A comparison of short-term and long-term air pollution exposure associations with mortality in two cohorts in Scotland.

BACKGROUND: Air pollution-mortality risk estimates are generally larger at longer-term, compared with short-term, exposure time scales. OBJECTIVE: We compared associations between short-term exposure to black smoke (BS) and mortality with long-term exposure-mortality associations in cohort participants and with short-term exposure-mortality associations in the general population from which the cohorts were selected. METHODS: We assessed short-to-medium-term exposure-mortality associations in the Renfrew-Paisley and Collaborative cohorts (using nested case-control data sets), and compared them with long-term exposure-mortality associations (using a multilevel spatiotemporal exposure model and survival analyses) and short-to-medium-term exposure-mortality associations in the general population (using time-series analyses). RESULTS: For the Renfrew-Paisley cohort (15,331 participants), BS exposure-mortality associations were observed in nested case-control analyses that accounted for spatial variations in pollution exposure and individual-level risk factors. These cohort-based associations were consistently greater than associations estimated in time-series analyses using a single monitoring site to represent general population exposure {e.g., 1.8% [95% confidence interval (CI): 0.1, 3.4%] vs. 0.2% (95% CI: 0.0, 0.4%) increases in mortality associated with 10-μg/m³ increases in 3-day lag BS, respectively}. Exposure-mortality associations were of larger magnitude for longer exposure periods [e.g., 3.4% (95% CI: -0.7, 7.7%) and 0.9% (95% CI: 0.3, 1.5%) increases in all-cause mortality associated with 10-μg/m³ increases in 31-day BS in case-control and time-series analyses, respectively; and 10% (95% CI: 4, 17%) increase in all-cause mortality associated with a 10-μg/m³ increase in geometic mean BS for 1970-1979, in survival analysis]. CONCLUSIONS: After adjusting for individual-level exposure and potential confounders, short-term exposure-mortality associations in cohort participants were of greater magnitude than in comparable general population time-series study analyses. However, short-term exposure-mortality associations were substantially lower than equivalent long-term associations, which is consistent with the possibility of larger, more persistent cumulative effects from long-term exposures.