Where can you breathe easy?

A home is no haven from pollution. Everyday items and activities give off pollutants that harm your health, says Paul Harrison

Many people worry about outdoor air pollution and the impact it can have on their health, but few seem to take an interest in their more immediate environment.

Up to 90 per cent of our time is spent inside buildings, 70 per cent at home. For the very young, the elderly and the sick, the percentage is even higher. Although polluted outdoor air can, of course, affect the quality of indoor air, there are some far more significant sources of pollution inside buildings that deserve attention by scientists, policy-makers and the public.

In 1991, a House of Commons select committee concluded an inquiry with the words: "There appears to be a worryingly large number of health problems that could be connected with indoor pollution that affect very large numbers of the population."

For the past six years, the Medical Research Council's Institute for Environment and Health in Leicester has investigated some of these pollutants and their associated heath effects.

The list is long, ranging from dust mites to carbon monoxide, from volatile organic com£ to man-made mineral fibres. There are also fine particles - particles less than 10 microns in diameter, usually referred to as PM10 - and radon, the natural radioactive gas that emanates from bedrock and is associated with lung cancer.

Most indoor pollution originates from cookers and heaters, cigarettes, building materials, furnishings and fabrics, insulation materials and consumer products.

Along with smoking, the burning of fuels in unflued cooking and heating appliances, where the effluent goes straight into the inside air, is probably the most significant source, yet it seems few people know or care about this. When outside levels of nitrogen dioxide in London rose above 450ppm - three times the official "safe" limit - during the winter of 1991, there was a huge amount of interest. But every Sunday, when gas cookers are going full blast and all the windows are closed, similar levels are reached in large numbers of kitchens across the United Kingdom.

Carbon monoxide is a particularly significant pollutant. We know that it can kill people suddenly and silently. That it can also poison people slowly is not so widely realised. There are probably thousands of people affected by the poorly understood chronic illness that carbon monoxide can cause. Misdiagnosis is worryingly common.

Dust mites, cats, dogs, insects and fungi contribute allergic substances to the air we breathe. The warm conditions that suit human beings also encourage dust mites to thrive - especially when combined with a touch of moisture, as found in bedding and seat cushions, for example. Mould growth is a well-known sequel of dampness, typified by black stains on bathroom walls and windows.

Fine particles have fascinated scientists since the early 1990s, when studies from the United States consistently linked PM10 levels with increased mortality and asthma and other respiratory disorders. Heart disease is also known to be linked to elevated levels of PM10, though the toxic mechanisms behind these effects remain unclear.

Levels of fine particles indoors tend to track those outdoors - but indoor sources can have a significant impact. Studies have again implicated heating appliances - especially open flued or non-flued appliances such as open wood and coal fires - as well as cookers and the process of cooking itself.

The impact of these exposures on health is unknown, but if outdoor epidemiological studies are reliable and can be extrapolated, it could be significant.

In 1996, the environment and health departments and the Medical Research Council joined forces to fund a series of wide-ranging research programmes that specifically included indoor air quality.

This initiative has involved a large number of researchers from many British universities, working on some 40 separate projects worth a total of about £5 million. The next step is to look at possible regulatory initiatives - such as setting emission standards for consumer products and guideline levels for key indoor pollutants - and to investigate effective, practical and achievable interventions.

To be successful and productive, this process will need to involve a wide range of stakeholders.

Paul Harrison is the acting director of the Medical Research Council's Institute for Environment and Health at the University of Leicester.