Mortality attributable to ambient fine particulate matter and nitrogen dioxide in Switzerland in 2019: Use of two-pollutant effect estimates

Abstract

Introduction: Air pollution health risk assessments have traditionally used single-pollutant effect estimates for one proxy ambient air pollutant such as PM_2.5. Two-pollutant effect estimates, i.e. adjusted for another correlated pollutant, theoretically enable the aggregation of pollutant-specific health effects minimizing double-counting. Our study aimed at estimating the adult mortality in Switzerland in 2019 attributable to PM_2.5 from a single-pollutant effect estimate and to the sum of PM_2.5 and NO₂ from two-pollutant estimates; comparing the results with those from alternative global, European and Swiss effect estimates.

Methods: For the single-pollutant approach, we used a PM_2.5 summary estimate of European cohorts from the project ELAPSE, recommended by the European Respiratory Society and International Society for Environmental Epidemiology (ERS-ISEE). To derive the two-pollutant effect estimates, we applied ELAPSE-based conversion factors to ERS-ISEE PM_2.5 and NO₂ single-pollutant effect estimates. Additionally, we used World Health Organization 2021 Air Quality Guidelines as counterfactual scenario, exposure model data from 2019 and Swiss lifetables.

Results: The single-pollutant effect estimate for PM_2.5 (1.118 [1.060; 1.179] per 10 μg/m³) resulted in 2240 deaths (21,593 years of life lost). Using our derived two-pollutant effect estimates (1.023 [1.012; 1.035] per 10 μg/m³ PM_2.5 adjusted for NO₂ and 1.040 [1.023; 1.058] per 10 μg/m³ NO₂ adjusted for PM_2.5), we found 1977 deaths (19,071 years of life lost) attributable to PM_2.5 and NO₂ together (23% from PM_2.5). Deaths using alternative effect estimates ranged from 1042 to 5059.

Discussion: Estimated premature mortality attributable to PM_2.5 alone was higher than to both PM_2.5 and NO₂ combined. Furthermore, the proportion of deaths from PM_2.5 was lower than from NO₂ in the two-pollutant approach. These seemingly paradoxical results, also found in some alternative estimates, are due to statistical imprecisions of underlying correction methods. Therefore, using two-pollutant effect estimates can lead to interpretation challenges in terms of causality.

Keywords: Air pollution; Concentration-response function; Multi-pollutant; risk assessment.