US heat deaths undercounted fivefold, NPR-Boston University investigation finds

Official US climate records show around 1,700 heat-related deaths yearly, but an NPR and Boston University analysis estimates the true toll is closer to 9,000 annually. The undercount, driven by heat being masked by other causes on death certificates, leaves policymakers with a blind spot.

Bottom line — The investigation estimates 9,000 heat-related deaths per year in the US — five times the official average.

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• The two-year investigation by NPR and Boston University analysed weekly all-cause mortality from 2018 to 2025 across 530 US counties representing 73% of the population, according to lead researcher Quinn Adams.
• Heat often exacerbates conditions like heart disease but leaves no obvious mark on the body, said Dr Sameed Khatana of the University of Pennsylvania, making it easy for coroners to record only the immediate cause.
• Florida and Texas have the highest raw numbers of heat-related deaths, the model found, but states like Michigan and Massachusetts also see far more than officially reported — over 100 and 60 respectively, compared to single-digit official counts.
• Maricopa County, Arizona, has developed sophisticated death-investigation protocols that look for signs of heat stress, but most US counties lack such procedures, said Nichole Quick of the Los Angeles County Department of Public Health.
• The heat wave season in the US has lengthened by six weeks since the 1960s and the number of heat waves in major cities has doubled since the 1980s, per the National Climate Assessment and EPA data cited by NPR.
• Adams noted that many Americans 'do not see themselves as vulnerable to heat,' but the data show heat kills across all regions and demographics.

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