Air pollution can cause adverse effects in humans across the life-course, including increases in rates of preterm birth, respiratory infections, asthma attacks, cardiorespiratory disease, COPD, dementia and early death. With lung growth and development continuing until early adulthood, children are particularly vulnerable to the effects of air pollution. Effective interventions are urgently needed to improve air quality. London’s Ultra Low Emission Zone (ULEZ), introduced to central London in April 2019, with proposed extensions to cover the whole of Greater London by 2021 represents a strategy to achieve this objective. Vehicles entering the ULEZ will need to meet minimum emission standards to avoid paying a daily charge. The Children’s Health in London and Luton (CHILL) study will test the hypothesis that the introduction of the ULEZ will improve lung growth and respiratory health in primary school children. We established and carried out baseline health assessments in two parallel cohorts of primary school-children (aged 6-9 years at baseline) in central London (Intervention; n=1606 from 44 schools) and Luton (Control; n=1652 from 38 schools) in the year prior to the commencement of the ULEZ. Annual follow up visits over the next three years will assess changes in lung function (post-bronchodilator forced vital capacity and forced expiratory volume in one second) using spirometry, and other respiratory health outcomes. Paediatric spirometry is more challenging than in an adult population. Initially assessed spirometry data at baseline (n=943) showed that younger participants were less likely to produce successful spirometry manoeuvres. Overall, 78% of participants performed acceptable spirometry. When characterised by age, success rates are higher in 8-9 year olds (school year 4; 84%) compared with 7-8 year olds (school year 3; 75%) or 6-7 year olds (school year 2; 76%). No differences were observed between genders (76% male, 77% female). These findings demonstrate a high level of spirometry success in this young cohort, against a rigorous set of QA criteria (Miller et al, 2005), and support maintaining the current approach in future years as the cohort matures. There is a need to evaluate the effectiveness of air quality mitigation schemes, both in terms of their capacity to improve air quality, but also to improve objective measures of population health. CHILL is designed to achieve this aim by following lung growth in a vulnerable group. The baseline year has demonstrated a high level of engagement by schools and initial analyses have provided evidence of improvements in spirometry performance in older children. The rate of successful spirometry is expected to increase as the cohort ages, leading to a higher overall rate of success across the four years of the study.