Distinguishing the associations between short-term exposure to outdoor nitrogen dioxide and adverse health effects from those of particulate matter

• Main Author: Mills, Inga Clair
• Published: St George's, University of London 2016
• Subjects: 363.739
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.703123

Epidemiological time-series studies have demonstrated associations between short term exposure to nitrogen dioxide (N02) and adverse health outcomes. It is unclear whether these associations are independent of particulate matter (PM) or other traffic related pollutants. This has led to uncertainties about the public health importance of N02. This thesis presents a comprehensive, systematic review and meta-analysis of time-series studies of N02 and daily mortality and hospital admissions which aimed to determine if N02-associations are independent of PM; evaluate heterogeneity; and calculate coefficients for use in health impact assessments (HIA). Peer-reviewed time-series studies of N02 were assembled from databases and existing reviews. Descriptive data and estimates from single-and two-pollutant models along with data on effect-modifiers were extracted from the studies. Random-effects summary estimates were calculated globally and stratified by different geographic regions, and tests for heterogeneity and small study bias were conducted. 239 peer-reviewed time-series studies of N02 were identified. 76 cause- and age specific combinations of mortality or hospital admissions were meta-analysed. Two pollutant model study estimates generally showed that the N02-associations are independent of PM mass. For all-cause mortality, a 10 ug/m3 increase in 24 hour N02 was associated with a 0.78% (95% CI: 0.47, 1.09) increase in the risk of death, which reduced to 0.60% (0.33,0.87) after control for PM, and heterogeneity between geographical region-specific summary estimates was removed (I2 from 66.9% to 0%). Few studies assessed N02 together with traffic-related PM. None of the factors examined, including daily PM10 and the NO2/PM10 concentration ratio, accounted for the substantial heterogeneity between region-specific N02 estimates. This review provides evidence of associations between N02 and adverse health outcomes that are independent of PM mass. However, the limited evidence on adjustment of N02-associations for traffic-related PM means that some uncertainty remains regarding possible confounding and HIA exercises should reflect this.