This is a plain English summary of an original research article
The Low Emission Zone covering much of Greater London was introduced between 2008 and 2012 to improve air quality. Some measures of air pollution have slightly reduced over that time, but measures of children’s respiratory health and lung development have not significantly improved.
This NIHR funded study assessed over 2,000 primary school children during the first five years of the Low Emission Zone, during which charges were phased in for several categories of commercial diesel vehicles. There was a great deal of annual variation in background nitrogen dioxide levels, but overall they fell by about 10%, or 5μg/m3, over the five years. There was no change in particulate matter levels. Overall, markers of respiratory health in 8- to 9-year old children did not improve during this time. However, this study did confirm previous findings that children, whose homes and schools were in areas of greater pollution, had smaller lung volumes, indicating impaired development.
The study did not demonstrate direct respiratory benefits from the small reduction in emissions achieved by the Low Emission Zones. It suggests that more ambitious air quality interventions are required to improve markers of child health.
Why was this study needed?
There is growing concern about diesel emissions and their impact on childhood development and a wide range of health harms. Children’s lungs are particularly susceptible, and the developmental damage caused in early years may bring serious consequences in later life.
In response to increasing concerns about poor air quality, London introduced the world’s largest city-wide Low Emission Zones (LEZs) in 2008, charging £100 to £200 daily for high-emission commercial diesel vehicles to drive in the zone. The intention was to encourage the use of cleaner vehicles, with the expectation that this would improve air quality and improve health. This study set out to assess whether the introduction of the LEZ improved the respiratory health of the children living within the area.
What did this study do?
This cross-sectional study involved 2,164 children attending 28 primary schools in the boroughs of Tower Hamlets, Hackney, Greenwich and the City of London from 2009 to 2014. The sequential design meant that different sets of 8- to 9-year olds were assessed in each period.
Child health assessments, including lung function tests, were taken on a single visit during winter months. Information about children’s asthma, allergies and rhinitis was collected from questionnaires sent to parents. This health data was compared against detailed pollution models based on air monitoring stations to estimate annual nitrogen dioxide (NO2) and particulate matter exposure at children’s home and school addresses over the previous year, as well as over the most recent hours and days.
The cross-sectional design was selected to reduce the risk of drop-out. However, a longer-term study including changes with the inclusion of comparator areas unaffected by the LEZ may have provided more robust data
What did it find?
- Over five years, average inner city background NO2 reduced from 50 to 45μg/m3 (a difference of by 5μg/m3), and at roadside locations from 75 to 68μg/m3 by (a difference of 7μg/m3). There was no significant reduction in concentrations of particulate matter with diameters of less than 2.5μm (PM2.5) or less than 10μm (PM10).
- Although no trend was found between years, children had smaller total lung capacity (FVC) if they had greater annual exposure to areas with higher levels of NO2 or PM10. The average exposure of children in the study is estimated by the authors to have caused a reduction in lung volume of approximately 5%. Higher exposure to particulate matter in the seven days before assessment was also associated with lower FVC.
- The volume of air that a child could exhale in one second (FEV1) was not associated with their annual NO2 or particulate matter exposure. However, higher exposure to particulate matter in the seven days before assessment was associated with lower FEV1. Tests were performed with and without bronchodilation.
- The health surveys completed by parents on behalf of children found that rhinitis was reduced in children with less pollutant exposure, but other allergic and respiratory symptoms did not differ.
What does current guidance say on this issue?
NICE guidance on air pollution has a number of recommendations to help improve health by tackling poor air quality. Clean air zones, including zero and low emission travel, are one of these.
NICE recommends implementing clean air zones with targets to progressively reduce NO2 levels below the EU limit (annual mean values of 40μg/m3), and which aim to meet World Health Organization air quality guidelines.
What are the implications?
Air quality has improved slightly since the introduction of London’s Low Emission Zone, but the respiratory health of children was not found to change in this study.
There are several remaining uncertainties. The study did not include comparator areas outside the LEZ and used a short time horizon of five years following introduction of the zone. It is possible that the small changes in air quality were insufficient to cause demonstrable improvement in the long-term lung development of children, but effects may become apparent in the longer-term with further air quality improvements and more extensive health evaluation.
In order to improve health, policy makers should consider additional interventions to reduce pollutant emissions from vehicles and other sources and should monitor changes in polluting behaviours to assess interventions’ effectiveness. The upcoming introduction of measures such as Ultra Low Emission Zones in London should further improve air quality and may bring greater health benefits.
Citation and Funding
Mudway IS, Dundas I, Wood HE et al. Impact of London's low emission zone on air quality and children's respiratory health: a sequential annual cross-sectional study. Lancet Public Health. 2019;4:e28-40.
National Institute for Health Research Biomedical Research Centre at Guy's and St Thomas' National Health Service (NHS) Foundation Trust and King's College London, NHS Hackney, Lee Him donation, and Felicity Wilde Charitable Trust.
DEFRA. National air quality objectives and European Directive limit and target values for the protection of human health. London: Department for Environment Food and Rural Affairs; 2005.
NICE. Air pollution: outdoor air quality and health. NG70. London: National Institute for Health and Care Excellence; 2017.
WHO. Air pollution and child health prescribing clean air summary. Geneva: World Health Organization; 2018.
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