Martin Williams
September 2015

The evolution of air quality policies over the past 20 years

As diesel emissions are now all over the news, thanks to the defeat device scandal which has been unearthed at Volkswagon in the USA (with implications for Europe also), we take a look back into the environmental SCIENTIST archive. For some time air quality scientists have been pointing out that real world emissions weren't improving at anywhere near the levels industry testing suggested should be the case. In this article from 2013, we see how scientists have actually been talking about the diesel problem for some time, but policy hasn't kept up...


environmental SCIENTIST | Air quality: should we have achieved more? | April 2013

During the 1980s, UK policy on air quality had begun to be driven by EU initiatives. The first air-quality directives were already in place1,2,3,4 giving the UK an obligation to achieve explicit levels of ambient air quality for the first time. There was a feeling – certainly in the UK – that urban air pollution was a problem solved, and in the 1980s attention had been more focused on ecosystem issues such as acid rain which had a high political profile.

In the 1990s the two main instruments for controlling air pollution in the UK were essentially source based: the Environmental Protection Act 1990 controlled industrial emissions (through the concept of integrated pollution control, which tries to ensure that any particular solution does not lead to problems elsewhere) and the Clean Air Act 1993 relied on smoke control. Motor-vehicle emissions were also recognised as important sources of urban air pollution and emission standards were in place, but these were essentially capping limits and did nothing of substance to force technological developments or drive emissions down.

At about the same time in the early 1990s, a series of what proved to be ground-breaking papers emerged from the Harvard School of Public Health in the USA through the work of Dockery, Schwartz, Pope and others5. They demonstrated associations between adverse health effects, including premature deaths, and relatively low levels of particulate matter (PM) that were previously considered to be harmless. These studies employed sensitive and sophisticated statistical techniques but were initially greeted with scepticism in some quarters. However, further studies by other groups and intensive reanalysis and scrutiny by independent researchers, notably the Health Effects Institute in the USA6, led to these studies being accepted and their findings used to inform policy development in the USA, the EU and elsewhere. Moreover, the health impacts of long-term exposure to fine particles, PM2.5 (defined as particulate matter that passes through a size-selective inlet with a 50-per-cent efficiency cut-off at 2.5 um), have now been shown to be substantial, and monetisation of these effects is now the dominant factor in cost-benefit analyses of ambient air-pollution policies.

Science and strategy

These scientific developments contributed to a resurgence in strategic thinking in the UK with work beginning to develop a coherent strategic approach to air quality that lead to the Air Quality Strategy (AQS) being published in May 1997. During the formulation of the AQS, legislation to incorporate the thinking was also being drawn up and this culminated in the ground-breaking Environment Act 1995, the first piece of primary legislation in the UK to incorporate the concept of ambient air-quality standards in a coherent manner. Space does not permit an adequate appraisal of the immense amount of scientific and medical research and assessments that have underpinned the UK Air Quality Strategy, its successors and the formulation of legislation and policy.

There has been a series of definitive synthesis reports from expert groups in both the environment and health departments. Over the past 20 years (and even earlier),  reports produced by the Photochemical Oxidants Review Group, the Quality of Urban Air Research Group, the Airborne Particles Research Group and more recently the Review of Transboundary Air Pollution and Air Quality Expert Group, together with the series of reports from the Department of Health Committee on the Medical Effects of Air Pollutants (COMEAP) and its forerunner the group on the Medical Aspects of Air Pollution Episodes (MAAPE), as well as the Defra group the Expert Panel on Air Quality Standards (EPAQS), have been tremendously influential in the wider European and international arenas, and have been highly regarded by both scientists and policy-makers.

Air quality management and Europe

At the same time as the UK was developing these policies, the EU also began working along similar lines, formulating a more strategic approach to air-quality policy through the so-called air quality framework directive (96/62/EC)7, which was agreed in 1996. This paved the way for 'daughter directives' the first of which (99/30/EC)8 dealt with PM, sulphur dioxide (SO2), lead and nitrogen dioxide (NO2) and was agreed during a UK presidency of the EU and then adopted in 1999. Subsequent directives dealt with carbon monoxide and benzene (2000/69/EC)9 and with ozone (2002/3/EC)10. The latter did not contain mandatory limit values like the other directives, reflecting the transboundary nature of ozone and the fact that a member state would not necessarily have complete control over ozone sources measured within its territory. The fourth in the series covered arsenic, nickel, cadmium, mercury and polynuclear aromatic hydrocarbons11, and like the ozone directive did not set mandatory limit values but incorporated target values for the pollutants concerned.

