Experts have ruled out the lagoon at Titikaveka in the Cook Islands as the source of a chemical irritant which has caused eye and nose problems and forced the closure of a school.
New Zealand scientists completed tests on an area of the lagoon but found no trace of an algae blamed for the irritations.
Health officials have closed Titikaveka school although some lessons will take place in a nearby Sunday School hall.
Investigators are now looking at the possibility that a barrel of pesticide has leaked.
It is thought the container may have been buried near the school site.