The switch to electric vehicles has other beneficial effects above and beyond reducing carbon dioxide emissions from personal transport. Conventional internal combustion engine vehicles also produce pollution, notable nitrogen oxides and particulate matter, that have seriously detrimental effects on local air quality. Poor air quality can affect people’s health, especially irritation of the respiratory system and lungs and some forms of cancer. Improving local air quality is therefore a major issue for many local councils, and encouraging the switch to EVs is a key means for addressing this. Below we look in more detail at these tailpipe pollution issues.
What are NOx emissions?
When fossil fuels are burned in an internal combustion engine, some of the nitrogen in the air is oxidised to form oxides of nitrogen. This is a mixture of nitric oxide (NO) and nitrogen dioxide (NO2), but the exact proportion of this mixture varies. The mixture is conventionally referred to as NOx emissions (where x denotes a number somewhere between 1 and 2).
Why are NOx emissions harmful?
Both nitric oxide (NO) and nitrogen dioxide (NO2) are hazardous to human health. As gases, they can be easily breathed into the lungs. NOx pollution can cause or exacerbate asthma, decrease lung function, increase the risk of respiratory conditions (such as flu and pneumonia), and increase response to other allergens. The UK’s Clean Air Strategy states “Air pollution is the top environmental risk to human health in the UK, and the fourth greatest threat to public health after cancer, heart disease and obesity.” In the UK alone, air pollution is responsible for 40,000 premature deaths annually.
The NOx emissions can also undergo chemical reactions in the lower atmosphere which form nitric acid and ozone. You may know this from the acrid smell that comes from photocopiers – this is ozone. Both nitric acid and ozone are known to damage lung tissue.
Nitrogen dioxide, NO2, is a pungent, brown gas – you may remember this from chemistry lessons at school, but always made in a fume hood. Nitrogen dioxide is important in the formation of smog and is responsible for the visible brown haze produced.
How do EVs reduce NOx emissions?
With no internal combustion engine, electric vehicles produce no NOx emissions. They therefore save the NOx emissions that would have been produced from an ICE vehicle.
Diesel engines produce particularly large volumes of NOx emissions, 0.96 g per mile. Petrol engines are less harmful, producing 0.48g per mile. This may not seem a lot, but with the average driver clocking up 10,000 miles per annum, a diesel car will emit 10kg of NOx every year.
An alternative way of seeing this would be that a typical urban A-road carrying 20,000 vehicles per day would be producing 3.5 kg of NOx emissions along every metre of road every year.
Clearly switching to an electric vehicle is a sensible means of reducing this pollution and associated health consequences. The Government’s Clean Air Strategy estimates that the costs of air pollution to society could be reduced by £1.7 billion every year.
What are particulates?
Particulates are very small solid particles that are suspended in the air. They are formed from the combustion of fossil fuels in ICE vehicles, with other particulates coming from road dust, and released from brake pads.
Particulates are classified by the size of the particles. Particles that are 10 micrometres (that’s 10/1000 of a millimetre) are called PM10s. Even smaller particles of 2.5 micrometres are called PM2.5s. For context, a human hair (and solar cells, incidentally) are about 100 micrometres.
Particulates accumulate on buildings and other surfaces, which has a visual impact by making the environment look dirty. It can be a considerable expense to clean buildings to remove accumulated soot and other particulate matter.
Why are particulates harmful?
Because particulates are so small, they can be inhaled deep into the lungs and even enter the bloodstream. The World Health Organisation recognises particulates as a carcinogen (that is to say, it causes cancer). They can also cause respiratory issues and heart attacks.
How do EVs reduce particulates?
Diesel vehicles produce the most particulates pollution at 0.064 g/mile. Petrol vehicles produce approximately 20 times less particulate pollution at 0.003 g/mile.
A typical diesel car will therefore emit 640 grams of particulate matter every year. Alternatively, a major A-road carrying 20,000 vehicles per day will release 230 grams of particulates every year, along every meter of the road.
It is important to note that particulate emissions from EVs are not zero, as small amounts of particulates are released from brake pads. This has been picked up by some elements of the media (we won’t publicise them with a hyperlink) as a means of criticizing the switch to electric vehicles. However – and this is so simple, it barely needs saying – particulate emissions from EV brake pads will be many times less than particulate emissions from a vehicle with both brake pads and an internal combustion engine!