Research over the past decades has consistently demonstrated adverse effects of ambient air pollution from traffic, fossil fuel combustion, industrial facilities and other sources on child respiratory health [1]. This research has directly informed ambient air quality standards for ground-level ozone (O₃), fine particulate matter (PM_2.5, particulate matter <2.5 μm), and other criteria pollutants regulated by the US Environmental Protection Agency and the European Union. These air quality standards are designed to protect sensitive subpopulations, including children [1]. In addition to studies showing the acute effects of air pollution on child respiratory health, a growing number of longitudinal studies have demonstrated adverse effects of long-term air pollution exposures on lung function growth during childhood [1]. In contrast, only very few studies have investigated the effects of improved air quality on children's lung function, although this "natural experiment" can make an even stronger argument for stricter air quality standards.