How protective is air conditioning in preventing deaths amid soaring temperatures and record heat levels? Yale researchers found that AC usage cut the amount of annual heat-related deaths by half in the U.S, saving more than 5,000 lives a year. However, while AC is a highly effective heat adaption tool, protective effects level off once usage passes a certain point. On days of extreme heat, risk levels tick up even with usage.
Previous studies have relied only on air conditioning units owned, but this is the first nationwide study to assess how much actual usage of air conditioning avoids heat-related mortality.
Heat stress is the leading cause of weather-related deaths and the number of people exposed to extreme heat is growing with climate change. The last three years have been the hottest on record. High temperatures are linked to heat exhaustion, heat stroke, cardiovascular, respiratory and renal diseases
The study, published in Nature Health , examined 3,108 counties across the contiguous U.S. from 2010 to 2020 using Yale's fusionACS, a data science platform which merges U.S. household surveys with American Community Survey microdata.
"Our dataset enabled us to relate AC use to mortality and temperature at a meaningful spatial scale," said study co-author Narasimha Rao , professor of energy systems.
The scientists found that air conditioning reduced heat-attributable mortality by 57%. The largest number of averted deaths was concentrated in Florida and the Southwest and Gulf Coast regions.
Cranking up the AC doesn't save additional lives. Instead, moderate usage provides the most benefits. Over-usage of AC on moderate heat days may explain the flattening of benefits, the authors said.
Air conditioning usage is also contingent on affordability. More than a quarter of households in the U.S. reported difficulty affording energy use.
"For years we have known that air conditioning protects people from extreme heat, but we could only guess by how much because the available data told us who owned an air conditioner, not who could actually afford to run one," said Kai Chen, associate professor of epidemiology at the Yale School of Public Health.
The strong protective effect of AC against heat-attributable mortality indicates the public health necessity of addressing urban heat by making AC affordable and greening urban spaces, the authors noted.
"This is one of many studies we are undertaking to understand how low-income households cope with extreme heat," Rao said. "While some choose to turn on their ACs, others make the uncomfortable choice of bearing the heat to save money for food."
The study was funded by a Yale Planetary Solutions' Seed Grant.
