Heat-related illness rates in the United States have risen sharply over the past several years, reaching their highest recorded level in 2025, according to an analysis of more than 246 million U.S. patients.

The analysis, conducted using data from Epic's Cosmos database, examined outpatient and emergency medical encounters between January 2017 and May 2026. Researchers tracked conditions including heat cramps, heat exhaustion, heat syncope, heat stroke, heat fatigue, and heat-related edema.

The data showed that heat-related illnesses follow a strong seasonal pattern, with rates increasing during the summer and generally reaching their highest point in July. During the summer months of June, July, and August, the rate increased from 24.4 cases per 100,000 patients seen in 2017 to 36.5 cases per 100,000 in 2025.

That represents an increase of roughly 50% over the period. The 2025 figure was the highest summer rate recorded in the analysis, although researchers noted that the severity of the seasonal peak varied from year to year.

The study drew on more than 246 million patients and included encounters from urgent care facilities, walk-in clinics, emergency departments, doctors' offices and emergency-to-inpatient admissions. The data covered patients across the United States and was used to calculate rates based on the number of patients receiving medical care.

The analysis also found significant differences across the country. The South recorded the highest rates of heat-related illness, with an average July rate of about 76 cases per 100,000 patients seen. Researchers found that the region also experienced the highest number of July days with National Weather Service heat alerts.

The West presented a notable exception. Although the region averaged nearly 20 heat-alert days during July, its heat-related illness rate was the lowest among the nine NOAA climate regions examined, at approximately 22.5 cases per 100,000 patients. Researchers cautioned that the regional comparison has limitations because heat-alert days were counted across entire climate regions rather than being weighted according to population or the number of patients exposed.

Researchers said heat-related illness can range from relatively mild conditions such as heat cramps and fatigue to serious medical emergencies including heat stroke. The body's ability to cool itself can be impaired by high temperatures and humidity, increasing the risk of illness during periods of extreme heat.

The researchers also examined the relationship between heat alerts and medical encounters. In most regions, higher rates of heat-related illness corresponded with more frequent heat alerts, although the relationship was not uniform across the country.

The study has several limitations. The researchers noted that not every case of heat-related illness results in a medical visit, meaning the analysis captures people who sought care rather than the total number of Americans who experienced heat-related symptoms. Differences in access to health care and changes in the volume of medical encounters can also affect the reported rates.

The findings nevertheless provide a large-scale look at how frequently heat-related conditions have appeared in medical encounters over the past decade. With the 2025 summer rate reaching a record 36.5 cases per 100,000 patients seen, the analysis shows a substantial increase compared with the levels recorded at the beginning of the study period.