AIR POLLUTION AND HEALTH

by Professor Anthony Seaton MD, FRCP, FRCPE, FFOM, Department of Environmental and Occupational Medicine, University of Aberdeen

Concern about the effects of poor air quality on health has been much publicised recently. The author explores these concerns and answers some commonly asked questions

Public anxiety about the effects of air pollution on health has been stimulated by media publicity and the actions of the green lobby. At a recent national conference of the Royal College of General Practitioners, I asked the audience how many had been asked for information about pollution by patients or parents, or told by a patient that their chest was made worse by pollution. Almost everyone in the hall put up a hand.

Pollutants and their sources

In the 1960s and before, winter smogs were a familiar feature of UK towns and cities. Control of coal burning in houses and factories, with removal of power generation to mainly rural areas, has eliminated the major source of such pollution. It has also contributed to our reputation for exporting pollutants (especially acid aerosols and sulphur dioxide) to northern Europe. But as coal burning in towns has been controlled, petrol and diesel vehicles have become much more widespread, and these are now the main sources of air pollutants outdoors. In addition, since most people now spend most of their time indoors, indoor sources of pollution may have a significant effect on health.

When you sit in your car with the heating on in a winter traffic jam, you are breathing a cocktail containing carbon monoxide, nitrogen dioxide, fine particles, and an assortment of hydrocarbons including benzene. All of these have recognised adverse effects on health. Carbon monoxide reduces the oxygen-carrying capacity of the blood and may provoke angina. Nitrogen dioxide irritates the airways (particles have been associated with excess deaths and asthma attacks in populations), and benzene can cause leukaemia.

Nitrogen dioxide and hydrocarbons, primary pollutants, drift with the wind and enter a series of sunlight-driven reactions, the most important outcome of which is formation of ozone. As these reactions take time, ozone tends to build up downwind of the source of primary pollutants, and is therefore mainly a summer rural pollutant. Although ozone in the stratosphere absorbs harmful ultraviolet rays and is therefore beneficial to health, at ground level it acts as a respiratory irritant.

Indoor pollutants include carbon monoxide and nitrogen dioxide. Carbon monoxide is the most important pollutant in the UK in terms of causing sudden death, and is generated by defective, inadequately vented gas apparatus. Nitrogen dioxide is generated in the home mainly by gas cooking, and reaches higher levels in unventilated kitchens than it does outdoors during pollution episodes.

Important health effects

With smoke control, improvements in fuel and engine efficiency, and the introduction of tighter emission controls and catalytic converters, much has already been done to reduce air pollution in the UK. However, the rise in numbers of cars, and in particular the increase in diesel vehicles (which cannot as yet be fitted with exhaust filters or catalytic converters), has meant that the downward trend in urban pollution has slowed. The trend could even be reversed without further action, some of which would undoubtedly have an impact on the liberties and pockets of vehicle users. It is desirable, therefore, to have some understanding of the likely health advantages from further control of outdoor pollution.

The evidence on adverse health effects comes from both experimental studies and epidemiology. Experimental studies have shown the irritant effects of nitrogen dioxide, sulphur dioxide, and ozone on the airways but, with the exception of ozone, only at concentrations well above those found in urban areas today. People with asthma are likely to be somewhat more susceptible. Ozone probably has effects on airways at concentrations reached during summer pollution episodes, and may be responsible for increased complaints of asthma and hay fever at such times. Levels of carbon monoxide are generally too low to cause concern. Benzene levels are unlikely to make a measurable contribution to the incidence of leukaemia in the UK, and are only of clinical concern in industrial situations.

Epidemiological studies have consistently shown a correlation between increases in air pollution in cities and adverse effects on health (seen as increases in general practitioner consultations and hospital admissions). These effects are relatively small, and studies need to involve populations of a million or more in order to detect them. Weather, and particularly temperature change, has a much larger effect and needs to be taken into account.

Air pollution and asthma

Some people with bad asthma, and most with chronic, smoking-induced airflow obstruction, often notice that their chest gets worse when the weather is bad and there is a lot of pollution around.

Those with lung problems who notice effects should be advised to increase their normal therapy appropriately. Older patients should be advised to keep warm and stay indoors during episodes as far as possible, remembering that temperature is a more important determinant of deterioration than pollution.

There is little evidence that an asthmatic child will suffer clinically significant effects from the sort of episodes that occur in the UK. Asthma should always be managed by adjusting the dose of inhaled drugs in order to control symptoms, rather than by restricting activities, and exercise is good for asthma.

A gas cooker itself does not cause asthma, but could be making matters worse if someone spends a lot of time in the kitchen. Any effects would come from a build-up of nitrogen dioxide, so the problem can be avoided by making sure that the room is well ventilated during cooking, preferably with an extractor to the outside. The problem does not arise with electric cookers or with other vented gas fires and central heating, although these may represent a threat to health if they are defective and releasing carbon monoxide. Such equipment should be serviced regularly.

When considering a holiday destination, asthmatics should avoid big cities with traffic problems in the summer. Athens, Bangkok, Cairo, and Calcutta are notorious. Keep to the seaside and you will be fine. Why not try Scotland?

Improving air quality

There is some evidence that long-term exposure to urban pollution may cause increased risks of developing some diseases, but these risks are very small compared with others suffered by the same people who have to live in polluted areas, and which relate mainly to poverty. Such long-term risks are relatively unimportant in individual cases. Short-term risks apply only to people with chronic cardiorespiratory disease. If the pollution is a problem, the environmental health department of the local authority should have a strategy for improvement of air quality.

A local factory polluting an area with smelly smoke may cause harm, or it may just be a nuisance. It is impossible to answer this without knowing what the smoke contains and what the factory does. An approach to the local environmental health department or the local Health and Safety Executive (the number should be in the local telephone directory) can result in appropriate action being taken. If this does not produce action, it is easy to interest local councillors or members of parliament in such matters.

A car producing the least pollution in terms of particles and nitrogen dioxide is one with a petrol engine fitted with a catalytic converter, but it uses more fuel and produces more carbon dioxide, a greenhouse gas. Diesels will undoubtedly become cleaner over the next few years. If you wish to be "green" and drive, use your vehicle only for longer journeys, drive within speed limits at a steady speed, drive a small, fuel-efficient car, and have it serviced regularly.

Conclusion

It is often difficult to attribute ill effects to one particular pollutant, since there are usually increases in several at the same time, but the current consensus is that small particles, possibly in combination with nitrogen dioxide, and ozone are the two most important pollutants.

Increases in particle levels in the air are associated with increased numbers of deaths from cardiovascular and respiratory diseases, more people going to their doctors with chest complaints, and increases in asthma attacks.

Increases in ozone levels have been associated with increased numbers of asthma attacks and decreases in lung function in patients with lung disease.

There is also some evidence that women and children spending time in the kitchen might be at increased risk of symptoms and impairment of lung function, as a result of exposure to nitrogen dioxide from gas cookers.