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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.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.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.Increased NH4+ exposure may be a risk factor for the onset of asthma in preschool children.