It urged nations to "redouble efforts to assess the quantities being used to kill pests on shipments of rice, maize, nuts and other big commodity export crops."

The ozone layer is a 20 mile high shield filtering harmful levels of the sun's ultraviolet rays. A number of chemicals, such as refrigerants and aerosols, have been phased out under the 1987 Montreal protocol.

But UNEP said there were lingering loopholes for methyl bromide, which kills everything from beetles to moths, with some experts estimating that a fifth of worldwide usage was excluded from controls.

"Pest-control purposes, involving exports of commodity crops, animal fodder, cut flowers, hides and consignments in wooden pallets, are exempted from the international phase out," UNEP said in a statement. Even so, Toepfer said that "efforts to repair the ozone layer have been one of the great environmental success stories."

"Scientists estimate that, by the middle of the century and as a result of the phasing out on numerous ozone damaging chemicals, the ozone layer will be repaired. But this is far from guaranteed," he said.