Birmingham’s clean air zone policy has successfully reduced levels of an air pollutant which adversely affects people’s health, a new study has found.
University of Birmingham scientists used a new, innovative technique to discover that during the CAZ’s first seven months of operation the policy resulted in "modest, but significant" reductions in nitrogen dioxide gas, which fell by up to 7.3%.
The new technique involved a method called "random forest machine learning", which strips out weather and seasonal effects on air pollution levels, and then compared the "de-weathered" air pollution data in the Birmingham CAZ with those from cities with no CAZ.
In their findings, which have now been published in Environmental & Resource Economics, the researchers discovered that over the first seven months since the CAZ launched in June 2021, the biggest reductions in nitrogen dioxide were at busy roadside locations within the zone.
They also found levels fell on roads outside the CAZ - which suggested that rather than traffic just moving to nearby areas, there could be behavioural changes that contributed to reduced air pollution in surrounding neighbourhoods too.
Lead author Dr Bowen Liu said their work provided the first comprehensive evaluation of the Birmingham CAZ, "an internationally significant policy to improve urban air quality in the UK’s second city".
"As predicted, Birmingham’s CAZ reduced [nitrogen dioxide] pollution, but it has no detectable impact in the concentrations of fine particles, PM2.5 – the air pollutant with greatest health effects.”
Professor Zongbo Shi, who oversaw the research, added that PM2.5 at monitoring sites in Birmingham still regularly exceeded WHO air quality guideline levels, "at which health impacts occur with significant health implications, including hundreds of premature deaths every year".
"More rigorous policy interventions - such as further local measures to reduce wood burning and agricultural emissions and nationally co-ordinated actions to mitigate secondary PM2.5 pollution - are needed to address non-vehicle sources of PM2.5 as quickly as possible.”
According to the WHO, air pollution is one of the largest risks to the health of urban populations, with short and long-term exposure shown to increase the risk of acute and chronic diseases, and reduce life expectancy. Professor Shi added: "Improving air quality reduces healthcare costs, including to the NHS, and boosts economic productivity with lower levels of pollution-related illness.”
The new study, which was funded by Natural Environment Research Council and supported by Research England, is part of the WM-Air project. The new techniques used could provide a blueprint for cities across the UK and beyond - such as London, Glasgow, Munich, Milan, and Amsterdam - to analyse the effectiveness of their own clean air interventions.
This comes just days after London's embattled ULEZ - or ultra low emission zone - was rolled out to cover the entire Greater London area.
The policy has faced some fierce opposition in the form of a High Court action by Conservative-lead councils and ULEZ camera vandals, but Mayor Sadiq Khan has maintained that while a difficult choice during a cost-of-living crisis, the expansion was "the right thing to do".