The synergism between asthma, respiratory infections, and pollutants: a syndemic perspective

Abstract

The experience of asthma in New Zealand is characterised by stark health inequalities. Māori, Pacific, and more deprived populations are most likely to be hospitalised for asthma, but the reason behind this disparity is not clearly understood. Due to the complex aetiology of the disease, it is likely that a combination of factors is responsible, rather than a single cause. A syndemic interaction describes how diseases can have synergistic effects when experienced concurrently, and the experience of this synergism is concentrated in select populations due to the asymmetrical distribution of risk. Using the syndemic perspective, I investigate how respiratory infections and pollution interact through multiple levels of causality to influence the severity of asthma. I use hospitalisation data to perform a time-series analysis, exploring how hospitalisations for asthma with a comorbid respiratory infection (ACRI) are affected by daily fluctuations in PM10 and NO2 in Auckland. Using a spatial analysis, I elucidate the effects of area-level variables, such as annual concentrations of PM10, on asthma and ACRI hospitalisation. Daily averages of PM10, but not NO2, are significantly associated with an influx of asthma and ACRI hospitalisations, with the risk of ACRI increasing more steeply with each elevation of PM10 than asthma overall. This suggests a synergistic effect, where asthma outcomes are significantly worse when pollution is high and the patient has a respiratory infection. Conversely, PM10 does not vary spatially with asthma and ACRI hospitalisation, though census area units (CAUs) with high hospitalisation rates experience greater deprivation and crowding. While not subjected to greater exposure, children living in more deprived CAUs are more vulnerable to the effects of pollution. This synergism occurs despite low ambient PM10 levels, as children encounter a variety of asthma triggers and are insufficiently buffered from the resulting respiratory inflammation. There are many opportunities for risk to be focused on marginalised populations through structural processes; inadequate building policy, unaffordability of quality rentals, differential prescription of preventer treatments, and the high costs of home heating contribute to the amplification of asthma and ACRI risk.