Research Evaluation of a Heat Vulnerability Index on Abnormally Hot Days: An Environmental Public Health Tracking Study Colleen E. Reid,1 Jennifer K. Mann,1 Ruth Alfasso,2 Paul B. English,3 Galatea C. King,3 Rebecca A. Lincoln,4 Helene G. Margolis,5 Dan J. Rubado,6 Joseph E. Sabato,2 Nancy L. West,7 Brian Woods,8 Kathleen M. Navarro,1 and John R. Balmes1,9 1Environmental Health Sciences, School of Public Health, University of California–Berkeley, Berkeley, California, USA; 2Massachusetts Department of Public Health, Bureau of Environmental Health, Boston, Massachusetts, USA; 3Environmental Health Investigations Branch, California Department of Public Health, Richmond, California, USA; 4Department of Health and Human Services, Maine Center for Disease Control and Prevention, Augusta, Maine, USA; 5Department of Internal Medicine, School of Medicine, University of California–Davis, Davis, California, USA; 6Office of Environmental Public Health, Oregon Health Authority, Portland, Oregon, USA; 7Division of Environmental Health, Washington State Department of Health, Olympia, Washington, USA; 8Environmental Health Epidemiology Bureau, New Mexico Department of Health, Santa Fe, New Mexico, USA; 9Division of Occupational and Environmental Medicine, Department of Medicine, University of California, San Francisco, California, USA Background : Extreme hot weather conditions have been associated with increased morbidity and mortality, but risks are not evenly distributed throughout the population. Previously, a heat vulner- ability index (HVI) was created to geographically locate populations with increased vulnerability to heat in metropolitan areas throughout the United States. o Bjectivs : We sought to determine whether areas with higher heat vulnerability, as characterized by the HVI, experienced higher rates of morbidity and mortality on abnormally hot days. Methods : We used Poisson regression to model the interaction of HVI and deviant days (days whose deviation of maximum temperature from the 30-year normal maximum temperature is at or above the 95th percentile) on hospitalization and mortality counts in five states participating in the Environmental Public Health Tracking Network for the years 2000 through 2007. r esult : The HVI was associated with higher hospitalization and mortality rates in all states on both normal days and deviant days. However, associations were significantly stronger (interaction p-value < 0.05) on deviant days for heat-related illness, acute renal failure, electrolyte imbalance, and nephritis in California, heat-related illness in Washington, all-cause mortality in New Mexico, and respiratory hospitalizations in Massachusetts. c onclusi : Our results suggest that the HVI may be a marker of health vulnerability in general, although it may indicate greater vulnerability to heat in some cases. k ey w ords : climate change, extreme heat, hospitalizations, mortality, vulnerable populations. Environ Health Perspect 120:715–720 (2012). http://dx.doi.org/10.1289/ehp.1103766 [Online 31 January 2012] There is considerable evidence of elevated mortality, and increasing evidence of increased morbidity, associated with heat waves and extreme hot weather conditions (Basu 2009; Basu and Samet 2002). Particular population subgroups are at increased risk of mortality during extreme heat, including the elderly (Fouillet et al. 2006; Medina-Ramon et al. 2006), people of lower socioeconomic sta- tus (Curriero et al. 2002; Naughton et al. 2002; Rey et al. 2009), people who live alone (Naughton et al. 2002; Semenza et al. 1996), people with less education (Medina-Ramon et al. 2006; O’Neill et al. 2003), people of races other than white (O’Neill et al. 2003; Schwartz 2005; Whitman et al. 1997), peo- ple with preexisting health conditions such as cardiovascular disease, diabetes, renal disease, nervous disorders, cerebrovascular disease, pul- monary conditions, and mental health condi- tions (Schwartz 2005; Semenza et al. 1996, 1999; Stafoggia et al. 2006, 2008), people without access to cooling devices such as air conditioning (Chestnut et al. 1998; Curriero et al. 2002; Semenza et al. 1996), and peo- ple in neighborhoods with less green space (Kilbourne et al. 1982; Tan et al. 2007). Heat waves are projected to increase in frequency, severity, and duration in many parts of the world because of climate change (Meehl and Tebaldi 2004). Municipal inter- ventions to prevent heat-related deaths have been shown to decrease mortality in subse- quent heat events (Ebi et al. 2004; Fouillet et al. 2008; Naughton et al. 2002). There is some question, however, as to whether the most vulnerable populations are being reached by these interventions (Bassil and Cole 2010). Although there is an increased understanding by city governments of the need to have heat warning plans, they have also expressed their desire for more information to develop and implement such plans (Balbus et al. 2008; O’Neill et al. 2010). Maps that identify which populations and areas within a city are most vulnerable to heat can help local governments allocate resources to the areas in greatest need (O’Neill et al. 2009). Reid et al. (2009) created a national heat vulnerability index (HVI) to locate popu- lations vulnerable to heat at the submetro- politan level using variables associated with vulnerability in previous studies. Although others have created heat vulnerability maps Environmental Health Perspectives • volume 120 | number 5 | May 2012 for specific metropolitan areas (Johnson et al. 2009; Lindley et al. 2006; Loughnan et al. 2009; Rinner et al. 2009; Sister et al. 2009; Vescovi et al. 2005), the HVI is the only vul- nerability map that is national in scope. The HVI suggests substantial variability in heat vulnerability across the United States as well as within metropolitan areas, but additional information is needed to confirm that higher rates of death and illness occur during abnor- mally hot days in areas that the HVI identifies as more vulnerable. People adapt physiologically and tech- nologically to the climate in which they live (Kinney et al. 2008). Heat waves earlier in the summer can be more hazardous to health than those later in the summer (Anderson and Bell 2011). Therefore, we investigated the impact of days that were much hotter than normal, for a given location and time of year, on hospitalization and mortality rates. Our study addresses the extent to which areas with higher HVI values experience higher morbid- ity and mortality on abnormally hot days. This study is the result of a data link- age project within the Centers for Disease Control and Prevention’s (CDC) National Address correspondence to C. Reid, School of Public Health, University of California, 50 University Hall #7360, Berkeley, CA 94720-7360 USA. Telephone: (408) 836-5929. Fax: (510) 642-5815. E-mail: creid@berkeley.edu Supplemental Material is available online (http:// dx.doi.org/10.1289/ehp.1103766). We acknowledge K. Wheeler and T. Matte, New York City Department of Public Health, who contributed to our study design and B. Siebuhr, Washington State Department of Health, who pro- vided statistical software support. This work was supported by Centers for Disease Control and Prevention through the Berkeley Center for Environmental Public Health Tracking (5U19EH000097). The heat vulnerability index was created using the U.S. Environmental Protection Agency Science to Achieve Results (grant R832752010). K.M.N. is employed part-time as a public health intern for the Natural Resources Defense Council, an environmental advocacy nonprofit organization. The other authors declare they have no actual or potential competing financial interests. Received 4 April 2011; accepted 31 January 2012. 715