Evaluating excessive heat warnings for public health sensitivities

Children and the elderly are both vulnerable to health effects from heat waves. Excessive heat events are projected to increase in frequency and duration in the United States. The main communication vehicle to the public about the dangers of heat events is the warnings and alerts issued by the National Weather Service (NWS) (“heat products”). There is no overall guidance issued to weather forecast offices on how to define criteria for heat products; therefore, each office is free to devise its own definition. Some offices use heat index thresholds and others use thresholds as defined by the heat watch warning system developed by Kalkstein et al. (2007). No systematic review has been conducted on whether current heat products meet criteria to protect public health. We evaluated eight heat wave events in the San Francisco Bay Area in 2008 and 2009 and one event in Los Angeles in 2009. Shapefiles of forecast zones were obtained from the NWS regional forecast office. Forecast boundaries were overlaid with ZIP code boundaries using GIS. A list of heat products issued in 2008 and 2009 were obtained from the San Francisco and Los Angeles weather forecast offices. Heat-related emergency room visits were tabulated for all ZIP codes that fell within forecast zones for defined heat wave periods. In the majority of cases, heat products were issued at appropriate times before peaks of emergency room visits. However, in two cases heat products were not issued and notable morbidity occurred. We make recommendations for further analysis in this area and suggestions for modifying NWS heat products to be more protective of public health.