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Executive summary
Conclusions

Executive summary

1.1 The Committee on the Medical Effects of Air Pollutants (COMEAP) was asked by the Department of Health (DH) to advise on the possible links between outdoor air pollution and asthma, excluding biological pollutants such as pollen. A Sub-Group was set up to review this area in detail and to draft the report. The report has been agreed by the Committee.

1.2 The following terms of reference were provided by the Department:

To advise on:

1. The time trends and geographical pattern of asthma in the United Kingdom (UK) and the relationship of air pollution to such trends and patterns.>/li>

2. The role of air pollution in aggravating existing asthma.
3. The possible mechanisms by which air pollution might cause or aggravate asthma.
4. Gaps in relevant information.
5. Recommendations for further work.

1.3 It was recognised that, while this report would focus on outdoor pollution, other forms of air pollution, including that found indoors (where most people spend the majority of their time), or associated with cigarette smoking, might also be relevant to the causation of asthma.

1.4 Asthma is a disease of the lungs in which the airways are unusually sensitive to a wide range of stimuli, including inhaled irritants and allergens. This results in obstruction to airflow which is episodic - at least in individuals with early or mild asthma - and which causes symptoms of tightness and wheeziness in the chest.

1.5 There has been an increase of about 50% in the prevalence of childhood asthma over the last 30 years, which corresponds to an increase in atopic diseases generally over this time. There has been at least a ten-fold increase in hospital admissions for asthma among children, which may partly reflect changes in medical practice.

1.6 Over the period during which asthma has been increasing, emissions of coal smoke and sulphur dioxide have fallen markedly while those of oxides of nitrogen and volatile organic compounds from motor vehicles have increased. During this time emissions of particles from coal smoke have fallen, whilst those from diesel vehicles have increased.

1.7 Data on trends and geographical variations in exposure to ozone, nitrogen dioxide and particles from vehicles are limited. The occurrence of ozone episodes in summer has probably increased over this century, but in the 15-20 years since measurements began, there is no clear trend in annual average concentrations. Annual average nitrogen dioxide concentrations have not increased in large urban centres, although there is some indication of a small increase in other urban areas.

1.8 It has been suggested that environmental factors such as air pollution could initiate asthma in previously healthy individuals or provoke or aggravate asthma symptoms in those who are already asthmatic.

1.9 While there is laboratory evidence that air pollution could potentially have a role in the initiation of asthma, there is no firm epidemiological or other evidence that this has occurred in the UK or elsewhere.

1.10 While there is some epidemiological evidence that air pollution may provoke acute asthma attacks or aggravate existing chronic asthma, the effect, if any, is generally small and the effect of air pollution appears to be relatively unimportant when compared with several other factors (eg, infections and allergens) known to provoke asthma.

1.11 There is some laboratory evidence that exposure to the common gaseous pollutants can enhance the response of asthmatic patients to allergens, though the effect does not seem to be large. There is no direct evidence for such an interaction as a result of exposure to outdoor air pollution in the UK.

1.12 There is no consistent relationship between trends in the prevalence of asthma and trends in emissions or ambient concentrations of air pollutants. A number of equally, if not more, plausible explanations for the trends in asthma have been hypothesised.

1.13 The epidemiological evidence concerning the short term effects of air pollution on asthma indicates that:

1. Day-to-day variations in air pollution are likely to have a small effect on the lung function of asthmatic adults and children. In general these changes are unlikely to cause symptoms. However, patients with severe asthma may be more affected because of their lower reserve of lung function. The main effects are observed in the elderly with chronic obstructive lung disease (which includes asthma).
2. Seasonal patterns of asthma bear little or no relationship to those of air pollution.
3. Based on studies from overseas, it is likely that the short-term fluctuations in levels of air pollution currently encountered in the UK are responsible for small changes in the numbers of hospital admissions and accident and emergency attendances for asthma. Limited experience from the UK during well defined air pollution episodes indicates that admissions may be increased by a small amount, along with similar increases in admissions for other respiratory diseases.

1.14 The epidemiological evidence concerning the geographical distribution of asthma indicates that:

1. There is little or no association between the regional distribution of asthma and that of air pollution.
2. Prevalence studies comparing high with low pollution areas have not found consistent associations between outdoor air pollution and asthma prevalence.
3. There is no convincing evidence that asthma is more common in urban areas than in rural areas of the UK. Limited evidence from the UK and other countries suggests a modest relationship between asthma prevalence and local traffic density. The extent to which this is due to air pollution has yet to be determined.

1.15 A number of recommendations for further work are made.

Conclusions

1.16 As regards the initiation of asthma, most of the available evidence does not support a causative role for outdoor air pollution. (This excludes possible effects of biological pollutants such as pollen and fungal spores.)

1.17 As regards worsening of symptoms or provocation of asthmatic attacks, most asthmatic patients should be unaffected by exposure to such levels of non-biological air pollutants as commonly occur in the UK. A small proportion of patients may experience clinically significant effects which may require an increase in medication or attention by a doctor.

1.18 Factors other than air pollution are influential with regard to the initiation and provocation of asthma and are much more important than air pollution in both respects.

1.19 Asthma has increased in the UK over the past thirty years but this is unlikely to be the result of changes in air pollution.

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copyright: © | last updated 17 July 2002

HMSO, 1995
ISBN 011321958x

Available from The Stationery Office