The researchers, from the Swiss Federal Institute for Environmental Science and Technology, said samples of rainwater they had studied contained levels of chemicals that - in some cases - were way above drinking water standards.

It appears the chemicals evaporate and are absorbed by clouds and then return to earth in rainwater which runs off into streams and rivers. Until now scientists had assumed pesticides only infiltrated groundwater directly from fields where they were sprayed.

The research, which will be published in Analytical Chemistry in the summer, also found that the highest concentrations of pesticides occurred in the first rain after a long dry spell, especially when local fields had been treated.

Drinking water

"Drinking water standards are regularly exceeded in rain," Stephan Müller, a chemist with the Dübendorf-based institute told New Scientist magazine.

Under EU and Swiss regulations, a litre of drinking water should not contain more than 100 nanograms of any particular pesticide.

However, Müller and his colleague Thomas Bucheli found one sample of rainwater that contained almost 4000 nanograms per litre of 2,4-dinitrophenol, a widely used pesticide.

Previously, the authors had shown that in rain samples taken from 41 storms, nine contained more than 100 nanograms of atrazine per litre - one sample even had around 900 nanograms.

Müller and Bucheli are concerned about the growing practice of using rainwater that falls onto roofs to recharge underground water. They say this water often contains dissolved herbicides that have been added to roofing materials to prevent vegetation growing.