A worm's-eye view of Birmingham city centre road

Policy measures to reduce air pollution have been thrust into the spotlight this week, with the expansion of the London Ultra Low Emission Zone (ULEZ) intersecting with national politics following the Uxbridge by-election result. With these polices politicians are trying to create healthier cities with enhanced well-being for all urban residents and workers. With so much political noise, the facts – and some interesting insights from new research into the Birmingham Clean Air Zone – risk getting lost.

Air pollution is the largest environmental risk to health, leading to around 35,000 excess deaths each year in the UK, with short- and long-term exposure shown to increase the risk of acute and chronic diseases. Improving air quality reduces healthcare costs and boosts economic productivity with lower levels of pollution-related illness.

As discussions continue on urban air quality and sustainable development, Birmingham’s CAZ has emerged as a focal point of debate within the regional political landscape.

Bowen Liu – Assistant Professor, Department of Management, University of Birmingham

To date, many prominent interventions, such as Low Emission Zones (LEZs) or Clean Air Zones (CAZs), aim to reduce air pollution arising from vehicles, as there is a legal obligation to reduce NO2 levels below a specific threshold level, and most NO2 in cities – where levels are highest – originates from road transport. As discussions continue on urban air quality and sustainable development, Birmingham’s CAZ has emerged as a focal point of debate within the regional political landscape.

Reducing air pollution is critical but needs behavioural change

Mitigating air pollution is critical for public health and urban sustainability. Yet, achieving this goal hinges not just on regulations, but on fostering behavioural change. Encouraging cleaner transport options, embracing energy efficiency, and promoting sustainable urban practices are essential for Birmingham’s journey toward cleaner air and a healthier city.

In our recent study published in Environmental & Resource Economics, an interdisciplinary team of atmospheric scientists and environmental economists from the University of Birmingham quantified the ‘causal’ impacts of the Birmingham CAZ on local air quality. Using an innovative method called ‘random forest machine learning’ to strip out the effects of weather, and other seasonal factors – such as changes in travel behaviour linked to holidays – on air pollution levels, we can quantify the change in NO2 attributable to the CAZ policy alone, if nothing else had changed.

This analysis found that during the first seven months of operation, the implementation of the Birmingham CAZ led to ‘modest, but significant’ reductions in NO2 of up to 7.3%. Interestingly, the study also found evidence of reductions in NO2 levels outside the CAZ area – suggesting that the footprint of benefits may extend well beyond the zone itself. The approach used in the study may now provide a blueprint for cities across the globe to investigate the effectiveness of their own Clean Air interventions.2

As predicted, the CAZ led to reductions in NO2 pollution but did not demonstrate any detectable impact on the other key pollutant in urban air - PM2.5 - fine particles in the air, which originate from a much broader range of sources, and which have the most significant health implications. The challenge here is that the Birmingham CAZ as a policy is designed to reduce one pollutant – NO2 – below a specific legal limit – while the science tells us that attention to all pollutants, especially PM2.5, is required. For NO2 health impacts occur at levels well below the current legal limit in the UK.

Solutions beyond CAZ needed

CAZ and ULEZ policies spark strong reactions as they challenge existing behaviour, impose immediate individual and organisational costs against longer term and societal benefits, and their effectiveness is mixed. Policy makers should make such strategies part of a comprehensive approach, currently missing in UK cities, to reduce health impacts from all air pollution, rather than just reduce levels of one pollutant, NO2, below a legal limit. Clean Air Zones may be necessary, but they are not sufficient.

We need an all-encompassing strategy to foster healthy cities and ULEZ/CAZ is just one piece of the puzzle. Initiatives should target the removal of high-polluting vehicles, especially those visibly emitting smoke. Reducing emissions from domestic solid fuel combustion is increasingly important. Effective spatial planning is important to avoid needless trips caused by poor urban design. We need an affordable, integrated public transport system and support for hybrid working models. Intelligent green infrastructure design is also vital; it not only beautifies urban spaces but also mitigates air pollution impacts.

Local air pollution and the global climate challenge

Many air pollution sources also cause carbon dioxide (CO2) emissions – as they involve burning fossil fuels – meaning that there are win-wins for air quality and for climate from their reduction. However, the spatial effects differ as CO2 lasts a long time in the atmosphere and wider earth system – climate change is a global challenge requiring reductions in carbon emissions everywhere. In contrast, many air pollutants are local or regional, as they only last a few hours or days in the air. This means that local and regional changes can improve local air quality and deliver local health benefits – irrespective of what happens much further afield. Understanding this difference may help local communities to assess the benefits, locally, from initiatives such as Clean Air Zones in their neighbourhoods, and to better engage with such policies in the future.