Reid et al. of mortality or hospitalization regardless of how hot the day is. However, some evidence of effect modification by deviant day was observed for heat-related illness, specifically in California [RRs (95% CIs): nondeviant day, 0.94 (0.90, 0.97); deviant day, 1.11 (1.05, 1.17)], Washington [nondeviant day, 0.91 (0.80, 1.04); deviant day, 1.22 (0.95, 1.56)], and Massachusetts [nondeviant day, 1.11 (1.01, 1.21); deviant day, 1.26 (1.10, 1.44); Figure 2A; see also Supplemental Material, Table 1 (http://dx.doi.org/10.1289/ ehp.1103766)]. In Oregon, rates for this outcome show no difference between non- deviant and deviant days, whereas in New Mexico, there was a significant decrease in heat-related hospitalizations on deviant days for a one-unit increase in HVI. A sig- nificant interaction was also found for all- cause mortality in New Mexico [RRs (95% CIs): nondeviant day, 1.18 (1.06, 1.31); deviant day, 1.30 (1.15, 1.47); Figure 2B]. We also found statistically significant inter- action terms for respiratory hospitalizations in Massachusetts and for electrolyte imbal- ance, nephritis and nephrotic syndrome, and acute renal failure in California; however, the magnitudes of the differences in estimated effects for deviant days and nondeviant days were small [Figure 2A; see also Supplemental Material, Table 1 (http://dx.doi.org/10.1289/ ehp.1103766)]. The association between HVI and cardiovascular mortality was stronger on nondeviant days than on deviant days in Washington [RRs (95% CIs): nondeviant day, 1.13 (1.09, 1.17); deviant day, 1.05 (1.00, 1.11); Figure 2B]. The patterns for Oregon, except for heat-related illness, were similar to those in the other states where we had more power to see an effect. A n a l y s i s o f e x t r e m e d a y s s h o w e d almost the same effect estimates and similar August September Table 3. Percentage of deviant days and extreme days in each month by state from May through September 2001 through 2007 (n = number of deviant days or extreme days). May 38.5a 26.9 44.1a 20.3 13.3 5.8 6.4 4.3 4.2 4.4 June 16.6 31.0a 6.5 33.5a 33.3a 11.9 20.8 19.6 15.6 19.2 July 14.5 5.6 11.1 22.5 31.0 33.5a 29.0 60.9a 39.6a 43.0a 6.1 18.1 24.0 11.2 10.1 23.1 38.9a 14.8 31.7 26.8 IinVH tsyad r d c n in no t in nd a ro i v foRR oden seae a ive 1.4 -enu tna 1.2 1.8 1.6 1.0 0.8 ovlume 24.3 18.4 14.3 12.5 12.3 25.7 4.9 0.3 10.0 6.7 patterns to that observed with deviant days [see Supplemental Material, Table 2 (http:// dx.doi.org/10.1289/ehp.1103766)]. The main difference is that for extreme days, there was consistency across all states for heat-related illness, with higher risk on extreme days com- pared with nonextreme days for a one-unit increase in the HVI. Deviant days were identified as extreme days about half the time in most states, with the lowest percentage of agreement in New Mexico (21.7% of deviant days were extreme days) and the highest in Washington (81.4% of deviant days were extreme days). Given the relatively low agreement, it is interesting that, for the most part, there were no large differ- ences in effect estimates for deviant days com- pared with extreme days in models without the HVI [see Supplemental Material, Table 3 (http://dx.doi.org/10.1289/ehp.1103766)]. When we analyzed tertiles of the HVI, we did not find evidence against the use of HVI as a continuous variable in our analyses (data not shown); however, because our study examined only five states, this should be further investigated in future studies. Discussion The main purpose of our study was to inves- tigate whether people living in more heat- vulnerable ZIP codes, as characterized by the HVI, experienced higher rates of mortality and morbidity during abnormally hot days than during other days. Overall, our find- ings indicate that HVI was consistently asso- ciated with most health outcomes on both normal (nondeviant days) and abnormally hot (deviant) days. Therefore, HVI may be a State Deviant days CA (n = 70,224) MA (n = 21,659) NM (n = 1,294) OR (n = 6,603) WA (n = 10,028) Extreme days CA (n = 74,388) MA (n = 27,661) NM (n = 1,479) OR (n = 7,909) WA (n = 15,063) 3.0 2.5 IinVH tsyad sea iva2.0 e ed r c n in no i t d -une tan1.5 n n ro ia v foRR oden1.0 0.5 718 California Massachusetts New Mexico Oregon Washington California Massachusetts New Mexico Oregon Washington Figure 2. RRs for a one-unit increase in the HVI for five hospitalization diagnoses (A) and three mortality categories (B), by state, on nondeviant and deviant days. ● ● ● ● ● ● ● ● ● ● 120 | number 5 | May 2012 • Environmental Health Perspectives Abbreviations: CA, California; MA, Massachusetts; NM, New Mexico; OR, Oregon; WA, Washington. aState’s month, with the highest percentage of deviant or extreme days. Electrolyte imbalance, nondeviant day Heat-related illness, nondeviant day Acute renal failure, nondeviant day Nephritis and nephrotic syndrome, nondeviant day Nephritis and nephrotic syndrome, deviant day Electrolyte imbalance, deviant day Heat-related illness, deviant day Acute renal failure, deviant day All-cause mortality, nondeviant day Cardiovascular mortality, nondeviant day Respiratory mortality, nondeviant day All-cause mortality, deviant day Cardiovascular mortality, deviant day Respiratory mortality, deviant day n n ● ● ● ● ● ● ● ● ● ●