Our analysis shows that nearly 210 million Americans—or two-thirds of the population—live in counties vulnerable to health threats from unexpectedly high summer temperatures,c which can cause heat exhaustion and heatstroke or worsen preexisting cardiovascular and respiratory conditions.d An estimated 1,300 excess deaths occurred annually during extreme summer heat from 1975 to 2004,e and more than 65,000 people end up in emergency rooms each summer with heat-related illnesses.

Older adults, young children, people with chronic illness, lower-income communities, some communities of color, and people who work or exercise outdoors are among the populations most vulnerable to heat-related health effects.g City residents also face a heightened risk because of warmer temperatures in cities from the urban heat island effect, caused by the mostly paved surfaces that absorb and re-radiate heat and the lack of green spaces and tree cover in these areas.

Through August, 2017 has been the third-hottest year on record in the continental United States, with Florida, Georgia, North Carolina, and South Carolina all registering new highs for the first eight months of a year.i The United States has warmed by about 1.5°F since 1895, with most of the rise occurring since 1970.j If the world continues to increase emissions of carbon pollution, the country could see 5°F to 10°F of additional warming by the end of the century.