THE MONTREAL Protocol to protect the ozone layer is the most successful
international environmental treaty in history.

But last week the Bush administration worked to undercut it by insisting on exemptions for US users of a pesticide that is harmful both to the ozone layer -- which protects the earth from ultraviolet radiation, a cause of skin cancer -- and to the farm workers who apply it.

The pesticide is methyl bromide, which is used as a fumigant to sterilize soil or to protect stored food. Now that the main culprit in ozone destruction, chlorofluorocarbons, have been largely eliminated thanks to the 1987 protocol, methyl bromide is the principal threat to the ozone layer. It has also been linked to prostate cancer in the farm workers who handle it.

Since the signing of the treaty by the United States during the Reagan administration, methyl bromide use worldwide has been reduced to 30 percent of its 1991 level. The treaty called for its total elimination by Jan. 1, 2005, except for certain limited uses that treaty signers could agree are "critical."

But growers of strawberries, tomatoes, and other crops pushed the Bush administration and other governments to seek exemptions during talks in Prague last week. The United States, the world's biggest user of methyl bromide, won permission last year to produce the pesticide at 35 percent of the country's 1991 level. Last week the protocol's technical committee agreed to increase that to 37 percent in 2005 but called for a provisional reduction to 27 percent in 2006.

Exemptions for less-than-critical uses of methyl bromide punish conscientious growers in the United States and abroad who use alternative methods that sometimes add to the cost of production. David Doniger of the Natural Resources Defense Council says one alternative is to cover soil with black plastic, which traps solar heat to kill soil organisms.

Methyl bromide is often used to fumigate wood packing crates, killing invasive pests in the wood. But there are chemical alternatives to that use, and heat treatment of the wood can achieve the same effect. Plastic packing has also been used to replace wood.

"Dramatic progress has been achieved over the past 15 years in eliminating CFCs and other ozone-destroying chemicals," said Klaus Toepfer, executive director of the United Nations Environment Program, earlier this year. "But the task remains unfinished, as demonstrated by delays in phasing out methyl bromide more completely."

Instead of seeking an end run around the treaty, the Bush administration should work harder with US growers to explore, test, and apply alternatives to methyl bromide. At a time when the United States is being criticized internationally for opposing treaties to curb global warming, land mines, and international crimes, it should at least be a leader, not a foot-dragger, on the ozone accord.