Connect the dots

The problem


1.The problem

What is air pollution?

Air pollution is ‘the presence in or introduction into the air of a substance which has harmful or poisonous effects’.1 The most common air pollutants are listed below (see table 1.1).

Emissions of these pollutants damage the environment and are significant determinants of human health. A large body of evidence exists that suggests air pollutants increase the prevalence of numerous health conditions, including lung cancer and asthma.

Table 1.1

Air pollutants and their health impacts

Pollutant name

Main sources

Health impact


Evaporation and combustion of petroleum products

Cancer, leukaemia.

Carbon monoxide (CO)

Road transport (particularly petrol), combustion, industry – CO arises from incomplete combustion

Headaches, nausea, dizziness, affects lung performance

Heavy metals

Combustion, industrial processes

Nausea, diarrhoea, abdominal pain, irritation of eyes, nose, throat, and lungs, brain and kidney damage, asthma, respiratory diseases, lung cancer

Nitrogen Dioxide (NO2)

Transport, combustion

Lung irritation, decrease lung function, and increase chance of respiratory infections – long-term exposure is associated with low birth weight babies and excess deaths

Ozone (O3)

Reaction of hydrocarbons, NOX, and volatile organic compounds (VOCs) in sunlight

Harms lung function and irritates respiratory system; can increase incidence and severity of asthma and bronchitis – long-term exposure can lead to cardiorespiratory mortality

Particulate Matter
(PM10 and PM2.5)

Transport (exhaust, tyre and brake wear), combustion, industrial processes and construction

Linked to asthma, lung cancer, respiratory and cardiovascular diseases, infant mortality and low birth weight

Sulphur Dioxide (SO2)

Combustion (coal) and road transport

Causes irritation of lungs, nose and throat, and exacerbates asthma

Source: WHO 2013

Of particular concern to human health are nitrogen dioxide (NO2) and particulate matter (PM10 and PM2.522), as both are recorded at high concentrations across the UK, and because the evidence linking them to poor health is strong (Defra 2015a). Indeed, experts estimate that 29,000 ‘equivalent’ deaths are caused annually from exposure to PM2.5 in the UK (COMEAP 2010) – a figure which increases to 40,000 when also considering the related effects of NO2 (RCP 2016). The subsequent costs to the NHS and the economy (in terms of working days lost) is estimated at more than £20 billion every year (RCP 2016).

Such impacts have led the World Health Organisation (WHO) to set out ‘guideline levels’ for the concentration of each pollutant (WHO 2005) (see table 2.1 below). However, in the case of PM, WHO has cautioned that, because health impacts are still found at very low concentrations, ‘no threshold has been identified below which no damage to health is observed’.

Table 1.2

Pollutant concentration limits within the European Union

Pollutant name

Averaging period

(micrograms per cubic metre, µg/m3)

Permitted exceedances (per year)


WHO guideline levels


1 hour

1 year

200 µg/m3

40 µg/m3



Jan 2010 (extended Jan 2015)

Jan 2010 (extended Jan 2015)

As per EU values

As per EU values


1 day

1 year

50 µg/m3

40 µg/m3



Jan 2005 (extended Jan 2011)

Jan 2005

As per EU values

20 µg/m3


1 year

1 year

25 µg/m3

20 µg/m3



Jan 2015

Jan 2020

10 µg/m3

10 µg/m3

Source: EC 2016
Notes: Concentration limits are expressed either in the form of an annual average concentration or as a restriction on the number of ‘exceedances’ over shorter time periods in a whole zone.

Regardless, the WHO guidelines are generally being seen as the standard by which success can measured, and have been translated into law via the European Union’s Ambient Air Quality Directive 2010 (and then into UK law through the Air Quality (Standards) Regulations 2010). This means that any area within the UK that breaches these limits is breaking both UK and EU law (pre-Brexit).

How is the UK performing?

According to the government’s latest draft National Air Quality Strategy for Tackling Nitrogen Dioxide, there are 31 zones across the country that are failing to comply with EU law on NO2 limit values (Defra 2017). WHO finds that there are 11 and 40 areas across the UK which are in breach of WHO safe ‘guidelines’ on PM10 and PM2.5 respectively (WHO 2016).

Furthermore, modelling by the Department for the Environment, Food and Rural Affairs (Defra) has previously shown that, without significant policy changes, most of the UK will remain in breach of legal limits into 2025 and beyond, with London not reaching compliance with legal limits until 2030 (Defra and DfT 2017a).

The UK is not the only country failing to comply with EU law; some other countries continue to breach concentration limits across the EU (ibid 2017). However, few countries perform as poorly as the UK in terms of the number of areas that are non-compliant, and the length of time this is likely to remain the case.

Figure 1.1

Areas in breach of EU limit values for NO2

Source: Defra and DfT 2017a

Table 1.3

Number of zones* projected to be non-compliant with the limit value for NO2 over time, assuming no additional policy interventions**









Number of zones
















Number of zones








Source: Defra and DfT 2017a
*Out of the total 43 reporting zones
**These projections are based on COPERT 5 emission factors. If Euro standards are less effective than predicted (as has been the case with historical real-world operations), the number of non-compliant zones will be higher.

What is causing the air pollution problem?

The causes of air pollution are many and complex, with significant contributions from gas combustion (including cookers and central heating boilers), as well as rail and air transport. However, one of the primary causes is road transport.

Figure 1.2

Breakdown of UK national average NOx roadside concentration into sources, 2015

Source: Defra and DfT 2017a

Cars, buses and lorries accounted for around 34 per cent of nitrogen oxide (NOx) emissions, and 14 and 13 per cent of PM10 (particulate matter) PM2.5 respectively in 2015 (Defra and DfT 2017a). However, this rises significantly when we consider concentrations at the roadside, where road transport contributes some 80 per cent of NOx concentrations (see figure 1.2).

