Commonly used indices completely disagree about the relative effect of moisture on heat stress

Abstract Irrigation and urban greening can mitigate extreme temperatures and reduce adverse health impacts from heat. However, some recent studies suggest these interventions could actually exacerbate heat stress by increasing humidity. These studies use different heat stress indices (HSIs), hindering intercomparisons of the relative roles of temperature and humidity. Our novel method compares the sensitivity of HSIs to temperature and humidity, independent of HSI units. Using this method, we explain the properties of different HSIs and identify the conditions under which they agree or disagree. We highlight recent studies where the use of different HSIs could have led to opposite conclusions. Our findings have significant implications for the evaluation of irrigation and urban greening as adaptive responses to overheating, and climate adaptation measures in general. We urge researchers to be more critical in their choice of HSIs; our method provides a useful tool for making informed comparisons.