"The way they are used in Colombia's eradication program, they do not present a significant risk for human health," said the study by the Inter-American Drug Abuse Control Commission, which is an agency of the OAS.

A large portion of the more than $3 billion in US aid since 2000 has gone into bankrolling Colombia's program of spraying coca crops using the herbicides glyphosate and cosmo-flux.

Critics of the program say spraying also kills peasants' food crops, although Colombian authorities say coca growers often hide their plants in banana and yucca plantations.

The area planted with coca crops in Colombia's jungles has fallen by a third since a peak in 2001, although crop dusters made no progress reducing area last year due to replanting, according to US government figures.

Colombia is the world's largest supplier of cocaine.

The government sees destroying the cocaine trade as key to ending Colombia's four-decade-old civil war, because Marxist rebels and far-right militias both use drug proceeds to buy weapons.

The OAS study was the first major international study into the health effects of spraying, although investigations in Colombia had drawn similar conclusions.