What are the effects on health of transport-related air pollution?

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Exposure to transport-related air pollution varies, as some groups may be more exposed, depending on how long they stay in polluted areas and what they do while there.

The issue

The effects on health of transport-related air pollution have become one of the leading concerns about transport. In the next few decades, road transport will remain a significant contributor to air pollution in cities across the European Region, and estimates indicate that 100 000 deaths a year in these cities could be linked to ambient air pollution, shortening life expectancy by an average of a year. A significant fraction of these deaths and a range of other adverse effects on health are attributable to transport-related air pollution.

In 2010, 90% of the urban population in the 15 countries that belonged to the European Union (EU) before 1 May 2004 are expected to be living in areas meeting the EU hourly air-quality limit value for nitrogen dioxide, carbon monoxide, benzene and lead. Also, exposure to particulate matter is expected to decrease, though it will still cause significant effects on health. The eastern half of the WHO European Region, however, is expected to have more difficulties in meeting air quality standards. In this part of the Region, the rapid increase in private cars and in goods transported by lorries, in combination with a decline in public transport, have turned road transport into an increasing contributor to air pollution.

The WHO 2005 report Health effects of transport-related air pollution provides the first comprehensive assessment of air pollution related to road transport and of the risks it presents to human health. Furthermore, it considers the entire chain of relevant issues: from patterns and trends in activities that determine the intensity of emissions from transport, to contribution of traffic to pollution levels, and finally to patterns of human exposure to such pollutants.


Emissions from the transport sector

The transport sector is an important source of emissions of a wide range of gaseous air pollutants and of suspended particulate matter (PM) of different sizes and compositions. Tailpipe emissions of primary particles from road transport account for up to 30% of fine PM (less than 2.5 µm in aerodynamic diameter: PM2.5) in urban areas. Other emissions from road transport (such as those from resuspended road dust and the wear of tyres and brake linings) are important contributors to the coarse fraction of PM (2.5-10 µm in aerodynamic diameter: PM10-2.5). Road transport is also the most important source of emissions of nitrogen dioxide and benzene in cities. In the future, alternative vehicle technologies – such as fuel cells, electric vehicles and hybrid vehicles – are likely to play a major role in the market and to have a significant impact on emission of the pollutants, but this is not expected to happen in the next decade.

Human exposure to transport-related air pollutants

Exposure to transport-related air pollution varies, as some groups may be more exposed, depending on how long they stay in polluted areas and what they do while there. For most pollutants, exposure concentrations appear to be two to three times as high near busy roads as at background measurement sites. Also, exposures inside vehicles are especially high for primary exhaust gases and PM. Nevertheless, patterns of exposure are often complex and vary substantially, depending on the particular pollutant and the lifestyle and behaviour of the particular population group. Moreover, the intake of pollutants differs among drivers, bicyclists and pedestrians, but it is difficult to separate their exposure to transport-related air pollution from exposure to the pollution from other sources.

Effects on health of transport-related air pollutants

Evidence from epidemiological and toxicological studies indicates that transport-related air pollution affects a number of health outcomes. Such pollution contributes to an increased risk of death, particularly from cardiopulmonary causes, and it increases the risk of non-allergic respiratory symptoms and disease. Experimental research indicates that the effects are linked to changes in the formation of reactive oxygen species, to changes in antioxidant defences and to non-allergic inflammatory processes. Laboratory studies suggest that transport-related air pollution increases the risk of developing an allergy and can exacerbate symptoms – in particular, in susceptible subgroups. The evidence from population studies, however, does not consistently support this view.

Laboratory studies indicate that fine PM (especially the soot content) and ozone are associated with an increased risk of mortality and respiratory morbidity, while exposure to nitrogen dioxide, ozone and PM has been correlated with the development of allergic responses. Some studies indicate a significant increase in the risk of myocardial infarction caused by transport-related pollution; however, only a few studies have been conducted on the issue. Other studies and experimental evidence indicate that exposure results in changes in the regulation of the autonomic nervous system and in elicited inflammatory responses.

Studies also indicate an increased risk of various types of cancer in people with prolonged exposure to higher levels of transport-related air pollution. In particular, occupational long term exposure of professional drivers and railway workers, increases the incidence of (and mortality from) lung cancer. Furthermore, evidence shows adverse effects on pregnancy, as the fetuses are considered to be highly susceptible to a variety of toxicants present in transport-related air pollution. Birth outcomes, such as an increase in postneonatal infant mortality, and a decrease in male fertility also seem to be affected by transport-related air pollution, although the number of studies addressing this hypothesis is small.

Only a few studies analyse the effects on health of specific interventions, and even fewer focus on transport-related air pollution. These studies indicate that reduced air pollution may directly reduce acute asthma attacks in children and also the medical care associated with these attacks. Long-term decreases in air pollution are associated with declines in bronchial hyperreactivity, in the average annual trend in deaths from all causes and in respiratory and cardiovascular diseases. Such decreases in air pollution appear to provide a gain in life expectancy.

Policy considerations

Implementation of technological improvements, such as particle traps, preheated catalytic converters and electronic vehicle controls, may have an impact on transport-related air pollution. Also, stricter exhaust-emission legislation (on PM and nitrogen oxides from conventional diesel and petrol engines) can also contribute to a decrease in transport-related air pollution. Alternative vehicle technologies and fuel substitutes may play an important role in substantially reducing the emission of hazardous air pollutants. However, many of the positive effects of technological improvements risk being offset by an increase in the number of vehicles, of the number of kilometres travelled, by a trend towards replacing smaller vehicles with more powerful engines and an by increased use of diesel fuel. That is why technological improvements alone may be insufficient to bring concentrations of transport-related pollutants below levels that pose a threat to human health.

There is also a need to consider measures that influence the amount of travel. For example, integrated urban planning, such as zoning offices, green areas and non-residential functions around urban highways, separating pedestrians and bicyclists from road traffic, and introducing measures that provide disincentives to using private vehicles (such as parking fees and congestion charges) seem to contribute to lowering emission rates. Such measures encourage a shift in favour of public transport and increase bicycling and walking, which have additional positive effects on health. Moreover, control mechanisms, such as mandatory car inspections, are needed to eliminate gross polluters and avoid badly maintained vehicles.

As the association between adverse effects on health and exposure to transport-related air pollution still needs to be adequately quantified, more research is needed – for example, to clarify which constituents of traffic emissions are responsible for the observed adverse effects.

Type of evidence

This report is based on epidemiological and toxicological evidence. Experts prepared the synthesis papers, and these were externally reviewed and discussed according to a consensus assessment of the strength of the evidence on the links between various health outcomes and transport-related air pollution.

The views expressed in this summary are based on a publication of a HEN Network member agency and do not necessarily represent the decisions or stated policy of WHO/Europe.


This report is based on the results of the project implemented by The Air Quality Programme of the WHO Regional Office for Europe.

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