Ambient Air Pollution and All-Cause and Cause-Specific Mortality in an Analysis of Asian Cohorts

Abstract

Introduction: Much of what is currently known about the adverse effects of ambient air pollution comes from studies conducted in high-income regions, with relatively low air pollution levels. The aim of the current project is to examine the relationship between exposure to ambient air pollution (as predicted from satellite-based models) and all-cause and cause-specific mortality in several Asian cohorts.

Methods: Cohorts were recruited from the Asia Cohort Consortium (ACC). The geocoded residences of participants were assigned levels of ambient particulate material with aerodynamic diameter of 2.5 μm or less (PM_2.5) and nitrogen dioxide (NO₂) utilizing global satellite-derived models and assigned for the year of enrollment (or closest available year). The association between ambient exposure and mortality was established with Cox proportional hazard models, after adjustment for common confounders. Both single- and two-pollutant models were generated. Model robustness was evaluated, and hazard ratios were calculated for each cohort separately and combined via random effect meta-analysis for pooled risk estimates.

Results: Six cohort studies from the ACC participated: the Community-based Cancer Screening Program (CBCSCP, Taiwan), the Golestan Cohort Study (Iran), the Health Effects for Arsenic Longitudinal Study (HEALS, Bangladesh), the Japan Public Health Center-based Prospective Study (JPHC), the Korean Multi-center Cancer Cohort Study (KMCC), and the Mumbai Cohort Study (MCS, India). The cohorts represented over 340,000 participants.

Mean exposures to PM_2.5 ranged from 8 to 58 μg/m³. Mean exposures to NO₂ ranged from 7 to 23 ppb. For PM_2.5, a positive, borderline nonsignificant relationship was observed between PM_2.5 and cardiovascular mortality. Other relationships with PM_2.5 tended toward the null in meta-analysis. For NO₂, an overall positive relationship was observed between exposure to NO₂ and all cancers and lung cancer. A borderline association between NO₂ and nonmalignant lung disease was also observed. The findings within individual cohorts remained consistent across a variety of subgroups and alternative analyses, including two-pollutant models.

Conclusions: In a pooled examination of cohort studies across Asia, ambient PM_2.5 exposure appears to be associated with an increased risk of cardiovascular mortality and ambient NO₂ exposure is associated with an increased cancer (and lung cancer) mortality. This project has shown that satellite-derived models of pollution can be used in examinations of mortality risk in areas with either incomplete or missing air pollution monitoring.

Figure 1.

The black dots represent the locations of Wuhan’s nine ambient air monitoring stations. Note that PM_2.5 was measured only at stations 2 and 8. (From Feng et al. . Used with permission of Springer Science+Business Media.)

Figure 2.

Flow chart for the selection of the cohort study population.

Critique Figure.

Comparison of results for PTB and LBW from cohort and case–control studies. Cohort continuous exposures (panels A and B): ORs were estimated based on per 5-μg increase in PM_2.5 and PM₁₀, 3-μg increase in NO₂ and SO₂, 10-μg increase in O₃, and 100-μg increase in CO. Exposures were modeled as continuous variables based on nearest monitor (source: IR Table 1). Case-control dichotomous exposures (panels C and D): Exposures were modeled as dichotomous variables based on IDW exposures. In model 1, ORs were estimated as the increase in the crude odds. Model 2 adjusted for covariates from the delivery data, including maternal age, maternal educational attainment, maternal occupation, gravidity, parity, infant sex, season of conception, and ambient temperature during the week of conception. Model 3 adjusted for covariates from the delivery data plus additional covariates from the survey, including total household income, SHS exposure, maternal depression during pregnancy, and vaginal bleeding during pregnancy. Model 4, a two-phase model, adjusted for covariates from both the delivery data and survey data in model 3 (source: IR Table 7). Case-control continuous exposures (panels E and F): Exposures were modeled as continuous variables based on nearest monitor and were adjusted for covariates from the delivery data, including maternal age, maternal educational attainment, maternal occupation, gravidity, parity, infant sex, season of conception, and ambient temperature during the week of conception. Crude models were not presented (source: IR Appendix Table O.36).