This is why electric cars won't stop air pollution

Dr Jody Muelaner

(Credit: Shutterstock)
(Credit: Shutterstock)

Greenhouse gases such as CO2 and methane cause climate change, while various localised air pollutants cause respiratory disease.

These two quite separate issues are often confused. 

Electric cars will reduce our contribution to climate change, assuming we increase renewable energy generation to meet the higher demand. Replacing internal combustion engines with electric motors will also reduce some of the urban air pollutants that cause localised health risks. However, one of the biggest contributors to respiratory disease, particulate matter (PM), is not predominantly produced by the combustion of fossil fuels. 

The erosion of tyres and brake pads, as well as road dust, are also major sources of PM. Changing from internal combustion engines to electric motors will not reduce these sources of particulates.

“The latest research is indicating that non-tailpipe PM emissions are now 55-60% of vehicle emissions and this proportion will continue to grow with electrification of the fleet,” said Professor Frank Kelly of King’s College London, chair of the UK government’s Committee on the Medical Effects of Air Pollutants. 

The committee estimates that between 28,000 and 36,000 deaths are caused by air pollution each year in the UK. Strong correlations between air pollution and increased mortality mean that this figure can be given with some confidence, although the uncertainty is reflected in the range of values. 

Problematic estimates

Attributing deaths to individual pollutants is much more difficult. Many pollutants are created by combustion, such as oxides of nitrogen, sulphur oxides, carbon monoxide and unburned hydrocarbon. Particulates include combustion products such as soot and ash, as well as other traffic-related sources such as tyre particles. 

There is good evidence from various sources that all of these pollutants have negative effects on health, for example animal studies showing the toxicity of individual substances. However, the different types of pollution are also all strongly correlated with volumes of traffic, and therefore with each other. This makes it very difficult to estimate how many deaths are caused by each individual type of air pollution.

A literature review carried out for the European Commission concluded that about half of all particulate matter results from brake dust, road dust and tyre erosion. It also found some evidence that particulates from these sources may be more toxic than those from diesel combustion, although there is no conclusive evidence of deaths caused by these forms of pollution. 

Zinc from tyres

Tyres contain 1-2% zinc by mass and it is this zinc in tyre dust that has been associated with much of the toxic effect when inhaled. Vaporisation of organic compounds in tyres may also release highly carcinogenic volatile compounds such as polycyclic aromatic hydrocarbons.

“Due to the uncertainties about the emission factors for each source it is difficult to state how many deaths are caused by non-tailpipe particulate matter,” said Kelly. “As a very rough estimate, road transport is responsible for 50% of PM emissions in cities and the latest research is showing that non-exhaust emissions are now over 50% of vehicle PM emissions. This suggests that non-tailpipe PM is responsible for between 7,000 and 8,000 premature deaths a year.”  

No panacea

The rapid development of electric vehicles can give the impression that road transport is becoming sustainable. Electric cars are sometimes seen as a panacea for the environmental impact of transport. They are not. 

This illusion diverts resources from more mature technologies that can reduce pollution today. Established urban transport technologies, such as rapid transit and bicycles, remain much better solutions to the multifaceted challenges of climate change, energy security and public health. 

As engineers our duty should not only be to develop products that follow the latest trends. It is our responsibility to identify and promote the most appropriate engineering solutions to the problems facing our society.

Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.

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