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Airborne environmental pollutants and asthma


Asthma is a disease in which the resistance to the flow of air in the airways within the lungs changes markedly over short periods of time. It differs from chronic bronchitis or other lung diseases. Although there are genetic factors which predispose to the development of asthma, exposure to certain environmental factors may contribute significantly to the risk of developing the disease. Once somebody has asthma, exposure to some kinds of environmental factors often increases the likelihood of suffering from attacks. Thus some people find that in the grass pollen season their asthma gets worse: 


This account considers those environmental factors which are inhaled and which are not infective. Therefore it is not intended to address other important issues, such as the role of diet, or of infection. The subject can be usefully sub-divided into a number of simple questions. Some of these appear to have a clear answer and others do not (as yet):

1. Has the frequency of asthma increased with time?

A range of research has shown that over the last few decades there has been an increase in the prevalence of asthma. Many of the studies have investigated children in various parts of the developed world. Within each locality, the better studies, have tended to use the same questionnaire applied to children sampled in the same way but after a time interval. There is evidence that changes in medical practice and in awareness of the disease by doctors, patients and the general public alike has been responsible for an increase in hospital admission rates from asthma, and probably also in the recognition of the disease and its symptoms. However the consensus of opinion is that changes in medical practice or perception do not account for all the apparent increase in prevalence of asthma - i.e. some of the increase is "real" although perhaps not as great as some of the crude data might initially suggest.  There is evidence that this increase is beginning to flatten out.

It may be easy to assume that the increase in asthma incidence could only be attributable to respiratory exposures, since asthma is a disease characterised and defined by respiratory symptoms. However, one should avoid rushing to this judgement , since non-respiratory 'exposures', notably diet have a very important role to play. 

2. Can inhaled pollutants cause asthma in people who did not have the disease and who otherwise would not have developed it?

The clear answer to this question from occupational epidemiology, i.e. systematic studies of symptoms and/or lung function tests in large numbers of workers, is 'yes'. Several such epidemiologic studies show an increased risk of asthma as a direct consequence of occupational exposure to various substances. For these and other substances there is also very clear clinical evidence that they can cause occupational asthma. Some, such as di-isocyanates (used for example in twin-pack spray painting) can cause asthma symptoms in sensitised individuals at concentrations even as low as a few parts per billion, and have even been responsible for a few fatalities, Epidemiological studies of workers exposed to hexachloroplatinates (intermediates in the refining of platinum) suggest that if workers are exposed to high enough concentrations for long enough, they might all develop asthma. Incidentally, occupational epidemiology data clearly show that, for some of the workplace exposures, tobacco smokers are at a higher risk of developing asthma, earlier on, than non-smokers. 

However, although occupational exposure is responsible for a few thousand new cases of asthma in Britain alone every year, it clearly does not explain the apparent increase in asthma prevalence, especially in children.

3. In parallel with the changes in asthma prevalence, have there been any changes in airborne pollutant concentrations? 

Yes, there have been changes in air quality, but in different ways and in different directions! Pollution from the burning of coal, which resulted in emissions of sulphur dioxide and particulate matter has decreased considerably over the last few decades. However, overall average concentrations of pollution from motor vehicles, notably oxides of nitrogen, particulate matter from diesel internal combustion engines, and some other compounds has surprisingly not tended to increase, as one might have expected, in parallel with the increase in numbers of motor vehicles. However, the frequency of peaks of traffic related pollution and the geographical extent of it have probably increased. Episodes of pollution from secondary pollutants, notably ozone, produced by photochemical oxidation, have also increased. Moreover, we must not forget that in tandem, there have also been changes in people's diet, lifestyle, and in homes and other indoor environments. For example, homes have tended to become warmer and, in this and other ways, much more appealing to cohabitation by dust mites.

4. Can current levels of outdoor air pollution aggravate asthma symptoms/precipitate attacks in people already suffering from the disease? 

The epidemiological data suggest that this happens, even when adjusted for confounding factors, and when compared with several other factors that contribute to asthma symptoms. It is known from studies in human volunteers suffering from asthma, that exposure to sulphur dioxide can cause narrowing of the airways of the lung. However, outdoor concentrations of sulphur dioxide seldom reach levels such as these and which may be high enough to provoke a worsening in asthma symptoms; but there can be notable exceptions for example in localities where sulphur dioxide emissions are high and climatic conditions do not permit rapid dispersal. Peaks of ozone and other pollutants may be associated with increased frequency of asthma symptoms.

5. Can air pollution generally (i.e. not occupationally) cause asthma in people who did not have and otherwise would not have developed it? 

When this page was first written (around the mid 90s) the available epidemiologic evidence did not clearly support this. The best types of studies to investigate this hypothesis are very expensive and difficult to conduct and would have to be followed up over a long period of time. 

Analysis of routine data on ill-health and on pollution collected in the current manner, could not answer this crucial question, because of bias and confounding factors. Some studies indicate that exposure to biological pollutants in the home (such as house dust mite antigen) or outdoors (such as certain pollen particles or fungal spores) might be associated with an increased risk of developing asthma. More recent work is beginning to suggest that, to a degree, ambient air pollution exposure can increase the risk of children developing asthma, although more work is needed to characterise the size of the effect, such as the multicentre 'ESCAPE' project (see link below).

The image alongside shows a dust mite of the same family as the house dust mite.