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Background: Previous studies found exposure to air pollution leads to exacerbations of asthma in paediatric and adult patients and increases asthma-related emergency hospital admissions (AREHA)., Methods: AREHAs and levels of air pollutants (PM 10 , PM 2.5 and NO 2 ) were obtained from Mexico City for the period 2017-2019. A time-series approach was used to explore the relationship between air pollutants and AREHA. Relative risks of AREHA were estimated using a negative binomial regression in young children (less than 5 years) and adults (greater than 18 years)., Results: There was a positive association between AREHA and PM 10 , PM 2.5 and NO 2 in adults, which remained after mutual adjustment for these pollutants. The relative risk (RR) of admission in adults increased by 3% (95% CI 1% to 4%) for a 10 µg/m 3 increase in PM 10 , 1% (0.03% to 3%) for a 5 µg/m 3 increase in PM 2.5 and by 1% (0.06% to 2%) for a 5 µg/m 3 increase in NO 2 . In contrast, in young children, AREHAs were negatively associated with PM 10 after adjustment for NO 2 (RR 0.97 (0.95 to 0.99) for a 10 µg/m 3 and with NO 2 after adjustment for PM 10 and PM 2.5 (RR 0.98 (0.96 to 0.99) and 0.97 (0.96 to 0.99), respectively, for a 5 µg/m 3 increase in NO 2 ). AREHAs in children were not associated with PM 2.5 after adjustment for NO 2 ., Conclusions: Ambient air pollution, within the previous week, was associated with emergency hospital admissions for asthma to public hospitals in adults in Mexico City. The relationship in children was less consistent. Further work is needed to explore why differences between adults and children exist to inform appropriate interventions to benefit public health., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)

The impacts of COVID-19 lockdown restrictions have provided a valuable global experiment into the extent of improvements in air quality possible with reductions in vehicle movements. Mexico City, London and Delhi all share the problem of air quality failing WHO guideline limits, each with unique situations and influencing factors. We determine, discuss and compare the air quality changes across these cities during the COVID-19, to understand how the findings may support future improvements in their air quality and associated health of citizens. We analysed ground-level PM 10 , PM 2.5 , NO 2 , O 3 and CO changes in each city for the period 1st January to August 31, 2020 under different phases of lockdown, with respect to daily average concentrations over the same period for 2017 to 2019. We found major reductions in PM 10 , PM 2.5 , NO 2 and CO across the three cities for the lockdown phases and increases in O 3 in London and Mexico City but not Delhi. The differences were due to the O 3 production criteria across the cities, for Delhi production depends on the VOC-limited photochemical regime. Levels of reductions were commensurate with the degree of lockdown. In Mexico City, the greatest reduction in measured concentration was in CO in the initial lockdown phase (40%), in London the greatest decrease was for NO 2 in the later part of the lockdown (49%), and in Delhi the greatest decrease was in PM 10 , and PM 2.5 in the initial lockdown phase (61% and 50%, respectively). Reduction in pollutant concentrations agreed with reductions in vehicle movements. In the initial lockdown phase vehicle movements reduced by up to 59% in Mexico City and 63% in London. The cities demonstrated a range of air quality changes in their differing geographical areas and land use types. Local meteorology and pollution events, such as forest fires, also impacted the results., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)