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Interpretation of measured aerosol mass scattering efficiency over North America using a chemical transport model
Jan 8, 2020Research report (Health Effects Institute)
Links Between Air Pollution and Death Rates in Areas with Low Pollution: Phase 1
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Abstract
Consistent associations were observed between fine particulate matter (PM) and nonaccidental mortality for concentrations as low as 5 μg/m.
- The hazard ratio for nonaccidental mortality increased by 1.053 for every 10 μg/m increase in PM across pooled cohorts.
- A stronger hazard ratio of 1.13 was found in the Canadian Community Health Survey cohort, indicating a significant relationship with PM.
- The shape of the PM-mortality relationship exhibited a supralinear association, with no apparent threshold for risk.
- Sensitivity analyses suggested that longer exposure averages and smaller spatial scales led to larger associations between PM and mortality.
- Immigrants showed similar or greater sensitivity to PM exposure compared to nonimmigrants, despite overall improved survival rates.
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INTRODUCTION: Fine particulate matter (particulate matter ≤2.5 μm in aerodynamic diameter, or PM) is associated with mortality, but the lower range of relevant concentrations is unknown. Novel satellite-derived estimates of outdoor PMconcentrations were applied to several large population-based cohorts, and the shape of the relationship with nonaccidental mortality was characterized, with emphasis on the low concentrations (<12 μg/m) observed throughout Canada. 2.5 2.5 3
METHODS: Annual satellite-derived estimates of outdoor PMconcentrations were developed at 1-kmspatial resolution across Canada for 2000-2016 and backcasted to 1981 using remote sensing, chemical transport models, and ground monitoring data. Targeted ground-based measurements were conducted to measure the relationship between columnar aerosol optical depth (AOD) and ground-level PM. Both existing and targeted ground-based measurements were analyzed to develop improved exposure data sets for subsequent epidemiological analyses. 2.5 2.5 2
UNLABELLED: Residential histories derived from annual tax records were used to estimate PMexposures for subjects whose ages ranged from 25 to 90 years. About 8.5 million were from three Canadian Census Health and Environment Cohort (CanCHEC) analytic files and another 540,900 were Canadian Community Health Survey (CCHS) participants. Mortality was linked through the year 2016. Hazard ratios (HR) were estimated with Cox Proportional Hazard models using a 3-year moving average exposure with a 1-year lag, with the year of follow-up as the time axis. All models were stratified by 5-year age groups, sex, and immigrant status. Covariates were based on directed acyclical graphs (DAG), and included contextual variables (airshed, community size, neighborhood dependence, neighborhood deprivation, ethnic concentration, neighborhood instability, and urban form). A second model was examined including the DAG-based covariates as well as all subject-level risk factors (income, education, marital status, indigenous identity, employment status, occupational class, and visible minority status) available in each cohort. Additional subject-level behavioral covariates (fruit and vegetable consumption, leisure exercise frequency, alcohol consumption, smoking, and body mass index [BMI]) were included in the CCHS analysis. 2.5
UNLABELLED: Sensitivity analyses evaluated adjustment for covariates and gaseous copollutants (nitrogen dioxide [NO] and ozone [O]), as well as exposure time windows and spatial scales. Estimates were evaluated across strata of age, sex, and immigrant status. The shape of the PM-mortality association was examined by first fitting restricted cubic splines (RCS) with a large number of knots and then fitting the shape-constrained health impact function (SCHIF) to the RCS predictions and their standard errors (SE). This method provides graphical results indicating the RCS predictions, as a nonparametric means of characterizing the concentration-response relationship in detail and the resulting mean SCHIF and accompanying uncertainty as a parametric summary. 2 3 2.5
UNLABELLED: Sensitivity analyses were conducted in the CCHS cohort to evaluate the potential influence of unmeasured covariates on air pollution risk estimates. Specifically, survival models with all available risk factors were fit and compared with models that omitted covariates not available in the CanCHEC cohorts. In addition, the PMrisk estimate in the CanCHEC cohort was indirectly adjusted for multiple individual-level risk factors by estimating the association between PMand these covariates within the CCHS. 2.5 2.5
RESULTS: Satellite-derived PMestimates were low and highly correlated with ground monitors. HR estimates (per 10-μg/mincrease in PM) were similar for the 1991 (1.041, 95% confidence interval [CI]: 1.016-1.066) and 1996 (1.041, 1.024-1.059) CanCHEC cohorts with a larger estimate observed for the 2001 cohort (1.084, 1.060-1.108). The pooled cohort HR estimate was 1.053 (1.041-1.065). In the CCHS an analogous model indicated a HR of 1.13 (95% CI: 1.06-1.21), which was reduced slightly with the addition of behavioral covariates (1.11, 1.04-1.18). In each of the CanCHEC cohorts, the RCS increased rapidly over lower concentrations, slightly declining between the 25th and 75th percentiles and then increasing beyond the 75th percentile. The steepness of the increase in the RCS over lower concentrations diminished as the cohort start date increased. The SCHIFs displayed a supralinear association in each of the three CanCHEC cohorts and in the CCHS cohort. 2.5 2.5 3
UNLABELLED: In sensitivity analyses conducted with the 2001 CanCHEC, longer moving averages (1, 3, and 8 years) and smaller spatial scales (1 kmvs. 10 km) of exposure assignment resulted in larger associations between PMand mortality. In both the CCHS and CanCHEC analyses, the relationship between nonaccidental mortality and PMwas attenuated when Oor a weighted measure of oxidant gases was included in models. In the CCHS analysis, but not in CanCHEC, PMHRs were also attenuated by the inclusion of NO. Application of the indirect adjustment and comparisons within the CCHS analysis suggests that missing data on behavioral risk factors for mortality had little impact on the magnitude of PM-mortality associations. While immigrants displayed improved overall survival compared with those born in Canada, their sensitivity to PMwas similar to or larger than that for nonimmigrants, with differences between immigrants and nonimmigrants decreasing in the more recent cohorts. 2 2 2.5 2.5 3 2.5 2 2.5 2.5
CONCLUSIONS: In several large population-based cohorts exposed to low levels of air pollution, consistent associations were observed between PMand nonaccidental mortality for concentrations as low as 5 μg/m. This relationship was supralinear with no apparent threshold or sublinear association. 2.5 3
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