Introduction Asthma is the most prevalent disease in children worldwide. 1 Concern is growing regarding the impact of outdoor ambient air pollution exposure on the risk of pediatric asthma. Several studies have shown that fine particulate matter less than 2.5 μm in diameter (PM 2.5 ) is associated with respiratory symptoms, exacerbation, and new-onset asthma in children. 2,3 Previous studies have focused almost exclusively on health effects associated with transport-and industrial-related PM

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... exposure, whereas surprisingly little attention has been paid to agricultural air pollutants and the role of particulate precursors and components such as gaseous ammonia (NH 3 ) and ammonium (NH 4 + ), although NH 4 + accounts for a substantial proportion of the PM 2.5 mass. 4 More information on the physical and chemical properties of NH 3 and NH 4 + is given in the online data supplement; http://links.lww.com/EE/A13. Results of the few existing epidemiological studies showed an association of NH 4 + concentrations and ambient ammoniumbisulphate exposure from lags of 0-4 days and increased respiratory hospitalizations in adults 5,6 and increased emergency department visits due to upper respiratory infections in children. 7 Furthermore, high ambient NH 3 exposure during the last day has been linked with declining lung function, but not asthma symptoms, 8 whereas annual average NH 3 exposure has neither been associated with increased use of asthma medication nor asthma 9 in 8-to 12-yearold children. A review of controlled human exposure studies on inorganic NH 4 + salts concluded that there was no evidence of acute adverse pulmonary health effects from sulfate or nitrate salts at environmentally relevant levels, but at concentrations well