Relationships between excessive heat warnings, advisories, and emergency department visits in central North Carolina

Extreme heat is the leading cause of weather-related mortality in the United States. Data from the Centers for Disease Control and Prevention reveal that as many as 700 people die each year as a direct result of the heat. Extreme heat also affects human health through heat stress and can exacerbate underlying medical conditions that lead to increased morbidity. To reduce these impacts, several states and cities across the United States have developed heat response plans or heat-health warning systems aimed at identifying oppressive weather conditions and implementing a mitigation plan to protect those that are most vulnerable. In general, most heat response plans and warning systems are activated using threshold temperatures defined by the National Weather Service (NWS) or previously established temperature-health relationships, such as excess mortality above a baseline temperature. This past year, the North Carolina Division of Emergency Management's State Emergency Response Team developed a heat emergency response plan with the goal of “providing timely, adequate, and sustainable heat emergency response operational planning guidance in coordination with state and federal agencies to counties in order to protect lives during a heat emergency.” The plan is to be activated in phases when the local NWS forecast office issues a heat advisory or excessive heat warning. To evaluate the effectiveness of these triggering activation criteria, the relationships between the issuance of heat advisories and warnings, and emergency department (ED) visits were examined across the Raleigh, NC NWS county warning area for the period 2007 to 2010. Over four million people reside in this area that covers 31 counties in central North Carolina. Results reveal that, although 10 percent of all summer days during the four year period met heat advisory or warning criteria, these days only accounted for 37 percent of all ED visits for heat-related illness (primary and secondary diagnoses). In other words, the majority of heat-related ED visits occurred on days that fell below the threshold for an advisory or warning, suggesting that the current activation criteria may be too restrictive. Additionally, relationships between temperature and ED visits were examined with respect to seasonality, urbanization, age, sex, and other demographic characteristics to determine if differences in these relationships exist between days above and below the current thresholds. Results indicate that differences do exist and it is therefore recommended that heat response plans and warning systems be flexible enough to address the health effects of heat at temperatures below current NWS thresholds.