Evaluation of the heat vulnerability index all five states, and for nephritis and neph- rotic syndrome in all states but New Mexico [Figure 2; see also Supplemental Material, T a b l e 1 ( h t t p : / / d x . d o i . o r g / 1 0 . 1 2 8 9 / ehp.1103766)]. The magnitude of the estimated effect of HVI on health was essen- tially the same on deviant and nondeviant days for almost all outcomes in all states, an indication that living in a ZIP code with a higher HVI score denotes increased risk Daily observed temperature Daily normal temperature An absolute temperature threshold determines extreme day designation (#) *,# *,# # # A difference between observed temperature and normal temperature determines deviant day designation (*) 30-year normal maximum temperature adjustment for one another. Daily ground- level ozone measurements were obtained from the U.S. Environmental Protection Agency’s Air Quality System (2010) for all monitors and days in the study area and time period. We calculated the daily maximum 8-hr aver- age ozone value in a 24-hr period starting at 0800 hours for each monitor. ZIP codes were assigned the daily ozone data from the nearest air quality monitor. To investigate potential nonlinear relation- ships between HVI and hospitalizations and mortality, we modeled indicator variables for low, medium, and high HVI categories based on tertiles of the HVI distribution for all ZIP codes included in this analysis. All analyses were done using SAS (versions 9.1 and 9.2; SAS Institute Inc., Cary, NC) and R (version 2.11.1; R Project for Statistical Computing, Vienna, Austria). Statistical significance was determined as 95% confidence intervals (CIs) that did not cross 1, which assumes an α level of 0.05. Results Mean HVI values were similar in each state (Table 2). Tertiles of HVI were driven heav- ily by the distribution of HVI values in California, which included 60% of the ZIP codes in our analysis. California and Washington each had a greater percentage of ZIP codes in the highest HVI tertile (38% and 40%, respectively) than the other states (5–20%). California had the highest daily mean counts of hospitalizations for each out- come. New Mexico had the fewest observa- tions (ZIP code-days) of any state because it had only 20 ZIP codes for which an HVI value could be calculated. Hospitalizations for heat-related illness were rare, occurring on < 2% of ZIP code-days in each state. Means and standard deviations of daily mortality and morbidity counts were similar across all five states. Washington had the fewest days classi- fied as deviant (Table 2) because it had more days that were categorized, initially, as devi- ant that had maximum temperatures < 85°F. Reclassification of deviant days to nondeviant ranged from 3.9% of days in New Mexico to 29.5% in Washington. No extreme days were reclassified in Massachusetts or New Mexico, but a high of 8.1% were reclassified in California. The days classified as deviant were most likely to be in May for California and New Mexico and in June for Massachusetts, Oregon, and Washington, whereas days clas- sified as extreme were most likely to be in July in all states except Massachusetts, where most extreme days occurred in August (Table 3). On nondeviant days, the