Investigating the impact of London's Ultra Low Emission Zone (ULEZ) on children's health: cohort description and baseline data from the Children's Health in London and Luton (CHILL) prospective parallel cohort study

Traffic-related air pollution (TRAP) poses significant health risks particularly for children, with adverse effects that may impact health in later life. Low emission zones are a public health policy designed to reduce TRAP in urban areas. The CHILL (Childrens Health in London and Luton) Study will evaluate the impact of Londons Ultra Low Emission Zone (ULEZ) on childrens health, using a prospective two-arm parallel longitudinal cohort design. CHILL will examine associations between air pollution metrics and lung function growth, plus secondary outcomes. Here we describe the characteristics of the CHILL cohort at baseline, prior to the introduction of the ULEZ. We recruited 3414 children aged 6-9 years attending 84 schools (London - intervention site: 1664 children, 44 schools; Luton - comparator site: 1750 children, 40 schools). Baseline health assessments were conducted in 2018-2019 (before the introduction of the ULEZ in London). 97.0% of recruited children were assessed (London 96.5%, Luton 97.4%), with the primary outcome measure of post-bronchodilator forced expiratory volume in one second being successfully measured in 76.7% (London 76.9%, Luton 76.5%). 92.1% returned a completed parental questionnaire (London 89.3%, Luton 94.7%), including data for analysis of the secondary outcomes. Demographic characteristics and outcomes were similar across the two sites. We established well-matched cohorts of school children, in our intervention (London) and comparator (Luton) sites. Data on primary and secondary outcomes have been successfully collected, which, combined with detailed air quality metrics, provides a robust platform for evaluating the impact of Londons ULEZ on childrens health and development. ClinicalTrials.gov NCT04695093 (05/01/2021)