Street hockey players compete in a pick-up game near the White House during an excessive heat wave on July 20, 2019 in Washington, DC.
Street hockey players compete in a pick-up game near the White House during an excessive heat wave on July 20, 2019 in Washington, DC. Alex Wroblewski/Getty Images

A report from Climate Central, a nonprofit science and news organization, analyzed 239 locations in the United States. It found that 198 cities have experienced an increase in the annual number of days with a heat index temperature of reaching over 90 degrees Fahrenheit (32.2 degrees Celsius) or higher over the last four decades.

The past four years have been the warmest in the United States since record keeping began in 1895.

Why this matters: Your body doesn’t handle these extremely hot and humid days well, because sweat — your natural cooling mechanism — doesn’t evaporate when it’s really humid and you can’t cool down as well. It can also be hard to breathe.

For both conditions, “danger” days and “heat index” days, it can be dangerous to exercise outside and can lead to heat exhaustion and heat stroke.

A number of sporting events around the world have had to cancel due to high temperatures, including the New York Triathlon in July.