Descriptive statistics
Of the 3144 counties in the contiguous US, we excluded those with no recorded hospital admissions or emergency department visits during the study period, resulting in a total of 2935 to 2939 counties included in the analysis, depending on the specific outcome (see supplementary table S2). Between 1 January 2010 and 31 December 2016, a total of 10.3 million hospital admissions and 24.1 million emergency department visits for natural causes were recorded among 50.1 million adults aged ≥18 years with commercial or Medicare Advantage health insurance (see supplementary table S3). The incidence rates of hospital admissions and emergency department visits for natural causes were 207.9 and 485.7 per million enrollees per day during the study period, respectively (see supplementary table S4).
Of these healthcare encounters, more than 50% of hospital admissions and 28% of emergency department visits were for cardiovascular and respiratory diseases, and the distribution varied considerably across different age groups (fig 1 and fig 2). For example, among adults aged <30 years, only 11.3% of all hospital admissions were attributed to cardiovascular or respiratory diseases, but this percentage increased to 92.7% among those aged ≥75 years. In terms of absolute numbers, the incidence rates for hospital admissions and emergency department visits increased with age and tended to be higher in women compared with men, except for cardiovascular disease (see supplementary table S4).
Incidence rates of hospital admissions and emergency department visits also varied across geographic regions. The highest incidence rates for hospital admissions related to natural causes were observed in the northern Great Plains and the northeast, whereas the highest incidence rates for emergency department visits for natural causes were documented in the southeast and midwest (fig 3). Supplementary tables S3 and S4 show the total number and incidence rates of hospital admissions and emergency department visits for natural causes and for cardiovascular and respiratory diseases across different geographic areas, respectively.
During the study period, only 0.1% of county days (8344 out of 7 949 713) recorded daily PM2.5 concentrations that exceeded the current national ambient air quality standards of 35 μg/m3; these counties were primarily located in central California, northwestern Utah, southwestern Montana, and east Idaho. Daily PM2.5 levels were below the new WHO air quality guideline limit of 15 μg/m3 in 92.6% of county days (7 360 725 out of 7 949 713) (see supplementary figure S2). Restricting our sample of events to days below this level resulted in the exclusion of 9.4% of hospital admissions and 9.1% of emergency department visits.
Regression results
Exposure to PM2.5 at concentrations below the new WHO air quality guideline limit was associated with an increased risk of hospital admissions for natural causes, cardiovascular disease, and respiratory disease. Specifically, each 10 μg/m3 increase in lag 0-1 PM2.5 was associated with a 0.91% (95% confidence interval 0.55% to 1.26%) higher relative risk of hospital admissions for natural causes, 1.39% (0.81% to 1.98%) higher relative risk of hospital admissions for cardiovascular disease, and 1.90% (1.15% to 2.66%) higher relative risk of hospital admissions for respiratory disease (fig 4, also see supplementary table S5). The corresponding excess absolute risk was 1.87 (95% confidence interval 1.14 to 2.59), 1.04 (0.61 to 1.48), and 0.85 (0.52 to 1.18) per million enrollees per day for hospital admissions related to natural causes, cardiovascular disease, and respiratory disease, respectively (fig 4, also see supplementary table S6).
The association between exposure to PM2.5 and risk of hospital admissions was most pronounced at lag 0, but with little evidence of continued higher risk at lag 1, except for respiratory diseases. For example, a 10 μg/m3 increase in PM2.5 was associated with a 0.86% (95% confidence interval 0.52% to 1.19%) higher relative risk at lag 0 and a 0.04% (−0.31% to 0.38%) higher relative risk at lag 1 for hospital admissions related to natural causes. The corresponding excess relative risk for respiratory disease was 0.86% (95% confidence interval 0.16% to 1.57%) at lag 0 and 1.03% (0.31% to 1.76%) at lag 1 (see supplementary table S7).
For emergency department visits, a 10 μg/m3 increase in lag 0-1 PM2.5 was associated with a 1.34% (95% confidence interval 0.73% to 1.94%) excess relative risk of emergency department visits for respiratory disease (fig 4, also see supplementary table S5), corresponding to 0.93 (95% confidence interval 0.52 to 1.35) additional emergency department visits per million enrollees per day (fig 4, also see supplementary table S6). The estimated association between exposure to PM2.5 and emergency department visits for natural causes or for cardiovascular disease was weaker and not statistically significant.
We performed a series of sensitivity analyses to evaluate the robustness of our findings. In analyses that allowed for a flexible exposure-response relation for the association between exposure to PM2.5 and morbidity, we found a monotonic association between exposure to PM2.5 and the relative risk of hospital admissions for natural causes, cardiovascular disease, and respiratory disease, with no indication of a threshold at lower concentrations (fig 5). We found a monotonic association between exposure to PM2.5 and relative risk of emergency department visits for respiratory disease, with the association appearing more pronounced at lower PM2.5 levels (fig 5). When we adjusted for daily maximum ambient temperature or daily minimum ambient temperature instead of daily mean temperature, the results remain consistent with our main findings, except for the association between PM2.5 level and emergency department visits for natural causes and cardiovascular disease, which became statistically significant when we adjusted for daily minimum temperature (see supplementary table S8). Our results were not materially different when we additionally included an interaction term between relative humidity splines and NCA4 regions in the models (see supplementary table S9). Additionally, when we expanded our analysis beyond days with daily PM2.5 concentrations <15 μg/m3, we generally found an attenuation in the association for PM2.5 compared with low level exposure to PM2.5, suggesting that PM2.5 concentrations are even more strongly associated with adverse outcomes <15 μg/m3 versus >15 μg/m3 (see supplementary table S10).
We found that the association between exposure to PM2.5 and hospital admissions for natural causes was statistically significant only among adults aged ≥65 years (fig 6). For example, a 10 μg/m3 increase in PM2.5 was associated with an excess relative risk of 0.36% (95% confidence interval −0.72% to 1.45%) among adults aged 18-29 years compared with 1.43% (0.60% to 2.26%) and 2.21% (1.52% to 2.91%) among those aged 65-74 years and ≥75 years, respectively. The corresponding excess absolute risks were 0.41 (95% confidence interval −0.84 to 1.67), 4.76 (2.04 to 7.48), and 14.57 (10.09 to 19.06) per million enrollee per day for adults aged 18-29 years, 65-74 years, and ≥75 years, respectively. The association between exposure to PM2.5 and hospital admissions for natural causes was more pronounced among men, those residing in the northeast US, and those with Medicare Advantage health insurance.
For emergency department visits, we found a statistically significant association between exposure to PM2.5 and respiratory disease (see supplementary figure S3). This association was most pronounced in young and middle aged adults and in the southern Great Plains. For example, we found that adults aged 40-49 years had the highest excess relative risk of 2.57% (95% confidence interval 0.87% to 4.30%) compared with the older population among whom the association was attenuated and not statistically significant. A 10 μg/m3 increase in PM2.5 was associated with an excess relative risk of 5.64% (3.77% to 7.54%) in the southern Great Plains versus 1.07% (−0.43% to 2.59%) in the US northeast.