Impact of London’s Ultra Low Emission Zone (ULEZ) on lung development in children: results from the CHILL cohort study

Traffic-related air pollution harms children’s lung development; suboptimal lung growth is a risk factor for chronic lung disease. Cities are introducing Clean Air Zones (CAZs) but their health impacts are poorly understood. London’s ULEZ was implemented in 2019, providing a natural experiment to assess these impacts. We hypothesised that improved air quality arising from ULEZ implementation would improve lung growth trajectories of children living in London. We established a cohort of 3,414 schoolchildren aged 6-9 years in London and Luton, a distant control site with comparable demography and air pollutant mix. Annual spirometry (2018-2023) was conducted at schools. Mixed effects regression models examined the relationship between modelled air pollution exposures and lung function growth (increase in forced expiratory volume in 1 second, FEV1) over the study period). At baseline, FEV1 was significantly lower in London than Luton (-38ml, P<0.0001). However, FEV1 growth was greater in London than Luton (233 vs 223ml per year, P<0.0001) and after 4 years FEV1 was similar (2283ml in London, 2282ml in Luton). Modelled annual exposure to NO2 decreased faster in London than Luton (3.77 vs 1.77µg/m3 per year, P<0.0001). Across the cohort, an IQR decrease in NO2 (16.5µg/m3) was associated with an increase in FEV1 growth of 16.5ml per year (P<0.0001). Children in London had significantly lower lung function before ULEZ implementation, but subsequently experienced faster growth than those in Luton, achieving similar lung function after 4 years. This novel evidence of accelerated lung development supports wider implementation of CAZs as a public health intervention.