NPR and Boston University spent two years investigating heat-related deaths in the U.S. and found the official count is wildly off—by a factor of five. The CDC reports around 1,700 heat deaths annually in recent years, but the NPR-BU analysis estimates roughly 9,000 deaths per year between 2018 and 2025, with some years exceeding 10,000. The undercount happens because heat often acts as a trigger rather than the primary cause. Someone with cardiovascular disease might have a heart attack during extreme heat, but doctors list only the heart attack on the death certificate. Since heat leaves no visible mark, coroners and medical examiners don't note it, and it vanishes from the official tallies that flow up to state and federal recordkeepers. Heat is killing people everywhere, not just in traditionally hot states. Florida and Texas have the most deaths overall, but the disparities are striking when you look at the numbers. Michigan officially reports about 100 heat deaths yearly, but the research suggests over 1,000. Massachusetts claims 5 annual deaths while the actual figure likely exceeds 60. Florida's gap is most dramatic: 1,900 estimated deaths versus 47 officially reported. The danger is especially acute in cooler regions where people lack heat tolerance and infrastructure. Washington state has air conditioning in only 53% of homes compared to nearly 100% in Alabama, yet heat risks are growing everywhere as climate change extends the heat season by a month and a half since the 1960s. Some places are getting better at counting. Maricopa County in Arizona, home to Phoenix, trains death investigators to spot heat's subtle fingerprints—checking whether someone had working air conditioning, could afford to run it, or took medications that increase heat sensitivity. But Maricopa is an exception. Los Angeles and Houston officially report very few heat deaths despite research showing hundreds likely occur there annually. Building these protocols takes years, and most doctors certifying deaths lack the training to confidently identify heat's role, making systemic improvement slow across the country.