Here is also a comparison of different non-tailpipe emissions:
(Extract below is from a submission to DFT as a result of its call for evidence on the benefit of trams)
Q4 What would the environmental, economic and congestion benefits be?
Pollution – tail pipe and non-tail pipe emissions.
Trams are free from the majority of pollutants common with other road vehicles. Trams are
normally electrically powered with zero emissions at point of use. The emissions at power stations
are remote and more efficient than diesel or petrol powered vehicles as modern power generation
is either emission free with hydro, wind or tidal power stations or now well controlled with fossil fuel
based power generation. Diesel and petrol powered vehicles emit harmful PM10 and PM2.5
particulates as well as NOx pollution, all harmful to human health.
Trams also do not produce the non-tailpipe emission common with normal road vehicles including
buses. Steel wheels on steel rails wear much more slowly than rubber tyres on tarmac road
surfaces. The erosion of rubber tyres and the road surface, particularly from heavy buses, lorries
and cars produces a very fine toxic dust at PM2.5 size that is particularly hazardous to health being
easily absorbed by humans and can affect the brain.
As well as the tyre/road erosion, there is a fine dust produced from brake pad wear on road
vehicles. Trams use electric braking with disc brakes only used in the final stages of braking
minimising such emissions.
A Paper by Timmers & Achten in 2018 “Non-Exhaust PM Emissions from Battery Electric Vehicle”
describes their research into non tailpipe illustrated in the figure below shows how significant the
non-tailpipe PM emissions are from cars.
In Figure 12.2 they detail the difference in weight between electric vehicles (EVs) and their internal
combustion engine (ICEV) counterparts showing that EVs are between 14.6% and 28.7% heavier.
The paper also notes the resuspension of the PMs as vehicle re-lift PMs already lying on the
In the conclusions of this paper it notes that there is a consensus that whilst there has been a
strong reduction in tailpipe PM emissions over the past decades and this will continue in the
coming years, non-tailpipe emissions will account for 90% of PM emissions from traffic by the end
of the decade. It also notes that claims that EVs are emission free are unjustified as the increase in
weight is linked to higher non-tailpipe emissions.
This paper really brings together research into non-tailpipe emission and challenges the assertion
that electric vehicles are the answer to transport emissions. The weight gain argument applies to
electric buses as well as cars.
Trams running with steel wheels on steel rails eliminate both the tailpipe emissions and road and
tyre wear emissions and the majority of brake pad wear as electric braking is used for the most of
the slowing of the vehicles.
The outcome from the above is that Trams will eliminate both tailpipe and non-tailpipe emissions
produced by vehicles with rubber types and that these PM10 and PM2.5 are particularly harmful to
health as noted in the World Health Organisation paper “Health Effects of Particulate Matter” which
“PM is a widespread air pollutant, present wherever people live.”
The health effects of PM10 and PM2.5 are well documented. There is no evidence of a safe level of
exposure or a threshold below which no adverse health effects occur.
Since even at relatively low concentrations the impacts of air pollution on health is significant,
effective management of air quality aiming to achieve WHO AQG levels is necessary to reduce
health risks to a minimum.
Monitoring of PM10 and/or PM2.5 needs to be improved in many countries to assess population
exposure and to assist local authorities in establishing plans for improving air quality.
There is evidence that decreased levels of particulate air pollution following a sustained
intervention result in health benefits for the population assessed. These benefits can be seen with
almost any decrease in level of PM. The health and economic impacts of inaction should be
assessed.” (WHO, 2013, page 12).
Defra have recently concluded a ‘Call for Evidence: Brake, Tyre and Road Surface Wear’
We await the results and how they will influence the Defra and DfT future strategies on Clean Air