstroke that led to a coma after running a 10K race in sweltering heat. He says that he’s fortunate to be alive now. “It’s so disappointing that we continuously have so many kids die from heatstroke when we have something so simple that can protect them,” Casa says.

The Climate Central study builds on more forward-looking research that was published in July in the journal Environmental Research Communications. It found that the average number of days each year with a heat index above 100 degrees Fahrenheit in the US is expected to double by midcentury compared to heat indices between 1971 and 2000. Thanks to climate change, the number of “danger” days that feel like 105 degrees Fahrenheit or more will likely affect up to a quarter of the US, compared to less than 1 percent of the population who experienced those conditions between 1971 and 2000.