Impact of the Ultra Low Emission Zone on lung function growth in children in London, UK: a prospective parallel cohort study

Background Traffic-related air pollution is a risk factor for lung disease and early mortality. Clean air zones are public health policy interventions used to reduce traffic-related air pollution in urban areas, but evidence of their health benefits is limited. We describe a natural experiment study evaluating the impact of the Ultra Low Emission Zone (ULEZ) in London, UK. The primary aim was to assess the impact of the ULEZ on children's lung function growth trajectories, by comparing forced expiratory volume in 1 s (FEV1) measurements over 5 years between children in London and Luton, UK. Methods The Children's Health in London and Luton (CHILL) study is a prospective, two-arm, parallel cohort study. We recruited children aged 6–9 years from primary schools in central London (the original area of the ULEZ implementation) and Luton (a comparator site with no clean air zone). Children were excluded if they had symptoms of lung disease (excluding asthma) or learning or physical disabilities preventing them from giving informed assent. Lung function was measured by spirometry at annual school visits at baseline (before ULEZ implementation) and over the following 4 years. Annual residential exposures to nitrogen dioxide (NO2) and particulate matter with aerodynamic diameters of less than 10 μm (PM10) and less than 2·5 μm (PM2·5) were estimated at each child's home address at 20 m2 resolution using a validated dispersion modelling system. The primary outcome was the annual rate of lung function growth, measured as post-bronchodilator FEV1 over the 5-year study period. We assessed growth trajectories against individualised residential exposure to NO2, PM10, and PM2·5. We used mixed-effects linear regression to compare lung function growth between London and Luton and to examine associations between air pollution exposures and lung growth. Findings Of 122 schools approached, 84 (69%) agreed to participate (44 schools in London; 40 schools in Luton). Of 9419 children invited, we recruited 3414 children (1664 in London and 1750 in Luton) between June 5, 2018, and April 4, 2019, before ULEZ implementation. 3209 (94·0%) of 3414 children were included in the FEV1 analysis (1557 children in London, 1652 children in Luton). Children were similar across the two sites in terms of age, height, and weight, with a higher proportion of girls in London (867 [56%] of 1557) than in Luton (809 [49%] of 1652). Baseline adjusted FEV1 was lower in London than Luton (difference –38 mL, 95% CI –58 to –18; p=0·0002). At baseline, children's modelled annual exposures to NO2, the pollutant most reflective of exhaust emissions, were 18·95 μg/m3 (95% CI 18·51 to 19·38; p<0·0001) higher in London than in Luton. Over the follow-up period, children's FEV1 growth increased by 10 mL/year (95% CI 5 to 15; p=0·0002) more in London than in Luton (233 mL/year vs 223 mL/year), with modelled residential exposures to NO2 decreasing faster in London than in Luton (decreases of –3·77 μg/m3 per year in London vs –1·77 μg/m3 per year in Luton; p<0·0001). After 4 years, mean FEV1 reached parity across sites: 2283 mL (2207 to 2354) in London; 2282 mL (2185 to 2376) in Luton. The proportion of children with clinically impaired lung function fell from 184 (14%) of 1280 children to 51 (9%) of 585 children in London, and from 126 (9%) of 1339 children to 41 (7%) of 606 children in Luton. Interpretation Introduction of the London ULEZ was associated with improved lung function growth trajectories in children in London compared with children in Luton, suggesting that previous deficits in lung development were restored. This evidence supports wider implementation of clean air zones as a public health intervention. Funding National Institute for Health and Care Research and Natural Environment Research Council UK Research and Innovation.