Twenty million U.S. computers became obsolete in 1998. Since the average life span of a computer today is less than three years and more than half of all U.S. homes have computers, dealing with discarded computers is becoming a serious problem. The toxic substances contained in computers make disposing of them very difficult.

Burn them and you will create carcinogens. Bury them and you will contaminate the ground water. The growing panic over what to do with used computers is forcing computer makers to become computer recyclers.

Gateway, for example, offers rebates for customers who turn in old computers when buying new ones. Gateway spokesman Greg Lund says many an obsolete computer can be upgraded to appear - good as new.

"The ones that we are getting back in the recycling effort are very early machines that long ago lost their ability to be useful because they are so slow," he said. "As long as they are pentium-based computers, they can be upgraded with more hard drive capacity, more processor capacity and continue to be used."

Those that can not be upgraded are sent off to be recycled.

Hewlett Packard recycles its own computers in partnership with a Canadian mining company. Environmental Director Renee St. Dennis says hazardous substances like cadmium and mercury are extracted first.

"We pull those items out and we send those to our hazardous materials management firm that has all the specialized equipment to extract and reuse those potentially hazardous materials," she said. "The mercury and cadmium can be reused. You just want to control how that is done."

Ms. St. Dennis says the rest of the computer is shredded down to the size of a coin and then put through a very complicated separation process.

"We run the material across an 8 mm screen where all the tiny particles fall off," she said. "From there the material travels across magnets that pull out the steel and stainless steel. So now (in the end) we have got nice clean streams of commodities that can be sold and reused to make new products of all kinds."

At the moment, the cost of retrieving that value is more than the metals are worth, so the company charges a fee for recycling. Hewlett Packard is considering setting up similar fee-based recycling programs in other countries.

As the problem of obsolete computers multiplies around the world, the technology developed to dispose of old computers may turn out to be as valuable as the technology developed to create new ones.