Associations between daily mortality in London and combined oxidant capacity, ozone and nitrogen dioxide

Both nitrogen dioxide (NO 2 ) and ozone (O 3 ) are powerful oxidants in ambient air that are intimately linked through atmospheric chemistry and which continuously interchange over very short timescales. Based upon atmospheric chemistry alone, there is a strong, a priori, reason for considering O 3 and NO 2 together in epidemiological studies, rather than either of the two pollutants separately in single-pollutant models. This paper compares two approaches to this, using O x , defined as O 3 + NO 2 , as a single metric and also using O 3 and NO 2 together in two-pollutant models. We hypothesised that the magnitude of the association between O x and daily mortality would be greater than for NO 2 and O 3 individually. Using collocated hourly measurements for O 3 and NO 2 in London, from 2000 to 2005, we carried out a time series analysis of daily mortality. We investigated O 3 , NO 2 and O x individually in single-pollutant Poisson regression models and NO 2 and O 3 jointly in two-pollutant models in both all-year and season-specific analyses. We observed larger associations for mean 24-h concentrations of O x (1.30 % increase in mortality per 10 ppb) than for O 3 (0.87 %) and NO 2 (0 %) individually. However, when analysed jointly in two-pollutant models, associations for O 3 (1.54 %) and NO 2 (1.07 %) were comparable to the O x association. Season-specific analyses broadly followed this pattern irrespective of whether the O x concentrations were driven by O 3 production (summer) or depletion (winter). This novel approach in air pollution epidemiology captures the simultaneous impact of both oxidants whilst avoiding many of the statistical issues associated with two-pollutant models and potentially simplifies health impact calculations.