US heat deaths run five times higher than official counts, NPR investigation finds

Heat contributed to roughly 9,000 deaths a year in the United States between 2018 and 2025, about five times higher than the official government tally for the same period, according to a two-year investigation published by NPR in partnership with researchers at Boston University.

The finding comes as federal data confirms the summer of 2026 was the hottest on record in the United States. The National Oceanic and Atmospheric Administration found that the June through August period averaged 74.4 degrees Fahrenheit nationwide, breaking the previous record set in 1936 during the Dust Bowl and matched again in 2021, according to CBS News. August alone averaged 75.6 degrees, with daytime highs reaching 88.6 degrees. California, Nevada, New Mexico and Colorado each recorded their warmest August on record.

The NPR and Boston University analysis used weekly all-cause mortality records from 2018 through 2025, excluding 2020 because of disruptions from the COVID-19 pandemic, and applied a statistical method known as space-time stratified case-crossover analysis to nearly every US county with a population over 50,000, covering about 73 percent of the country’s population. Researchers cross-referenced that mortality data against temperature records from the ERA5-Land climate dataset.

The Centers for Disease Control and Prevention’s official count attributed an average of about 700 deaths a year to heat between 2004 and 2018, rising to an average of about 1,700 a year, and sometimes topping 2,000, in more recent years. The NPR and Boston University estimate of roughly 9,000 deaths a year for 2018 through 2025 is about five times the CDC’s more recent average, with some individual years estimated to exceed 10,000 deaths.

Researchers attributed the gap to inconsistent death investigation practices across the country. Heat’s role in a death is often masked because it can trigger other conditions, such as a heart attack, that are then listed as the official cause of death rather than heat itself.

Dr. Sameed Khatana of the University of Pennsylvania’s Perelman School of Medicine told NPR that heat is “what people often call the silent killer, compared to things like hurricanes,” explaining that extreme heat forces the heart to work beyond its capacity, which can trigger cardiovascular collapse.

Quinn Adams of Boston University’s Center for Climate and Health told NPR that “the frequency and the duration of these heat events has been expanding rapidly,” and said the trend is expected to continue as future decades bring longer and more intense heat events.

Nichole Quick of the Los Angeles County Department of Public Health acknowledged to NPR that official county-level heat death numbers “are vast undercounts.”

Maricopa County, Arizona, home to Phoenix, has developed more detailed death investigation protocols than most jurisdictions, training investigators to check whether a deceased person had working air conditioning, took medications that increase sensitivity to heat, or could afford to keep utilities running. NPR reported that most counties in the country lack comparable systems, which researchers said helps explain why heat deaths are so significantly undercounted nationwide.

By early September, drought covered 59.1 percent of the continental United States, an expansion of more than 10 percentage points since the start of August, according to NOAA data reported by CBS News.