Impact of London’s Ultra Low Emission Zone on lung development in children: the CHILL cohort study

Abstract Background Traffic-related air pollution (TRAP) impairs children's lung development, increasing the risk of chronic lung disease and early mortality. To reduce TRAP, cities worldwide are introducing Clean Air Zones (CAZs), but evidence of their health impacts is limited. London's CAZ, targeting all petrol and diesel vehicles and called the Ultra Low Emission Zone (ULEZ), was implemented in 2019, providing a natural experiment to assess the health impacts of such pollution-mitigation strategies. We hypothesised that the ULEZ would improve air quality and thereby improve lung growth trajectories of children living in London. Methods Prior to ULEZ implementation we established a cohort of 3414 schoolchildren aged 6-9 years living in London and Luton, a demographically similar city which does not have a CAZ. Lung function was assessed (by spirometry) annually between 2018-2023. Mixed-effects regression models examined the association between modelled annual nitrogen dioxide exposures (NO2, the pollutant most reflective of tailpipe emissions) and lung function growth. Results In 2018, children's estimated forced expiratory volume in 1 second (FEV1) was significantly lower in London than in Luton (-38ml, 95%CI -58, -18). Modelled annual levels of nitrogen dioxide (NO2) decreased faster at household locations in London than Luton (-3.77 vs -1.77µg/m3 per year, p < 0.0001), with total estimated reductions of 15.1 vs 7.1µg/m3 over the study period. Children's estimated FEV1 growth was greater in London than Luton (233 vs 223ml per year, p < 0.0001) and by 2023, mean FEV1 was similar between the two cohorts (London: 2283ml, 95%CI 2207, 2354; Luton: 2282ml, 95%CI 2185, 2376). Conclusions In the 4 years following the implementation of London's ULEZ, children's lung function growth was faster in London than Luton, associated with a greater decrease in residential NO2 exposure. As a result, children's lungs - initially smaller in London - had caught up with those in Luton after 4 years. Key messages • The detrimental effects of traffic-related air pollution on children’s lung development are reversible. • Clean air zones that effectively reduce NO2 can improve children’s lung growth trajectories.