Even though climate change is bringing more frequent and severe heat waves, there is no standard, global way to measure heat-wave severity, and existing indices have different thresholds for defining dangerous heat-stress conditions. Researchers report that five out of six existing heat-wave indices were unable to capture the severity and spatial distribution of recent lethal heat waves in India, Spain, and the USA. The sixth index — the lethal heat-stress index — was better able to identify dangerous heat-stress conditions, particularly in low-humidity regions.