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

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. This study uses calculus of variations to compare the sensitivity of HSIs to temperature and humidity, independent of HSI units. It explains the properties of different HSIs and identifies conditions under which they disagree, highlighting examples where different HSIs led to opposite conclusions. The findings have significant implications for evaluating irrigation and urban greening as adaptive responses to overheating and climate adaptation measures. The authors urge researchers to be critical in their choice of HSIs, especially in relation to health outcomes, and provide a method for making informed comparisons.

Charles H. Simpson, Oscar Brousse, Kristie L. Ebi, Clare Heaviside