Property: Innocent buyers and evil cellars: Asbestos, lead, mercury . . . thousands of homes are perched on toxic landfill sites. Amanda Seidl digs deep to learn the worst

Before 1986, contaminated land was a subject that few householders thought about. Then a house in Loscoe, Derbyshire, was destroyed and its owner killed by an explosion caused by methane leaking from decomposing rubbish in the landfill site underneath the property. Suddenly, contaminated land was an issue of concern, particularly for people living in houses built on such sites.

Until the Loscoe incident, reclaimed landfill was considered suitable for housing if it was shown to be 'stable'. But more recent research showed that landfill can remain active even after reclamation, producing methane gas and a toxic cocktail of chemicals that leaches into ground water. Housing estates all over the country have been found to have been built on land with dangerous levels of noxious substances, including asbestos, lead and methane. There are an estimated 30,000 hectares of heavily contaminated land in the UK, mostly in former industrial areas in the inner cities, but some in rural areas.

Protection of the green belt has forced housebuilders to use former industrial land for housing. Fairview Homes, for example, estimates that 90 per cent of its sites in greater London require some form of decontamination.

Land used for apparently innocuous purposes such as baking, repairing lorries or making paper may leave hazardous compounds and metals in the soil. The most common forms of contamination are poisoning from arsenic, cyanide and toxic metals such an lead, mercury and cadmium. Lead can cause severe health problems, including brain damage in young children; cadmium compounds have been shown to cause lung cancer, and exposure to mercury can lead to kidney disease and mental health problems. Many chemicals can be taken up by plants, which makes growing vegetables risky.

In Sittingbourne, Kent, the discovery of arsenic, cadmium and lead deposits five years ago on a housing development at Church Milton caused alarm to householders. There had been a brickworks on the site, but by the Twenties, the works had closed, and the land was being farmed again. When it was bought by the housebuilders they assumed they were acquiring virgin green fields, and it was only in the later stages of the development that the contamination was discovered.

The local council, Medway Health Authority and the Department of the Environment launched an investigation: children living on the estate were tested for lead in their blood, and householders were advised to cover their garden soil with plastic sheets. Finally, after a year of research, the estate was pronounced 'clean' and the development resumed. But although confidence has returned to the estate - along with building societies and some would-be purchasers - house prices remain low compared with other similar parts of Sittingbourne.

In Greenwich, south-east London, residents on one housing estate realised something was wrong when blue patches appeared in their gardens; but contamination is not usually so obvious. Short of employing a scientist to drill a borehole in the backyard, there is no fail-safe way of finding out.

If you think your house might be on a contaminated site, you can call in your local environmental health department to carry out an investigation. Where they suspect that a site is contaminated, they will monitor the levels and advise on remedial works.

Contamination is not necessarily man- made: recently, the presence of radon, a radioactive gas produced by certain rocks, has caused alarm in south-west England, in north-east Scotland, Derbyshire and Northamptonshire. In Northampton, up to 20 per cent of houses could be affected by dangerous levels of carcinogenic radon.

Radon or methane gases can be reduced by ventilation under the floorboards. In houses with suspended floors this costs around pounds 100, but for houses with solid floors, ventilation may cost more than pounds 1,000.

In Wolverhampton, the local authority has started a programme to install gas monitoring devices on 44 former landfill sites it owns. On privately owned sites, the landowner or householders must install monitors at their own expense.

Soil contamination by toxic chemicals and metals calls for more drastic measures. Often the only certain solution is to dig up the topsoil to a depth of one metre, and replace it with clean soil. The contaminated soil must then be dumped at a licenced tip, at a cost of around pounds 100 per tonne.

When buying a house, it is essential to find out a history of the site. Since conventional searches only reveal notifiable former uses, such as mineworking, it may be necessary to consult local archives. Even this, however, may not reveal potential contamination.

If vendors cannot provide information about a site - and there is no legal obligation for them to do so - try quizzing the oldest local inhabitant you can find. A buyer's guide for identifying polluted sites, published by Friends of the Earth, may also help.

Tighter regulations introduced in 1989 make it unlikely that landfill sites will be used for future housing development. But for houses built before the new regulations were introduced, the developer is not liable for contamination problems that could not have been identified at the time of construction. The local authority that inspected the properties on completion also escapes liability.

There is no compensation fund for homeowners faced with large bills for dealing with contamination or a blighted property.

The warning caveat emptor has never been so apt.