The European Commission consolidated this earlier work in a Thematic Strategy on Air Pollution under the Sixth Environmental Action Programme adopted on 21 September 2005. The objective of the thematic strategy were achieving “levels of air quality that do not give rise to significant negative impacts on and risks to human health and the environment”12. The strategy set interim objectives for 2020 as follows:

  • 47 per cent reduction in loss of life expectancy as a result of exposure to particulate matter;
  • 10 per cent reduction in acute mortalities as a result of exposure to ozone;
  • Reduction in excess acid deposition up to 74 per cent in forest areas and 39 per cent in surface freshwater areas; and
  • 43 per cent reduction in areas or ecosystems exposed to eutrophication.

It is worth noting here that the thematic strategy is a Commission document, not a directive, and is not binding on member states.

The most recent development in Europe has been the agreement of a revised Air Quality Directive that brought together for revision the first three 'daughter directives'. The new directive (2008/50/EC)13, which was published in May 2008, incorporated some significant developments in air-quality management, including the concept of exposure reduction for PM2.5, foreshadowed the UK Air Quality Strategy of 2007, and due in no small part to UK input into the European process.

This novel approach was intended to drive air-quality policy to result in improved public health over a much larger part of the population (in principle most of the urban population) as compared to the more difficult, expensive and inefficient (in terms of public health) approach of incrementally approaching single-point limit values in smaller and smaller areas. As the directive turned out however, the final text fell between two stools: the negotiations on the exposure-reduction approach resulted in the driving obligations for PM2.5 being nonmandatory, but, contrary to the original concept, with a mandatory limit value that is relatively easily achieved over most of the EU. At the time of writing, the directive seems unlikely to be reviewed as part of the European Commission’s review of air-quality legislation, so this state of affairs will continue for some years.

Limits and targets

As noted above, the UK Environment Act of 1995 established the conepts of standards and objectives for ambient air for the first time in legislation. Initially these were based on the work of EPAQS and related advice from other expert groups mentioned above. Accordingly, a series of UK standards was formulated and as the EU process began to develop, a parallel stream of air-quality limit values and target values emerged, mostly, but not completely, for the same pollutants. During the 2000s this led to a relatively cumbersome set of standards (in the broad sense of the word) embodying both UK and EU values. In recent years these have become increasingly harmonised, through the process of transposing the most recent EU directive into UK law, although there are still strong arguments for rationalising even the set of EU limits and targets, not least for PM. This of course should not be seen as a means of avoiding the non-compliance positions in which many member states currently find themselves, particularly for NO2 and to a lesser extent for PM10 (defined as particulate matter that passes through a size-selective inlet with a 50-per-cent efficiency cut-off at 10 um).

Vehicle emissions

So far this discussion has centred on policy on ambient air quality. A large amount of activity has taken place over the past 20 years on source-related emission regulation. Perhaps the most significant in terms of improving air quality has been the series of so-called Euro standards for vehicle emissions. The first directive dated from 1970 (70/220/EEC)14 and applied to light-duty vehicles. This and subsequent amendments simply set a cap on emission performance rather than forcing technological improvements and large emission reductions, and it was not until the amendment embodied in Directive 88/76/ EEC15 – the so-called Luxembourg Agreement – that significant reductions were achieved. This directive, which has become known as the Euro 1 standard, effectively mandated the use of three-way catalysts (which reduce levels of all three regulated pollutants: carbon monoxide, hydrocarbons or volatile organic compounds (VOCs), and oxides of nitrogen). A fully functioning catalyst can reduce emissions by more than 90 per cent compared to an uncontrolled vehicle.

A subsequent series of directives limiting vehicle emissions, covering light- and heavy-duty vehicles, and extending to particles as well as gaseous emissions, has since been agreed and subsequent improvements are now up to Euro 6 for light-duty vehicles, due to come into force in September 2014. The parallel series of heavy-duty standards are designated with Roman numerals and the most recent level is Euro VI, which came into force in January 2013. The introduction of three-way catalysts in response to the Euro 1 standard for petrol vehicles, the subsequent tightening of the standards and the change to the test cycle sampling to require sampling to include the cold-start emissions, have been responsible for the most significant improvements in urban air quality in the UK and Europe in the last two decades.

However, for diesel vehicles regulation has been less successful. It is now well known that although the emission limits have reduced consistently over the past two decades, the emissions of oxides of nitrogen (NOx) from diesel cars in real-world driving have not reduced. This has been largely due to the fact that the regulatory test cycle does not adequately reflect driving conditions in the real world. However, the lack of reduction in ambient NO2 levels is also due to the increase in the fraction of NOx emitted as NO2 at the tailpipe due to the use of oxidation catalysts (to reduce hydrocarbon and particle emissions) followed by particulate traps (which use NO2 generated from NO (nitric dioxide) in the exhaust to oxidise the particles on the trap).

These facts, coupled with the increase in the number of diesel cars, not only in the UK but elsewhere in the EU in response to targets for CO2, has meant that ambient levels of NO2 (and NOx) have not decreased in the last 10 years or so to the extent expected, and in many cases have not decreased at all. This has contributed significantly to the wide extent of non-compliance across most of the EU with the ambient limit values for NO2 in the 2008 Air Quality Directive.

