2.1 Local air pollution

The main local air pollutants from burning transport fuels in the UK are:

• Carbon monoxide (CO) – this is a highly toxic gas that can impair brain function and, in sufficient concentrations, kill. Transport was a significant source of CO in the 1970s but tightened emission standards have dramatically reduced emissions and it now only contributes about one sixth of the UK total.
• Nitrogen oxides (NO_x) – mainly the acidic gases NO, nitric oxide and NO2, nitrogen dioxide. These cause respiratory problems and contribute to low-level ozone formation and acid rain. In 2019 transport produced about 40% of all UK NO_x emissions.
• Particulate matter (PM) – this is responsible for respiratory problems and is thought to be a carcinogen. Small particulate matter, known as PM10 and PM2.5 (particles smaller than 10 μm and 2.5 μm in diameter respectively), is particularly dangerous as it can penetrate deep into the lungs.
• Volatile organic compounds (VOCs) – benzene (which can be a component of petrol) and 1,3-butadiene (a product of incomplete combustion of fuels) are both carcinogens. They are easily inhaled owing to their volatile nature. VOCs are also ‘ozone precursors’. In the presence of bright sunlight their presence can assist the oxidation of oxygen in the air to ground-level ozone, which is toxic, even in low concentrations.

Other pollutants are:

• Sulfur dioxide (SO₂) – this is an acidic gas that can affect health and damage vegetation. It is produced by the combustion of sulfur compounds in the fuel. Emissions of this have been dramatically reduced in the UK by refining the sulfur content out of road vehicle fuel. However, sulfur is still permitted in shipping diesel fuel.
• Lead dioxide dust – lead is a heavy metal that can cause brain damage. In the past, tetraethyl lead was a common additive to improve the octane rating of petrol. Its use was banned in the UK in 2000 and this ban is now almost worldwide.

The level of air pollution in a particular location or city can be reported as a combined Air Quality Index (AQI) figure, the higher the number, the worse the air quality. Different countries have slightly different AQI definitions, but generally, they attempt to reflect the overall likely negative effect on health.

This is a global problem. In the USA, California has long had serious vehicle air pollution problems. These have led to progressively tightening emission standards since the 1970s, which have necessitated the development of new ‘clean-up’ technologies.

In other countries such as India and China where emission standards are less developed, air pollution is particularly severe.

Total emissions of SO₂, NO_x and particulates in the UK have fallen significantly since 1990. However, in 2019 it was estimated that there were still 28,000 – 36,000 deaths each year attributable to human-made air pollution in England alone (PHE, 2019).

NO_x and particulate pollution are intensely localised around main roads. This can clearly be seen when monitored and then plotted or mapped on to the traffic grids of major cities.

Figure 2 shows estimated concentrations of a single pollutant, NO₂, in London in 2019, extrapolated from measurements at a number of sites.

Figure 2: A map of estimated annual average NO₂ levels in London in 2019

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This is a map showing nitrogen dioxide concentrations in London. It covers an area approximately 50 kilometres from east to west and 30 kilometres from north to south. Central London is shown in the middle with Heathrow Airport visible at the lower left. The annual mean nitrogen dioxide concentration is shown by colour. A legend at the right shows yellow as indicating a level of between 15 and 20 microgrammes per cubic metre, light orange 20 to 25 microgrammes, orange 25 to 30 microgrammes, light pink 30 to 35 microgrammes, pink 35 to 40 microgrammes, light purple 40 to 45 microgrammes, mid-purple 45 to 50 microgrammes, dark purple 50 to 55 microgrammes, dark purple brown 50 to 55 microgrammes and dark brown over 55 microgrammes. There is an arrow to the right of the legend indicating a limit of 40 microgrammes. The map shows that the annual mean is only below 25 microgrammes in the outer suburbs and increases towards the centre of London. Most of central London has a level of 30 microgrammes or higher. The main roads are visible on the map as purple lines, indicating high levels of nitrogen dioxide, above the limit of 40 microgrammes. There are also high levels around Heathrow airport.

Figure 2: A map of estimated annual average NO₂ levels in London in 2019

A gram of NO₂ emitted in London’s Oxford Street can do far more health damage than one emitted on a rural road because there is a higher density of people there to breathe it in. The financial damage cost of a gram of NO₂ thus depends on where and when it is emitted. This is unlike the damage cost for CO₂ which has a global effect and largely independent of where it is emitted.

Note that Figure 2 is a snapshot of NO₂ levels in 2019. London has had a series of progressively tightening ‘Low emission zone (LEZ)’ and ‘Ultra low emission zone (ULEZ)’ policies since 2008. These aim to make sure that only vehicles meeting the latest low emission standards enter the city and have resulted in significant reductions in pollution levels. Many other cities worldwide have introduced similar policies.