Many of the 4.1 million laptop batteries recalled by Dell last Monday will end up in landfill sites around the world, but experts agree the environmental impact will be minimal.

Lithium ion batteries are benign compared to the toxic ingredients in other rechargeable batteries with nickel-cadmium or small sealed lead-acid chemistries. Those heavy metals include cadmium, mercury and lead, elements that cause human and environmental health threats when they leach into ground water or filter into the air after incineration, according to the US Environmental Protection Agency.

Recycling lithium ion batteries is easier to do. Dell is encouraging customers to return the potentially flammable batteries - the company will send a stamped envelope and address label to users who request a replacement battery.

But many customers will never take either step - claiming a new battery or returning the old one. Dell has not forecast the number of customers who will respond to the recall, but a spokeswoman suggested the 80/20 rule would apply, with 80 percent of customers making the change.

"We certainly do not advise people to throw away the old batteries; we've made it extremely easy to return them," said Anne Camden, a spokeswoman for Dell.

The number of people who return the batteries could be much lower.

"When you offer people a $50 (about £27) or $100 (£54) coupon in a mail-in rebate, you get about 50 percent compliance. When you offer them a $30 (£16) coupon, you get about 15 percent compliance. And that's when you're trying to give them cash," said Roger Kay, an analyst with Endpoint Technology. "So if you're asking people to put a battery in an envelope, the likelihood is they probably won't do it."

Even with curbside recycling programs, many homeowners don't bother to separate batteries from other rubbish, he said.

"Americans' attitudes about landfills are still pretty primitive. But Dell, like a lot of American companies, is pretty enlightened about how to handle toxic materials, so a lot of it will get recycled and purified and reused," he said.

And compared to poisonous material such as the arsenic derivative used in gallium arsenide microprocessors, the lithium in Dell's Sony-built batteries is nontoxic.

"Even if a customer places it in the trash can, and it enters the municipal solid waste stream, nothing's going to happen," said Norm England, president of the Rechargeable Battery Recycling, which has processed faulty batteries from some of Dell's past recalls.