Effect modification of ambient air pollution in the associations between aeroallergens and allergic disease medication relievers in Ontario, Canada

Abstract

Background: Ambient air pollution may modify the effect of aeroallergens on allergic diseases through enhanced inflammatory processes. However, further evidence is needed in order to clarify this effect modification influence. Aims: Investigate the effect modifying influence of outdoor air pollution in the associations between daily levels of ambient aeroallergens and daily numbers of medications sold to relieve allergic diseases such as asthma, allergic rhinitis and conjunctivitis. Methods: This study uses a multi-city time series approach by including seven cities in the province of Ontario, Canada. In each city, daily levels of aeroallergens (tree, pollen, mold) taken from fixed site monitoring stations from years 2007 to 2011 were obtained. Daily numbers of prescription and over-the-counter (OTC) medication sales used to relieve symptoms of asthma, allergic rhinitis and conjunctivitis were also obtained for the same time period for each city. A distributed lag Poisson regression model adjusted for seasonal trends, day-of-the-week, temperature and relative humidity was used by including interaction terms between aeroallergen levels and a dichotomized variable (low/high) characterizing levels of specific air pollutants (NO2, PM2.5, CO, O3 and SO2). Results were pooled across all cities using random effects model. Results: The relative risks characterizing the associations between daily pollen levels and daily medication reliever sales for asthma and allergic rhinitis were stronger on days with elevated levels of PM2.5 compared to days with lower levels (P value for interaction < 0.05). Relative risks (95% confidence interval) for an interquartile increase in tree pollen levels ranged from 1.15 to 1.30 on days of high PM2.5 levels. Conclusions: Results of this study suggest that ambient air pollution enhance the effect of pollen on asthma and allergic rhinitis. A reduction in ambient air pollution levels could reduce allergic exacerbations of asthma and allergic rhinitis.