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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11869-014-0249-8) contains supplementary material, which is available to authorized users.