Implementation of regulatory standards has reduced exhaust emissions of particulate matter from road traffic substantially in the developed world. However, nonexhaust particle emissions arising from the wear of brakes, tires, and the road surface, together with the resuspension of road dust, are unregulated and exceed exhaust emissions in many jurisdictions. While knowledge of the sources of nonexhaust particles is fairly good, source-specific measurements of airborne concentrations are few, and studies of the toxicology and epidemiology do not give a clear picture of the health risk posed. This paper reviews the current state of knowledge, with a strong focus on health-related research, highlighting areas where further research is an essential prerequisite for developing focused policy responses to nonexhaust particles.

This research quantifies current sources of non-exhaust particulate matter traffic emissions in London using simultaneous, highly time-resolved, atmospheric particulate matter mass and chemical composition measurements. The measurement campaign ran at Marylebone Road (roadside) and Honor Oak Park (background) urban monitoring sites over a 12-month period between 1 September 2019 and 31 August 2020. The measurement data were used to determine the traffic increment (roadside–background) and covered a range of meteorological conditions, seasons, and driving styles, as well as the influence of the COVID-19 “lockdown” on non-exhaust concentrations. Non-exhaust particulate matter (PM)₁₀ concentrations were calculated using chemical tracer scaling factors for brake wear (barium), tyre wear (zinc), and resuspension (silicon) and as average vehicle fleet non-exhaust emission factors, using a CO₂ “dilution approach”. The effect of lockdown, which saw a 32% reduction in traffic volume and a 15% increase in average speed on Marylebone Road, resulted in lower PM₁₀ and PM_2.5 traffic increments and brake wear concentrations but similar tyre and resuspension concentrations, confirming that factors that determine non-exhaust emissions are complex. Brake wear was found to be the highest average non-exhaust emission source. In addition, results indicate that non-exhaust emission factors were dependent upon speed and road surface wetness conditions. Further statistical analysis incorporating a wider variability in vehicle mix, speeds, and meteorological conditions, as well as advanced source apportionment of the PM measurement data, were undertaken to enhance our understanding of these important vehicle sources.

With vehicle fleets transitioning from internal combustion engines (ICE) to electric powertrains, we have used friction brake power simulations, for different vehicle classes and driving styles, to predict the impact of regenerative braking systems (RBS) on brake wear particulate matter emissions (PM₁₀ and PM_2.5). Under the same powertrain, subcompact (SC) vehicles were predicted to require between 38 and 68% less friction brake power than heavier sports utility vehicles (L-SUV). However, despite electric and hybrid vehicles being heavier than ICE vehicles, the results show that RBS would reduce brake wear by between 64 and 95%. The study highlights the effect of aggressive braking on the amount of friction brake power required, with electric powertrains more likely to require friction braking to perform short, but aggressive braking compared with longer, slower braking events. Brake wear reductions varied under different driving conditions, as the level of mitigation depends on the complex interaction of several variables, including: vehicle speed, deceleration rate, regenerative braking technology and vehicle mass. Urban brake wear emission factors for electric powertrains ranged from 3.9 to 5.5 mg PM₁₀/km and 1.5-2.1 mg PM_2.5/km, providing an average reduction in PM emission factors of 68%. Rural and motorway driving conditions had lower brake wear emission factors, with plug-in hybrid electric vehicles (PHEV) and battery electric vehicles (BEV) emitting negligible PM₁₀ and PM_2.5 brake wear. Although electric powertrain uptake, vehicle mileage driven and driving styles are dependent upon national policies and strategies, by 2035, we project that total UK brake wear PM emissions would reduce by up to 39% compared with 2020 levels. This analysis supports the transition towards electric and hybrid vehicle fleets to reduce brake wear emissions, however increases in tyre wear, road wear, and resuspension due to increased vehicle mass may offset these benefits.

The relationship between indoor air quality and public health remains under-researched. WellHome is a transdisciplinary community-based study that will engage with residents to co-design feasible and acceptable research to quantify air pollution exposure in 100 homes in West London and examine its potential to exacerbate asthma symptoms in children. Sampling strategies such as using air quality monitors and passive samplers placed in kitchens, children’s bedrooms, and living rooms, will be developed in collaboration with local ambassadors and participating households to measure multiple physical, chemical, microplastic, and biological contaminants. This will provide a comprehensive understanding of indoor air quality across the city’s socio-economic gradient. Other data collected will include housing types and tenure, ventilation practices, occupant behaviours, time-activity, and airway symptoms. Epidemiological analysis will examine air pollution exposure impacts on children’s respiratory health. The particulate mixture’s relative hazard will be evaluated in toxicity studies based on source profiles and activity patterns of participants, focusing on asthma exacerbation related pathways. The study’s findings will be communicated to participants through co-designed reports and inform evidence-based recommendations for reducing indoor air pollution in London and urban areas worldwide. By raising awareness and providing actionable insights, WellHome seeks to contribute to global efforts to improve the health and well-being of vulnerable communities.

The recent United Kingdom (UK) Environment Act consultation had the intention of setting two targets for PM_2.5 (particles with an aerodynamic diameter less than 2.5 μm), one related to meeting an annual average concentration and the second to reducing population exposure. As part of the consultation, predictions of PM_2.5 concentrations in 2030 were made by combining European Union (EU) and UK government's emissions forecasts, with the Climate Change Committee's (CCC) Net Zero vehicle forecasts, and in London with the addition of local policies based on the London Environment Strategy (LES). Predictions in 2018 showed 6.4% of the UK's area and 82.6% of London's area had PM_2.5 concentrations above the World Health Organization (WHO) interim target of 10 μg m^-3, but by 2030, over 99% of the UK's area was predicted to be below it. However, kerbside concentrations in London and other major cities were still at risk of exceeding 10 μg m^-3. With local action on PM_2.5 in London, population weighted concentrations showed full compliance with the WHO interim target of 10 μg m^-3 in 2030. However, predicting future PM_2.5 concentrations and interpreting the results will always be difficult and uncertain for many reasons, such as imperfect models and the difficulty in estimating future emissions. To help understand the sensitivity of the model's PM_2.5 predictions in 2030, current uncertainty was quantified using PM_2.5 measurements and showed large areas in the UK that were still at risk of exceeding the WHO interim target despite the model predictions being below 10 μg m^-3. Our results do however point to the benefits that policy at EU, UK and city level can have on achieving the WHO interim target of 10 μg m^-3. These results were submitted to the UK Environment Act consultation. Nevertheless, the issues addressed here could be applicable to other European cities.