A Comparison of Heat Indices for Quantifying Heat-Health Risk

Heat stress is one of the largest drivers of heat-related illnesses, especially amongst children and the elderly, as well as those who live in cities. Numerous heat indices have been developed to quantify heat stress and the apparent temperature during heat events, but there is no consensus on the most suitable index for identifying and quantifying heat-health risk. In the United States, the most commonly used index for estimating apparent temperature is the National Weather Service's heat index, while in Canada it is the Humidex. Both of these indices account for the combined effect of temperature and humidity to quantify heat stress and to issue extreme heat watches and warnings. However, there are hundreds of different heat indices that have been published, and to date there has not been a systematic evaluation of which indices are the most strongly linked to heat-related illnesses. This study will compare more than 20 different heat indices against to determine which are most suitable for quantifying heat stress and predicting adverse health outcomes. First, the 20 indices will be calculated using hourly meteorological data for 7 cities. All of the indices will be evaluated to determine how much they common variance they share with the standard heat indices (NWS Heat Index and Humidex) versus how much unique information they provide during extreme heat events. This will allow us to determine which subset of indices to evaluate using health data. The subset of relatively independent heat indices will compared with emergency medical service (EMS) call data from seven cities across the United States and Canada to identify which indices are most suitable for predicting adverse health outcomes during heat events. This study will help identify the most suitable heat indices (and the associated thresholds) for use in heat warning systems in North America.