Ammonia, ammonium, and the risk of asthma: A register-based case-control study in Danish children

Abstract

Background: Evidence concerning the health effects of exposure to fine particulate matter less than 2.5 μm in diameter (PM2.5) and particulate components such as ammonium (NH4+) is limited. We aimed to investigate gaseous ammonia (NH3), particulate ammonium (NH4+), the total concentration of these pollutants (NHx), and PM2.5 and their association with asthma in Danish preschool children. Methods: Estimates of incidence rate ratios associated with exposure to air pollution were calculated for children followed for the development of childhood asthma (n = 335,629) from their first to their sixth birthday during the time period 2006-2012 using a time-matched case-control design by combining register information. Modeled average concentrations of NH3, NH4+, NHx, and PM2.5 (5.56 km × 5.56 km grid resolution) during the past 3 months prior to the date of first diagnosis of asthma for cases. Results: We identified 12,935 incident cases of asthma. The base adjusted hazard ratio of incident asthma was 1.74 for children exposed to the highest decile of NH3 exposure (95% confidence interval = 1.60, 1.89) compared to children exposed to the lowest decile of NH3 exposure during the past 3 months prior to first diagnosis. Similar hazard ratios were found for the highest levels of NH4+ (2.33; 2.04, 2.65) and NHx (1.82; 1.68, 1.96). The positive associations were slightly attenuated, when adjusting for socioeconomic status, but disappeared when additionally adjusting for region, except in a two-pollutant model including NH4+ and PM2.5. No association was found with PM2.5. The findings were robust when using different exposure time windows. Conclusions: Increased NH4+ exposure may be a risk factor for the onset of asthma in preschool children.