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

Both nitrogen dioxide (NO₂) and ozone (O₃) 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 O₃ and NO₂ 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 O₃ + NO₂, as a single metric and also using O₃ and NO₂ together in two-pollutant models. We hypothesised that the magnitude of the association between O_x and daily mortality would be greater than for NO₂ and O₃ individually. Using collocated hourly measurements for O₃ and NO₂ in London, from 2000 to 2005, we carried out a time series analysis of daily mortality. We investigated O₃, NO₂ and O_x individually in single-pollutant Poisson regression models and NO₂ and O₃ 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 O₃ (0.87 %) and NO₂ (0 %) individually. However, when analysed jointly in two-pollutant models, associations for O₃ (1.54 %) and NO₂ (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 O₃ 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.