Vehicle emissions in the next two decades

This article provides a very brief summary of most of the main features of the air-quality policy over the past 20 years or so. What of the next 20 years? In terms of urban air quality and public health, much depends on the success of the Euro 6 and Euro VI standards in delivering real reductions in everyday use, both for NO2 and for PM. Work is already underway within the European Commission to devise a ‘not to exceed’ test procedure to be imposed after the improved regulatory test to address real-world driving. Early indications from tests on a very small number of Euro 6 diesel vehicles suggest that improvements over Euro 5 are likely but real emissions will not be at the level of the regulatory standard in grams per kilometre. 

The Euro 5 and 6, and V and VI standards have now stimulated the use of selective catalytic reduction (SCR) on vehicles and there is a concern that urban driving may not generate high-enough temperatures for the catalyst to work effectively. Likewise there are already some indications that the diesel particulate filters employed in response to the Euro 5 and 6 standards may not get hot enough to regenerate effectively, leading to increased fuel consumption and reduced performance. Challenges such as these will need to be overcome to realise fully the potential benefits of tighter emission standards.

Future air quality management

What of the future for approaches to managing ambient air quality more generally? Successive improvements will clearly become more difficult, and much depends on the economic circumstances of the UK and the EU in the future, as ambitious environmental improvements are notoriously difficult to achieve in recessions. There will probably be a review of the ambient air-quality directive in a few years' time. Will the philosphy of air-quality management and policy move further away from a heavy reliance on limit values to a more balanced blend of source-related regulation and ambient controls based on a form of exposure reduction across large areas and populations? There are strong arguments for this approach.

What improvements are likely by aligning policies on climate change and air quality? Ambitious targets for greenhouse-gas emission reductions, such as those in the UK Climate Change Act and the declaration of intent of the G20 (namely reductions of the order of 80 per cent by 2050), could result in large improvements in air quality and public health, but will the actions to achieve the targets materialise on a large-enough scale? Air quality management over the past 20 years has become increasingly regional and international – will this trend continue and result in hemispheric/global action on air quality? To what extent will local actions be important? As a hypothetical example, if the health effect evidence on PM suggests that the primary combustion emissions are the most important in toxicity terms, the balance shifts to a very large degree from the present position where local authorities can do very little on their own about total PM10 or PM2.5 mass. These are all speculative questions, and there are many others, but what is certain is that the next 20 years will be as challenging and fascinating as the last 20.


Martin Williams is a professor of air quality at King’s College, London, UK. He is Chairman of the Executive Body of the UNECE Convention on Long-range Transboundary Air Pollution (CLRTAP). Prior to this, he was Head of the Air Quality programme in Defra, with responsibility for air-quality policy, industrial pollution control and research. At King’s College, his research interests lie in the application of science to policy in air quality and climate change. He holds a Visiting Professorship at the University of Urbino, Italy. He delivered the IES Burntwood Lecture in 2007.


1. Council Directive of 15 July 1980 on air quality limit values and guide values for sulphur dioxide and suspended particles (80/779/EEC).

2. Council Directive 82/884/EEC of 3 December 1982 on a limit value for lead in the air.

3. Council Directive 85/203/EEC of 7 March 1985 on air quality standards for nitrogen dioxide.

4. Council Directive 92/72/EEC of 21 September 1992 on air pollution by ozone.

5. Dockery, D.W., Pope III, C.A., Xu, X., Spengler, J.D., Ware, J.H., Martha, E.F., Ferris Jr. B.G., Speizer, F.E., 1993, An association between air pollution and mortality in six U.S. cities, New England Journal of Medicine, 329, pp. 1753-1759.

6. Health Effects Institute, Reanalysis of the Harvard Six Cities Study and the American Cancer Society Study of Particulate Air Pollution and Mortality, Special Report, Health Effects Institute, July 2000.

7. Council Directive 96/62/EC of 27 September 1996 on ambient air quality assessment and management.

8. Council Directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air.

9. Directive 2000/69/EC of the European Parliament and of the Council of 16 November 2000 relating to limit values for benzene and carbon monoxide in ambient air.

10. Directive 2002/3/EC of the European Parliament and of the Council of 12 February 2002 relating to ozone in ambient air.

11. Directive 2004/107/EC of 15 December 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air.

12. Communication from the Commission to the Council and the European Parliament, Thematic Strategy on air pollution, COM(2005) 446 final.

13. Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe.

14. Council Directive 70/220/EEC of 20 March 1970 on the approximation of the laws of the Member States relating to measures to be taken against air pollution by gases from positive-ignition engines of motor vehicles.

15. Council Directive 88/76/EEC of 3 December 1987 amending Directive 70/220/EEC on the approximation of the laws of the Member States relating to measures to be taken against air pollution by gases from the engines of motor vehicles.