Pollution from the wear of brakes and tyres could eclipse all other sources of transport-related particle pollution by 2020, a foremost expert in environmental health has warned.
Professor Roy Harrison of the University of Birmingham said particulate matter (PM) emissions from electric and supposedly clean vehicles is likely to end up being a greater contributor to this type of pollution than fumes from diesel exhausts.
Prof Harrison, who has contributed to World Health Organisation guidelines on air quality and has advised the government on the implications of bad air, told E&T: "By the end of the decade, we expect pollution from traffic particles will be about 90 per cent from non-exhaust sources.
"This is a source that is growing in relative importance. It is probably growing in absolute magnitude as well as we get more traffic on the roads year on year."
PM is a term used to describe particles of varying minuscule sizes, some of which can penetrate lungs or enter the bloodstream when inhaled.
Some contain metals which could potentially enter the brain and may play a role in the development of neurological disorders.
Exhaust emissions such as nitrogen oxides have so far been the main focus of media attention, with electric vehicles seen as offering a ready-made fix.
But Harrison said: "It has been argued that electric vehicles are traditionally heavier than fossil-fuelled vehicles, and therefore that they have to be braked that much harder.
"If that is the case, these emissions would be as large as those from conventional, non-electric vehicles.
"The latest generation of diesels, with particle filters on the exhaust, emit virtually nothing from the exhaust. So, if an electric vehicle is emitting as much in the way of wear emissions as a diesel vehicle, switching to electric is not going to make a lot of difference."
Wear and tear is particularly acute in the case of lorries. However, it is a problem for all vehicles, be they petrol, diesel or electric, and regardless of whether they are private cars or public transport buses or trains.
"PM is the overlooked villain in the air quality piece," said Bridget Fox from London-based lobby group the Campaign for Better Transport. "We quite rightly focus on diesel, which is the major source of air pollution. It's the major source, but not the only source."
She was quick to stress, however, that using public transport was "part of the solution".
Liza Selley, a PhD student at Imperial College London who is studying pulmonary responses to traffic pollutants, said: "If you think about some of the ways of reducing exhaust emissions, for example switching to an electric or hybrid vehicle, that car still needs to have tyres, and it still has brakes, so the switch isn't really dealing with the issue."
Update on 12 May 2017:
Several readers of E&T have questioned Roy Harrison's comments, based on the fact that they do not appear to take into account the use by electric vehicles of regenerative braking. Professor Harrison responds:
"I am pleased to set the record straight on this issue. I made it clear when talking to E&T that I was basing my comments on a paper by VRJH Timmers and PAJ Achten, published recently in the journal Atmospheric Environment, 'Non-exhaust PM emissions from electric vehicles'. Timmers and Achten report the weight of a number of electric vehicles in comparison with their internal combustion engine equivalent. In all cases, the weight of the electric vehicle was greater, the range being from 14.6 per cent to 28.7 per cent heavier. Non-exhaust emissions from road vehicles arise from brake wear, tyre and road surface wear, and resuspension of road surface dusts. All are in general terms enhanced by increased vehicle weight. Timmers and Achten acknowledge the benefits of regenerative brakes on electric vehicles and made a conservative estimate of zero brake-wear emissions for electric vehicles. Hence, their claim that electric vehicle particulate matter emissions are comparable to those of conventional vehicles was based upon the greater tyre and road surface wear, and resuspension associated with a greater vehicle weight. Some electric vehicles are lighter than their internal combustion engine counterparts; consequently the issue is likely to be considerably more complex than suggested by this research."
