Low emission zones are now in place in 200 cities across Europe, but London findings suggest that interventions that deliver larger reductions in emissions may be needed.
The introduction of the low emission zone in London, UK, has only contributed to modest reductions in exposure to nitrogen oxides from diesel vehicles, and these improvements appear to have little effect on the lung health of children, according to an observational study published in The Lancet Public Health journal.
The observational study including over 2,000 primary school children (aged 8-9 years) living in highly polluted areas of London's low emission zone confirms that chronic exposure to common traffic-related pollutants is linked with reduced lung function.
Moreover, despite improvements in air quality following the introduction of the low emission zone, the researchers found no evidence of a reduction in the proportion of children with small lungs or asthma symptoms over the 5 years since the scheme was implemented.
The study highlights that despite European air quality policies and local measures, London's air pollution may be putting children at risk of lifelong health problems, and underscores the need for more radical solutions to tackle high levels of air pollution. Early in 2019, London will introduce an Ultra Low Emission Zone, which is predicted to deliver major improvements in air quality.
"In many areas of London, air pollution still remains a major issue," says author Professor Chris Griffiths from Queen Mary University of London, UK. "Some improvements in air quality have been made despite the diesel vehicles emitting higher levels of pollutants in the real world than in tests. Even so, many areas of inner and outer London are still breaching EU air pollution standards and are unlikely to meet them without a substantial tightening of current emission controls." 
Air pollution is linked to 7 million premature deaths worldwide every year (of these, 3.7 million are linked to outdoor air pollution). In the UK, air pollution contributes to 40,000 deaths each year, nearly a quarter of them in London . Across Europe, where more than half of new cars are fuelled by diesel, nitrogen oxide has become a major problem, and has been linked to asthma and impaired lung development in children.
Low emission zones have been widely promoted as the best way to tackle traffic pollution, with around 200 in operation across Europe. But little research has been done on the effectiveness of this approach at reducing air pollution and improving public health. London introduced the world's largest citywide low emission zone in stages during 2008 and 2012, requiring diesel vehicles entering Greater London to meet certain emission standards or pay daily charges--and providing a unique opportunity to investigate how effectively the initiative has improved air quality and children's respiratory health. 
The study included 2,164 children aged 8-9 years from schools close to air quality monitoring stations in four inner city London boroughs (Tower Hamlets, Hackney, Greenwich, City of London) in breach of EU annual nitrogen dioxide limits (40 μg/m³).
Between 2009-10 and 2013-2014, children were given yearly winter health checks that included measuring the size and function of their lungs by blowing into a machine called a spirometer . Parents were also asked to complete a medical history questionnaire for their child, including questions on respiratory and allergic symptoms (eg, asthma, eczema, hay fever).
Annual average exposure concentrations of nitrogen oxides and particulate pollution (PM 10 and PM 2.5) were measured at the home and school addresses of each child over the 5-year study. The researchers also modelled pollution exposures for the 3 hours (0600-0900h), 24 hours, and 7 days before each child's annual health check. This allowed them look at the effects of both short-term and annual average exposures simultaneously. They also took into account other factors that can affect respiratory health including age, sex, height, body mass index, ethnicity, socioeconomic status, and exposure to environmental tobacco smoke.
Results showed that introduction of the low emission zone had little impact in reducing levels of ultrafine particles (PM2.5) or course particulate matter (PM10) over the study period.
In contrast, levels of nitrogen dioxide at both the roadside and monitoring sites within and surrounding the study area fell by around 1 μg/m³ per year (figure 3). Additionally, the proportion of children exposed to the EU annual nitrogen oxides limit fell from 99% (444/450 children) in 2009 to 34% (150/441) in 2013.
Nevertheless, average exposure levels of nitrogen dioxide over the 5-year study remained high (median 40.7 μg/m³) and the researchers estimate that children's lung capacities were reduced by around 5%. They also noted some evidence of a reduction in rhinitis, but not asthma symptoms over the study period.
"Although changes of this magnitude are unlikely to cause problems in healthy children, we urgently need to know whether these lung deficits will impact lung function and health in later life", says Professor Griffiths. "Until longer-term impacts are known, doctors should consider advising parents of children with clinically significant lung disease to avoid living in highly polluted areas, or to adopt personal mitigation measures to limit their exposure. More research is needed to identify factors that lead to increased susceptibility or protection." 
According to lead author Dr Ian Mudway, King's College London, UK: "There is an urgent need to improve our air quality, especially within our congested cities. Policies such as the Low Emission Zone strive to do this, but their effectiveness needs careful and objective evaluation, not only in terms of whether they improve air quality, but more importantly, whether they deliver better health. As the evidence base grows demonstrating that air pollution impacts on the health of children born and growing up in our cities, so the justification for decisive action increases." 
Co-author Professor Frank Kelly, National Institute for Health Research (NIHR) Health of Environmental Hazards HPRU, UK, adds: "These new findings linking air pollution and children's lung growth provide further support for the introduction of the ultra Low Emission Zone in London early next year." 
The authors note that the study is observational, so no firm conclusions can be drawn about cause and effect. They also point to several limitations, including that the lack of a control group and pre-intervention health data mean that attribution of the changes in air quality and health to the low emission zone, and a direct evaluation of the health impact of the scheme, were not possible. They also note that the study began after the introduction of the initial phases of the scheme so the full effects might have been underestimated.
Writing in a linked Comment Dr Hanna Boogaard, Health Effects Institute, USA, discusses the challenges of conducting research into the health effects of air quality interventions. She concludes: "A key question that remains largely unresolved is whether NO? [nitrogen dioxide] is a causal agent or only an indicator of traffic-related air pollution, given that correlations in space and time between concentrations of NO? and other traffic-related air pollutants are often high. The study by Mudway and colleagues adds to this quandary, and shows that more stringent measures to improve urban air quality and children's health might be needed."