Investigating the independent and mixture effects of short-term exposure to particulate matter (PM) chemical components on mortality and the potential modifying effect of extreme temperature: A time-series analysis in London

Particulate matter (PM) is linked to adverse health outcomes, yet the roles of specific PM components and potential effect modification by extreme temperature remain unclear. Although most epidemiological studies have focused on total PM mass concentrations, increasing evidence suggests that PM components from different emission sources may exhibit differential health effects. This study used routinely collected daily mortality data from the Office for National Statistics, and PM chemical component measurements from a London urban background supersite to examine the independent and mixture effects of ten PM chemical components on mortality in Greater London between 2015 and 2018 and to investigate effect modification by heatwaves and cold spells. PM components include inorganic aerosols (black carbon from solid fuel combustion (BCsf), liquid fuel combustion (BClf), sulphate, nitrate, and ammonium) and organic aerosols (OA) (hydrocarbon-like OA (HOA), biomass burning OA (BBOA), cooking-like OA (COA), more and less oxidized oxygenated OA (MO-OOA and LO-OOA)). Quasi-Poisson generalized additive models were used to estimate single- and two-pollutant associations, and weighted quantile sum (WQS) regression was applied to assess the combined effects of PM component mixtures. All ten components showed positive associations with all-cause mortality in single-pollutant models, with stronger effect estimates observed for respiratory mortality, particularly for sulphate, ammonium and COA. In mixture analyses, the WQS index was positively associated with all-cause mortality (RR = 1.015, 95% CI: 1.006-1.023 per 25th percentile increase). COA and MO-OOA contributed most to all-cause mortality, while BBOA and BCsf dominated respiratory effects. Heatwaves consistently amplified respiratory risks in both single-pollutant and mixture models, with limited evidence of associations for cardiovascular mortality. These findings suggest that the health effects of PM vary by emission sources, supporting the development of source-specific air pollution control and climate adaptation strategies on reducing mortality risks associated with ambient PM exposure.