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

Both nitrogen dioxide (NO2) 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 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 O-3 + NO2, as a single metric and also using O-3 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 O-3 individually. Using collocated hourly measurements for O-3 and NO2 in London, from 2000 to 2005, we carried out a time series analysis of daily mortality. We investigated O-3, NO2 and O-x individually in single-pollutant Poisson regression models and NO2 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 NO2 (0 %) individually. However, when analysed jointly in two-pollutant models, associations for O-3 (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 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.