Whoever set fire to the huge Heyope tyre dump near Knighton, Powys, in 1989 could have had little idea that the action would enter the record books. Almost 13 years later, Britain's longest burning tyre fire smoulders gently on.

The tyres - all 10m of them - lie in a deep wooded valley in the Welsh borders. They are packed too densely for firefighters to extinguish and there are no flames, but temperature readings confirm the intense heat generated below the surface. Wisps of acrid black smoke occasionally drift up from the mass of rubber.

Scrap tyres are about to become the latest headache for a government still smarting from the debacle over its newly-created fridge mountain. A European directive will ban landfills of whole tyres by next year and shredded tyres by 2006. The option of dumping tyres in places like Heyope will be closed and new ways will have to be found to dispose of the 13m tyres that are stockpiled or put in landfills every year. The problem is huge. The number of tyres in use is forecast to increase by up to 60% by 2021, as the number of vehicles rises. Every day, 100,000 are taken off cars, vans, trucks, buses and bicycles. It is widely estimated that there are now more than 200m lying around.

"By their very nature, tyres are difficult to dispose off," says David Santillo, from the Greenpeace research laboratories, based at Exeter University. "They are designed not to fall apart while you're driving along the motorway, so they are one of the more intractable issues."

Although tyres remain substantially intact for decades, some of their components can break down and leach. Environmental conern centres on the highly toxic additives used in their manufacture, such as zinc, chromium, lead, copper, cadmium and sulphur.

The environment agency is launching a campaign later this month to alert the public and indus try to the need to prolong the life of existing tyres and find new recycling methods. "You can find landfill sites that cover an entire valley, with black as far as the eye can see," says an agency spokesman. "We have always viewed tyres as a resource, rather than something to be dumped."

The best use of tyres is probably to retread them, but this is now expensive, and fewer than ever are recycled in this way. According to the Used Tyre Working Group, a joint industry and government initiative sponsored by the main tyre industry associations, just 18% of Britain's tyres are retreaded. A further 48,500 tonnes are converted into "crumb rubber", used in carpet underlay and to make surfaces such as those on running tracks and children's playgrounds.

More controversially, a further 18% are burnt as a "replacement fuel" in the manufacture of cement. This is fast becoming the most popular way of disposing of them, but it is of increasing concern to environmentalists and scientists.

"Tyre burning emits ultra-fine particles that have a toxicity all of their own," says Vyvyan Howard, senior lecturer in toxicopathology at Liverpool University. "The toxicity is even stronger if this contains metals such as nickel and tin, which you get when you throw the whole tyre into the furnace. If the metal content of the particles goes up, then there is going to be an increasing impact on health."

The cement companies deny that they are affecting people's health.

Meanwhile, the UK sends 26% of its tyres to landfill, far less than some other EU countries. France sends almost half, Spain 58%, but Holland sends none. The industry is now racking its brains as to how to dispose of the extra 13m tyres that will accumulate from the end of next year.

Santillo believes the onus is on the manufacturers to produce tyres that lend themselves to greater recycling. "Tyre burning is a very attractive short-term option, but it is disingenuous," he says. "In the medium term, we have to look at the options for recycling, but longer term we must look at the sort of hazardous materials that are going into tyres in the first place."

Burning issues over old tyres

Blue Circle, Britain's largest cement maker, uses scrap tyres as a replacement fuel at its plant at Westbury, Wiltshire. The company argues that this is a "win-win" situation for the environment: fewer fossil fuels are burned and the tyres are re-used instead of being deposited in landfill sites; and Blue Circle benefits, since the 4m tyres it plans to burn every year help save the company an estimated £6m a year.

The company says that trials at the plant in 1999 showed that burning tyres reduces the company's impact on the local environment by 27% and decreases emissions of smog-causing oxides of nitrogen. Tyres burned at intense temperatures (1,450C to 2,000C), it says, produce no black smoke or acrid smell.

A local pressure group, The Air That We Breathe, disagrees. They say that tyre burning causes emissions of sulphur dioxide to rise tenfold and emissions of dust particles to increase 500%.

They also claim that carcinogenic dioxins are produced by the burning of the chlorinated elements in the tyres - and they are now seeking leave for a judicial review of the decision to allow Blue Circle to burn tyres.

Underwater attraction?

One alternative re-use of tyres that has been tested around the world is tyre reefs. These are made up of old tyres bound together and dropped in the sea. It is widely hoped that they can become breeding grounds for fish and crustaceans.

Scientists at Southampton Oceanography Centre (SOC) put 500 tyres together to form a reef the size of a tennis court and dropped it into Poole Bay, Dorset, in 1998. The reef has thrived and now boasts species such as lobsters and wrasse.

"The marine life growing on the reef is not affected by the substances that come out of the tyre," says Ken Collins, senior research fellow at the SOC. "The growth of life on the tyres is as prolific as on an ordinary reef."

The findings may suggest a potential use for scrap tyres in coastal defences around the UK and elsewhere.

"Are tyres under water harmless? No, but they are not very harmful," says Collins. "Tyres in the sea are extremely stable. Not a lot comes out of them. Most pollution in the sea caused by tyres comes from the pollutants that come off working tyres and are washed from the road into the sea by rainwater."