The United States plans to seek a waiver in international talks this week that would allow American farmers to continue using methyl bromide, a pesticide slated to be banned in 2005 because it contributes to destruction of Earth's protective ozone layer.

The pesticide, which has also been linked to prostate cancer in farm workers and neurological problems in people exposed to large quantities, was scheduled to be phased out in 2005 under the Montreal Protocol, the 1987 treaty struck under the Reagan administration to restrict the use of ozone-destroying chemicals. The pact, which has been updated several times and signed by more than 160 nations, is widely considered the most successful international environmental treaty in history.

The treaty allows petitions for "critical use" exemptions to the methyl bromide ban, and the Bush administration has already secured a one-year reprieve for 2005 at the behest of U.S. growers of tomatoes, strawberries and other produce.

The administration's repeated push for waivers has been a source of controversy abroad: negotiations on the issue broke down late last year, though treaty participants reached a compromise this March. The U.S. exemption for 2005 is twice as large as the combined 11 waivers obtained by other treaty signers.

The pesticide -- a fumigant injected into the soil to kill insects, weeds and disease -- remains popular with farmers because it is highly effective. It is also used to fumigate food processing and storage areas, such as grain bins and flour mills, to kill insects and rodents.

Although U.S. growers have cut their use to 30 percent of 1991 levels, thousands of farmers still use methyl bromide across the country, said Rebeckah Freeman, congressional relations director for the American Farm Bureau.

The U.S. exemption allows consumption to rise to 35 percent next year, and State Department officials will be seeking to raise the waiver to 37 percent of 1991 levels in 2006.

Administration and agriculture industry officials say the United States cannot afford to comply with the impending ban.

"This particular chemical, methyl bromide, is a tough one," said Claudia A. McMurray, deputy assistant secretary of state for environment, who is leading the U.S. delegation to treaty talks in Geneva this week. "What we have to do is balance our desire to aggressively phase out all ozone depleting chemicals with the fact that our farmers need this chemical."

But environmental and farm worker advocates question this assertion, arguing that alternatives exist and that methyl bromide use is dropping.

Ozone, a form of oxygen, is a harmful pollutant at ground level, but high in the atmosphere it forms a thin layer that blocks dangerous ultraviolet rays from the sun. In the 1970s, scientists discovered that several man-made chemicals were filtering into the upper atmosphere and thinning the ozone layer over Antarctica, allowing through more ultraviolet radiation that increases the risk of skin cancer and kills key ocean organisms at the base of the food chain. In recent years the size of the southern "ozone hole" has reached historic proportions, and ozone thinning has also appeared over the Arctic.

"This is a dangerous chemical that stands in the way of the healing of the ozone layer," said David Doniger, policy director for the Natural Resources Defense Council's climate center. "It's being phased out for a reason."

Doniger said Environmental Protection Agency figures, obtained by the NRDC under a Freedom of Information Act request, show that U.S. consumption of methyl bromide -- counting domestic production, imports and quantities taken from existing stockpiles, minus exports -- meets the current Montreal Protocol mandate. In other words, he said, farmers do not use as much methyl bromide as they are seeking in the waiver talks starting today.

A key unknown is the amount of methyl bromide in stockpiles held by U.S. producers, distributors and users. The EPA has not released this data, in part because there is just one significant methyl bromide manufacturer, Indiana-based Great Lakes Chemical Corp. James Nicol, Great Lakes' business manager for agricultural products, said the size of its inventory is confidential business information.

EPA officials said its figures on methyl bromide consumption are an estimate, and actual use may be well above 30 percent of the 1991 levels. "It captures the bulk [of use] but not 100 percent," said Adam Sharp, associate assistant administrator of the EPA's Office of Prevention, Pesticides and Toxic Substances.

Sharp noted that agricultural interests initially asked for an exemption to 60 percent of 1991 levels for 2006, rather than the 37 percent the United States is now seeking. "We really pared it back," Sharp said.

The Agriculture Department has spent $150 million on research into methyl bromide alternatives over the past decade, but growers say they have yet to find an acceptable substitute. Terry Kelley, a horticulturist at the University of Georgia's extension service, said a total methyl bromide ban would cost farmers in that state $100 million to $140 million a year.

"We don't have what you call economically and technologically feasible replacements for it," Kelley said. "It's not for lack of effort."

But some farmers and pesticide distributors say the agricultural community could adjust if necessary by rotating crops and using other fumigants. Indiana-based Fumigation Service and Supply has started using two other chemicals to kill pests in food processing warehouses, said general manager Patrick Kelley.

"Realistically, someone with an open mind who's willing to learn [can find] there are viable alternatives," he said.