The main source of road transport based pollution – making up over three quarters of emissions in total – are diesel vehicles. This increase in diesel-related emissions is driven largely by two main factors.

  1. A growing proportion of the fleet is made up of diesel vehicles in the UK, from 1.6 million (7 per cent) in 1994, to more than 10.7 million (over 36 per cent) today (DfT 2016).
  2. Diesel engines have failed to deliver the expected reductions in emissions under real world driving conditions compared to test conditions, with some studies showing that Euro 6 diesels produce between two and a half and seven times the published standard when driven on the road (Weiss et al 2011, Transport and Environment 2016).

What has the government been doing about it?

Awareness of the air pollution problem has been growing among the public and policymakers, but both national and local government have yet to respond with adequate measures to address the challenge.

As a result, in 2015 the UK Supreme Court ordered the government to introduce measures within the national action plan for NO2 to meet compliance across air pollution zones in the shortest time possible (UKSC 2015).

To meet this requirement, Defra produced a new plan for improving air quality in the UK. This included the introduction of ‘clean air zones’, in which the most polluting vehicles may face additional charges and regulation (with severity of charges and types of vehicles included dependent on the scale of their breach of EU limits).

Clean air zones

A clean air zone is ‘an area where targeted action is taken to improve air quality and resources are prioritised and coordinated in a way that delivers improved health benefits and supports economic growth’ (Defra 2017).

Any local authority can implement a clean air zone to address a local air quality issue. However, following a consultation in 2015, the UK government modelled NO2 levels across the UK, which indicated which areas might need to implement a charging clean air zone to achieve compliance with legal limits for nitrogen dioxide. This led to five cities – Birmingham, Derby, Leeds, Nottingham and Southampton – in addition to London exploring the implementation of such a zone.

Clean air zones fall into two categories.

  1. Non-charging clean air zones: These are defined geographic areas, used as a focus for action to improve air quality. This action can take a range of forms, including:
    • exploring innovative retro fitting technologies and new fuels
    • buying ultra low emission vehicles (ULEVs), and encouraging local transport operators to do the same
    • encouraging private uptake of ULEVs by ensuring adequate charge points
    • encouraging use of public transport, cycling, walking, park and ride schemes and car sharing
    • improving road layouts and junctions to optimise traffic flow, for example by considering removal of road humps
    • working with local businesses and neighbouring authorities to ensure a consistent approach.
  2. Charging clean air zones: These are zones where, in addition to the above, designated vehicles are required to pay a charge to enter or to move within the zone, if they are driving a vehicle that does not meet the particular standard for their vehicle type in that zone. Depending on the extent of the air pollution problem in the city, these zones may include different vehicles (set out in the table below).

Table 1.4

Charging clean air zone classes which local authorities may choose to deploy

Charging clean air zone class

Vehicles potentially included


Buses, coaches, taxis and private hire vehicles


Buses, coaches, heavy goods vehicles (HGVs), taxis and private hire vehicles


Buses, coaches, HGVs, large vans, minibuses, small vans/light commercials, taxis and private hire vehicles


Buses, coaches, HGVs, large vans, minibuses, small vans/light commercials, taxis and private hire vehicles, cars, motorcycles and mopeds

Source: Defra and DfT 2017a

However, while these plans did bring forward the date of compliance for many UK cities, Defra modelling suggested that, even with these changes, the majority of cities in the country would only reach compliance in 2025 or beyond (Defra and DfT 2017a). Under this plan, these zones also only covered a small number of areas. Many additional areas that were still failing to comply with UK and EU law were not required to take further action.

This led many environmental groups to conclude that the plans are still inadequate, and in November 2016 the government lost their second case in the supreme court, with the court ordering them to re-draft the air quality plan. This was published in consultation form in May 2017 (Defra and DfT 2017b). It sets out a range of possible additional options to reduce air pollution, including an expansion of the number of clean air zones to 27 and a scrappage scheme. However, it is worth highlighting that it does not commit government to implementing these measures.

Where next?

It is our view that the scale of the air pollution problem in the UK requires a bolder and more holistic approach. The evidence from places like London (Laybourn-Langton et al 2016) is that, to deliver the kind of improvements needed in air quality, two significant shifts will be needed in the way we travel.

  1. A move away from diesel vehicles towards petrol and, ultimately, hybrid and electric alternatives.
  2. A move away from private cars towards car sharing schemes, public transport, walking and cycling.

Achieving this transition will be difficult. It took nearly two decades for diesel cars to grow from a small minority of the fleet to two in every five cars on the road today. We now need to reverse this trend, and in a considerably shorter period of time than it took us to get to the current status quo. Evidence from cities that have managed it suggests that modal shift in transportation requires significant commitment from policy makers, as well as upfront investment in public transport.

Yet there is little doubt that the prize is worth it. Not only could these shifts save thousands of lives by reducing air pollution, they could also drive improvements in two of the government’s other transport objectives: a reduction in congestion, which is estimated to cost the UK economy over £61.8 billion over the next decade (INRIX 2016), and a reduction in road-based CO2 emissions, which currently make up a quarter of all UK emissions (DECC 2014).

However, delivering on this potential will require a bold new approach to the air pollution problem. Government will have to reach for all the available tools in its policy arsenal, including providing incentives to drive behaviour changes amongst businesses and individuals.

We will now explore the central tenets of such an approach.

2 Air pollutants with a diameter of 10 and 2.5 micrometres (μm) or less, respectively.