Asthma hospitalisation is associated with natural environments and their interaction with long-term pollutant exposure

Abstract

Introduction Whilst the contribution of socio-economic factors to geographical variation in asthma is well documented, there has been less consideration of the role of natural environments. Greening urban areas may alleviate air pollutant impacts, yet may also increase allergenic pollen emissions; natural environments may influence ambient particulate matter (PM) components. Method An ecological analysis of 760,000 asthma emergency hospitalisations in England (1997-2012) regressed population standardised small area admission rates (n = 32,000) against period mean NO2/PM10 exposure, natural environment measures, and interactions between them, adjusting for area deprivation and large area region (n = 9). Results Unadjusted models showed higher NO2/PM10 was associated with more admissions, and higher greenspace with fewer admissions. Adjusted and interacted models of pollutants with natural environments showed, for example, higher greenspace was significantly associated with fewer admissions when NO2 was held constant at the 10th percentile (+1% greenspace = -0.0015 log admissions per 100,000 (95% -0.0018, -.0011)), but not at the 90th. Further, the rate of increased admission with increased air pollution was substantially greater 0-1 km from the coast. Supplementary analysis showed vegetation type was important. For example, lower admissions were associated with greater broadleaf coverage and the effect was greater at low NO2, whilst arable was associated with increased admissions at high NO2, and reduced admissions at low NO2. Conclusions Residential area exposures to natural environments have significant associations with local rates of hospitalisation, and interact with long-term mean pollutant exposures. We speculate that the mechanisms involved may include salutogenic exposure to nature; effects of allergenic pollen and the potential for protein nitration to enhance allergenicity; the potential for greenspace to absorb air pollutants; sea source salt component in PM.