Extreme heat is the leading cause of weather-related death in the United States, and both the frequency and intensity of extreme-heat events have been rising.1 Heat makes people sick when the body can no longer cool itself, often because high humidity slows the evaporation of sweat, and the resulting illness can progress from mild heat cramps to heat exhaustion and, in serious cases, life-threatening heat stroke.1 To warn the public of increased risk, the National Weather Service (NWS) issues heat advisories and warnings based largely on the heat index, a combined measure of temperature and humidity rather than temperature alone.2 There is no single, fixed temperature at which an alert is issued: the thresholds are set locally, higher where hot weather is common, and they also shift through the season as people acclimate to heat as summer wears on.3 Knowing when heat-related illness occurs across the year, how its intensity has changed over time, and how it compares with where heat alerts are actually issued can help health systems and public health agencies anticipate demand and target prevention.
We examined outpatient and emergency encounters among more than 246 million U.S. patients between January 2017 and May 2026, identifying heat-related illnesses (heat cramps, heat exhaustion, heat syncope, heat stroke, heat fatigue, and heat edema). We examined these rates overall and across the nine National Oceanic and Atmospheric Administration (NOAA) U.S. climate regions based on where care was delivered. To place the regional patterns alongside heat exposure, we paired the illness rates with counts of how many days each month a NWS heat advisory, warning, or watch was in effect anywhere in each region, drawn from a publicly available archive of NWS alerts.4
Heat-related illness in the U.S. follows a sharp, repeating seasonal cycle, rising from roughly 1 to 2 cases per 100,000 patients seen in the winter months to a pronounced peak each summer, almost always in July. The intensity of the summer peaks varied year to year with a slight upward trend: the summer rate (June, July, and August) was 24.4 per 100,000 patients in 2017 and reached a high of 36.5 per 100,000 patients in 2025.
Across most climate regions, heat-related illness rates and heat-alert frequency rise together, as seen in Figure 2. The South stands out on both measures, with the highest illness rates (about 76.0 per 100,000 patients seen in July) and the most July days under a heat alert (averaging about 23 of the month’s 31 days), while lower illness regions such as the Northeast and Upper Midwest see correspondingly fewer alert days. The clear exception is the West, which was under a heat alert on about 20 days in July on average, yet had the lowest heat-illness rates of any region, about 22.5 per 100,000. That exception might reflect how the heat alert measure is built rather than an absence of heat risk: alert days are counted for each region as a whole, regardless of where within the region the alert applied or how many patients are seen there, so a large, geographically varied region can register many alert days on areas that contribute few patients.
