Extreme heat harms health, from dehydration and heat exhaustion to heat stroke, organ failure and death. People with heart disease, respiratory conditions such as asthma, kidney disease or other chronic illness are especially vulnerable. The 2026 heat season brought some of the highest temperatures on record.
The heat dome
From June 28 to July 7, 2026, a heat dome — a strong, persistent high-pressure system that traps hot air — sat over much of the eastern, midwestern, southern and south-central United States for about 10 days.
Maximum HeatRisk levels by county, June 28–July 7, 2026. Image from CDC's report
About 65% of the U.S. population — 221 million people in 42 states and D.C. — had at least one day of major or extreme risk on HeatRisk, a five-level forecast (none/low, minor, moderate, major, extreme) built by the National Weather Service and CDC that combines weather data with health measures.
What happened in emergency departments
CDC compared heat-related illness (HRI) visits per 100,000 emergency department visits during the heat dome with the same dates in 2018–2025, using the National Syndromic Surveillance Program:
- By September 10, 2026, EDs had recorded 145,321 heat-illness visits for the year; about 15% of them came in HHS Regions 1–7 during the heat dome's 10 days.
- Daily visit rates in those regions far exceeded past years, and in most regions topped the 95th percentile benchmark for 2018–2025.
- In HHS Regions 2, 3 and 5, the highest daily rates since 2018 came on or just before the July 4 holiday weekend.
- Rates were highest among men aged 26–54 in Regions 1, 3, 4, 5, 6 and 7, and among men 55 and older in Region 2.
The heat dome was one of several events in 2026 that drove heat-illness visits up.
Staying safe in extreme heat
- Watch heat warnings and advisories, and use tools like HeatRisk to plan outdoor activities ahead of time.
- No air conditioning at home? Follow local and state emergency guidance, including on cooling centers and other air-conditioned places.
For public health agencies, near real-time surveillance and early warning systems are key to spotting surges in heat illness quickly and targeting help to the people and regions at highest risk.
Sources
Based on Ambarish Vaidyanathan, Abigail Gates, Paul Iniguez, Jessica Lee and Aaron Bernstein, "Notes from the Field: Heat-Related Emergency Department Visits — June 28–July 7, 2026," MMWR, volume 75, Centers for Disease Control and Prevention; a work of the United States government in the public domain.
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