Time magazine reported on Saturday that investigators had found a crop-dusting manual during a search for those responsible for the attcks, triggering concern crop-dusting planes might be used for chemical or biological assaults.

In Europe, aerial crop-dusting of fields with insecticides and pesticides takes place on a much smaller scale than in the United States, given smaller fields and concerns over wind-blown contamination of residential areas, officials say.

Limited aerial crop-spraying takes place over paddy rice fields in Spain, wine-growing areas of the Mosel Valley in Germany, olive trees in the Greek island of Corfu and a few upland areas of Scotland and Wales to control bracken.

Aerial crop-dusting is banned in Italy where ground-based equipment is used, even in the soaked paddy fields of the Po Valley.

"There is not very much crop-dusting by air in Europe. The practical problem for a terrorist would be finding suitable equipment," said Theodor Friedrich, senior officer for farm mechanisation with the U.N. Food and Agriculture Organisation.

"Most spraying is done with ground-based equipment, which is more efficient and poses fewer environmental risks," he added.

"With tractors, 90 percent of pesticides get to the target area. From the air, only 50-70 percent reach the target area."

Aerial crop-dusting requires spraying of more chemicals than ground equipment, aircraft are costly, and there is a greater risk in Europe where fields are smaller than in the United States that the wind can blow chemicals into housing areas.

"Spraying from the air is less efficient because you are further away from the target area," Friedrich said. "If you release chemicals higher in the air, the spray evaporates and can come down far away."

TOUGH LICENSING

Another factor reducing risks for citizens is that in Europe, regulation of crop-dusting operations is already tight, including checks on the quality of spraying equipment and stringent licensing rules for the small band of operators.

German, French, British and Italian officials had no immediate reports of heightened vigilance in their countries' small crop-dusting operations.

"We are not aware of any changes to measures on the very minimal crop spraying activity here," said a spokeswoman with Britain's Environment, Food and Rural Affairs Department.

"It would not be easy for some strange character to charter a plane and swap its contents," she added.

A spokeswoman for Germany's Federal Ministry of Consumer Protection, Food and Agriculture also downplayed the threat.

"The aircraft involved are very small and we do not view this as posing any terrorist threat," she said.