With an estimated 150 million CFLs sold in the United States in 2006 and with Wal-Mart alone hoping to sell 100 million this year, some scientists and environmentalists are worried that most are ending up in garbage dumps.

Mercury is probably best-known for its effects on the nervous system. The Mad Hatter in the classic children's book "Alice in Wonderland" was based on 19th-century hat makers who were continually exposed to the toxin.

Mercury can also damage the kidneys and liver, and in sufficient quantities can cause death.

US regulators, manufacturers and environmentalists note that, because CFLs require less electricity than traditional incandescent bulbs, they reduce overall mercury in the atmosphere by cutting emissions from coal-fired power plants.

But some of the mercury emitted from landfills is in the form of vaprous methyl-mercury, which can get into the food chain more readily than inorganic elemental mercury released directly from a broken bulb or even coal-fired power plants, according to government scientist Steve Lindberg.

"Disposal of any mercury-contaminated material in landfills is absolutely alarming to me," said Lindberg, emeritus fellow of the US Department of Energy's Oak Ridge National Laboratory.

The mercury content in the average CFL -- now about 5 milligrams -- would fit on the tip of a ballpoint pen, according to the US Environmental Protection Agency, and manufacturers have committed to cap the amount in most CFLs to 5 milligrams or 6 milligrams per bulb.

The majority of Philips Lighting's bulbs contain less than 3 milligrams, and some have as little as 1.23 milligrams, said spokesman Steve Goldmacher.

To prevent mercury from getting into landfills, the EPA, CFL makers and various organizations advocate recycling.

Besides commercial recyclers and some municipal waste collection services, some retailers accept used CFLs.

IKEA, the Swedish home furnishings chain, has free drop-off programs at all of its 234 stores, 29 of which are in the United States. Spokeswoman Mona Astra Liss said response was slow at first, but has since picked up.

Now advocacy groups are calling on Wal-Mart Stores Inc. and other big chains to get involved.

Andy Ruben, vice president for corporate sustainability at Wal-Mart, said the company was working with the EPA's Office of Solid Waste and others to find mercury and recycling solutions.

RECYCLING HURDLES

One problem with recycling is that it isn't cheap.

Larry Chalfan, executive director of the Zero Waste Alliance environmental group, said the value of the metal, glass and mercury reclaimed from recycling fails to offset the cost of the process. "Someone has to pay," he said.

Costs can range from 20 cents to 50 cents per bulb -- not a paltry sum when some CFLs sell for less than US$2 at Wal-Mart.

But, compared with the overall lifecycle cost of buying and using a bulb, recycling would be less than 1 percent, said Paul Abernathy, executive director of the Association of Lighting & Mercury Recyclers, "a small price to keep the mercury out of the environment."

Another obstacle lies in the fragility of the bulbs and their mercury content.

"People who are going to accumulate these things from the public are going to have to address the fact that breakage will happen," Abernathy said. "There's the potential for contamination, and I think right now people are a little hesitant to volunteer to take on this liability."

The US government has no single recycling plan in mind, said Matt Hale, director of the EPA's Office of Solid Waste.

Among the alternatives are special curbside collections by municipalities, mail-back programs by manufacturers and drop-off programs at various places, including retail stores that sell CFLs, he said.

Some methods lend themselves to certain geographic areas more than others, Hale said, because of differences in population density, transportation infrastructure and proximity to recycling sites.

State laws are also a factor.

Federal regulations mandate recycling of fluorescent lighting, while exempting households and other small users. Some states, however, are strict. For example, California no longer allows anyone to throw CFLs in the trash, while Massachusetts requires manufacturers to implement recycling programs and meet certain targets.

As technology advances, however, mercury could become less of an issue, at least as far as light bulbs are concerned.

Last month General Electric Co. said it was working on doubling the energy efficiency of incandescent lights and eventually developing versions comparable with CFLs. These bulbs, which the company hopes to begin marketing in 2010, will cost less than fluorescents but they won't last as long.

Meanwhile, some environmentally minded consumers are embracing CFLs and doing their best to dispose of them responsibly.