AQEG Advice - on the concept of ‘zero emission vehicles’

With a drive to reduce the emissions from road transport, attention has turned to vehicles that produce lower emissions than those powered by conventional internal combustion engines. This has given rise to the term “zero emission vehicle” which is, for example used to describe battery electric vehicles. While some recognise that this is intended to mean that the vehicle has no exhaust emissions it implies that there are no emissions associated with the running of the vehicle at all and this is misleading. The impact of a vehicle on air quality arises through both exhaust emissions (often referred to as ‘tailpipe’ emissions) and non-exhaust emissions. The latter come from such processes as tyre and brake wear [1] and road surface abrasion [2].  For electric vehicles, while it is true these are zero emission vehicles in terms of exhaust emissions, they still produce substantial non-exhaust emissions of particulate matter [3, 4] much like conventionally fuelled vehicles. Some designs of electric buses also incorporate diesel powered heating systems which will be an additional source of emissions [5]. This is not just a technicality but an issue of effective communication which has the potential to damage consumer trust and make discussion of non-exhaust emissions more challenging.

With electric vehicles, there is a further issue to be considered.   The electricity produced to charge the vehicle may, depending on its source, also produce emissions of pollutants that affect air quality.  Electric vehicles can thereby displace emissions from the kerb-side to the power station [6].  Clearly, the emissions associated with electricity generation will vary according to the electricity generating mix. For example, vehicles charged with a purely renewable energy source, such as wind or solar power have the potential to have effectively zero emissions in terms of power generation. However, both electric and conventionally-fuelled vehicles may have additional emissions when considering a full life-cycle analysis of the vehicle and its usage.  Conversely, the increased use of small-scale combustion plants within urban areas to generate the electricity required could have implications for urban nitrogen oxide (NOx) emissions.

Electric vehicles have the potential to greatly reduce emissions of air pollutants, particularly NOx, at the roadside through eliminating exhaust emissions. However, to avoid any confusion or misunderstanding and considering the points made above, AQEG recommends that the term ‘zero emission vehicle’ with reference to electric vehicles should not be used and should be replaced by ‘zero exhaust emission vehicle’.

Evidence Base:

  1. Grigoratos, T. and G. Martini, Brake wear particle emissions: a review. Environmental Science and Pollution Research, 2015. 22(4): p. 2491-2504.
  2. Thorpe, A. and R.M. Harrison, Sources and properties of non-exhaust particulate matter from road traffic: A review. Science of The Total Environment, 2008. 400(1): p. 270-282.
  3. AQEG, Fine particulate matter (PM2.5) in the UK. 2012, Defra: London. p. 203.
  4. Timmers, V.R.J.H. and P.A.J. Achten, Non-exhaust PM emissions from electric vehicles. Atmospheric Environment, 2016. 134: p. 10-17.
  5. Partnership, L.C.V. The low emission bus guide. 2016  [cited 2018 19th February ]; Available from: http://www.lowcvp.org.uk/assets/reports/LowCVP%20LEB%20Guide%202016%20interactive%20V3.pdf (PDF).
  6. Nopmongcol, U., et al., Air Quality Impacts of Electrifying Vehicles and Equipment Across the United States. Environmental Science and Technology, 2017. 51(5): p. 2830-2837.