Daily nonaccidental mortality data for 108 U.S. urban communities (1987–2005) were obtained from an extended version of the National Morbidity, Mortality, and Air Pollution Study data set (originally 1987–2000) (Anderson and Bell 2009 (link); Internet-Based Health and Air Pollution Surveillance System 2005 ). Communities were defined as contiguous counties of a metropolitan area (Bell et al. 2004 (link)). We omitted data for noncontinental communities, those with population < 200,000 (Dominici et al. 2006 (link); Peng et al. 2008 (link)), and those without weather data for ≥ 99.5% of study days, leaving 59 communities.
Weather data came from the National Climatic Data Center (2011) . Daily relative humidity was calculated from dew point temperature (Bosen 1958 ) and used to calculate daily apparent temperature, a metric incorporating air temperature and relative humidity to better approximate the physical experience of heat (Robinson 2001 ). Dew point temperature is correlated with air temperature and was adjusted for daily mean temperature to limit collinearity in the model (after adjustment, average community correlation between daily dew point temperature and maximum temperature = 0.12) (Bell et al. 2004 (link)). We limited analysis to the warm season (1 May–30 September).
A heat wave consists of consecutive days with temperatures above a threshold temperature that can either be physiologically based (absolute threshold) or location based (relative threshold) (Robinson 2001 ). We used a relative threshold based on the community’s own long-term weather, to allow for regional acclimatization to temperatures normal for a community. We identified heat waves as ≥ 2 consecutive days with daily mean temperature (Tmean) higher than the community’s 95th percentile warm season Tmean (for 1987–2005), a definition similar to those used previously (Anderson and Bell 2009 (link); Hajat et al. 2006 (link)).
Sixteen of the 59 communities had very mild summers (90th percentile summertime mean apparent temperature < 80°F). Under our heat wave definition, the threshold temperature for a heat wave was low in these communities (e.g., 67.3°F in San Francisco, CA). Even though such temperatures are rare for these communities, we did not categorize days with these low temperatures as heat waves. Therefore, we excluded these communities with mild climates. Our final data set included 43 communities [see Supplemental Material, Table 1 (doi:10.1289/ehp.1002313)].
Weather data came from the National Climatic Data Center (2011) . Daily relative humidity was calculated from dew point temperature (Bosen 1958 ) and used to calculate daily apparent temperature, a metric incorporating air temperature and relative humidity to better approximate the physical experience of heat (Robinson 2001 ). Dew point temperature is correlated with air temperature and was adjusted for daily mean temperature to limit collinearity in the model (after adjustment, average community correlation between daily dew point temperature and maximum temperature = 0.12) (Bell et al. 2004 (link)). We limited analysis to the warm season (1 May–30 September).
A heat wave consists of consecutive days with temperatures above a threshold temperature that can either be physiologically based (absolute threshold) or location based (relative threshold) (Robinson 2001 ). We used a relative threshold based on the community’s own long-term weather, to allow for regional acclimatization to temperatures normal for a community. We identified heat waves as ≥ 2 consecutive days with daily mean temperature (Tmean) higher than the community’s 95th percentile warm season Tmean (for 1987–2005), a definition similar to those used previously (Anderson and Bell 2009 (link); Hajat et al. 2006 (link)).
Sixteen of the 59 communities had very mild summers (90th percentile summertime mean apparent temperature < 80°F). Under our heat wave definition, the threshold temperature for a heat wave was low in these communities (e.g., 67.3°F in San Francisco, CA). Even though such temperatures are rare for these communities, we did not categorize days with these low temperatures as heat waves. Therefore, we excluded these communities with mild climates. Our final data set included 43 communities [see Supplemental Material, Table 1 (doi:10.1289/ehp.1002313)].
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