HVI was asso- ciated with higher rates of hospitalizations for electrolyte imbalance, acute renal failure, respiratory hospitalizations, and all-cause and cardiovascular mortality (p < 0.05) in Environmental Health Perspectives • volume 120 | number 5 | May 2012 95 * * * 110 105 100 ) (°F 90 e r u t rae 85 p m e T 80 75 70 65 Observed maximum temperature Variable Mortality counts (mean ± SD) All cause Cardiovascular Respiratory Hospitalization counts (mean ± SD) Internal causes Cardiovascular diseases Respiratory diseases Cerebrovascular disease Electrolyte imbalance Heat-related illnesses Nephritis and nephritic syndrome Acute renal failure Exposure variables (%)a Deviant days Extreme days Vulnerability variables (mean ± SD) HVI (continuous) Social/environmental (factor 1) Social isolation (factor 2) Air conditioning prevalence (factor 3) Preexisting conditions (factor 4) HVI (categorical) Low (%) Middle (%) High (%) 8-hr ozone (ppm) 6/1/2060/32/20063/3/20063/4/20063/5/20063/6/2060/37/20063/8/2060/39/2060/130/2060/311/2060/312/260/0133/2060/134/2060/135/260/0136/2060/317/2060/318/260/0139/2060/320/2060/231/2060/232/2060/323/2060/234/2060/325/260/0236/2060/327/2060/238/2060/239/2060/330/2003 Figure 1. Deviant days and extreme days by daily observed temperature and daily normal temperature for one ZIP code for June 2003, as defined in “Materials and Methods.” For this specific ZIP code, a day was designated as a deviant day (*) if its observed maximum temperature was ≥ 14°F greater than the 30-year normal maximum temperature for that day, whereas a day was designated an extreme day (#) if its observed maximum temperature was ≥ 99°F. Table 2. Study variables for ZIP code-days by state. CA MA NM OR WA (n = 1,474,920) (n = 479,808) (n = 24,480) (n = 145,656) (n = 259,488) 0.40 ± 0.71 0.16 ± 0.43 0.04 ± 0.20 6.85 ± 6.75 1.90 ± 2.18 1.42 ± 1.77 0.35 ± 0.67 0.90 ± 1.27 0.002 ± 0.05 0.38 ± 0.76 0.20 ± 0.49 5.11 5.39 14.5 ± 1.74 3.49 ± 0.83 2.99 ± 0.79 4.21 ± 1.14 3.83 ± 0.58 2.12 ± 0.81 27.9 34.1 37.9 0.05 ± 0.02 0.29 ± 0.61 0.10 ± 0.34 0.03 ± 0.18 4.88 ± 5.42 1.57 ± 2.01 1.15 ± 1.62 0.22 ± 0.52 0.61 ± 0.61 0.001 ± 0.03 0.28 ± 0.28 0.16 ± 0.44 4.59 5.88 13.3 ± 1.38 2.74 ± 0.68 3.53 ± 0.69 4.01 ± 0.03 2.98 ± 0.69 1.57 ± 0.67 53.1 36.5 10.4 0.04 ± 0.02 0.45 ± 0.78 0.15 ± 0.42 0.05 ± 0.22 5.97 ± 5.63 1.23 ± 1.54 1.14 ± 1.44 0.23 ± 0.52 0.68 ± 1.01 0.001 ± 0.03 0.06 ± 0.40 0.13 ± 0.38 5.39 6.20 13.3 ± 1.20 3.25 ± 0.64 3.68 ± 0.92 4.00 ± 0.00 2.40 ± 0.48 1.55 ± 0.59 50.0 45.0 5.0 0.06 ± 0.01 0.29 ± 0.61 0.10 ± 0.33 0.03 ± 0.16 3.65 ± 4.15 1.02 ± 1.43 0.72 ± 1.12 0.18 ± 0.46 0.45 ± 0.82 0.001 ± 0.03 0.19 ± 0.50 0.10 ± 0.34 4.79 5.47 13.9 ± 1.13 2.80 ± 0.48 3.37 ± 0.60 5.00 ± 0.003 2.73 ± 0.59 1.82 ± 0.74 38.5 41.8 19.7 0.03 ± 0.01 0.30 ± 0.60 0.11 ± 0.34 0.03 ± 0.17 4.01 ± 3.77 1.09 ± 1.35 0.86 ± 1.15 0.18 ± 0.45 0.56 ± 0.88 0.001 ± 0.02 0.22 ± 0.53 0.12 ± 0.37 3.97 5.50 14.5 ± 1.58 2.85 ± 0.51 3.39 ± 0.85 5.57 ± 0.77 2.67 ± 0.66 2.16 ± 0.78 24.1 36.3 39.6 0.03 ± 0.01 717 Abbreviations: CA, California; MA, Massachusetts; NM, New Mexico; OR, Oregon; WA, Washington. aThe percentages for deviant days and extreme days by state can be > 5% because the temperature was equal to the 95th percentile value on many ZIP code-days.