Road vehicles are the main cause of the capital’s air pollution.
They produce around half of all nitrogen oxides and emit particulate matter.
What are nitrogen oxides?
Nitrogen dioxide is a brown gas with the chemical formula NO2. It is chemically related to nitric oxide, a colourless gas with the chemical formula NO.
Together, they are known as NOx, and it is NOx that is released into the atmosphere when fuels are burned, for example, petrol or diesel in a car engine.
The concentration of NO2 is measured in micrograms in each cubic metre of air (μg m3).
A microgram (μg) is one millionth of a gram. A concentration of 1μg m3 means that one cubic metre of air contains one microgram of pollutant.
Should I worry more about nitrogen dioxide or particulate matter?
There is evidence that high levels of particulate matter can inflame the airways in our lungs and, over a long period, affect how well they work.
People with asthma are particularly affected. The effect of nitrogen dioxide is not dissimilar to that of particulate matter (PM).
Audrey de Nazelle, senior lecturer in air pollution management at Imperial College London, says: “When you change the vehicle fleet from petrol or diesel to electric it tends to be very effective at reducing nitrogen dioxide but it’s not effective at reducing particulate matter because a lot of that comes from the tyres and brake wear from vehicles. The NO2 emissions come from the exhaust.”
She says that the UK tends to be more concerned with NO2 because we have higher compliance standards for NO2.
“That’s only because the standards for particulate matter are a lot more lax. It’s not because NO2 is more dangerous, it’s just the standards are more stringent.”
Campaigners have been lobbying the government to set limits for particles that are smaller than 2.5 micrometres (PM2.5). They are the most dangerous to human health as they can enter the bloodstream.