Richard Hollingham, of BBC Science: "Process going on all over the world"

Scientists from the University of Alberta studied snow in some of Canada's most remote regions - and found high levels of industrial pollutants and agricultural pesticides.

The team, writing in the journal Nature, say the pollutants were transported from developed areas and deposited on the ground as snow or rain when the air cooled.

And they say that, since it is likely the same process is going on all over the world, food and water supplies need to be monitored in parts of the globe previously thought to be free of pollution.

The BBC's Paul Johnson: No place on earth is pollution-free

The chemicals concerned, organochlorides, are a type that tends to build up in the food chain, so although they might be harmless to a fish, they could be more toxic to humans who eat the fish.

Polluters: Spreading their reach even wider than previously thought

David Schindler, from Alberta University, says his team has shown for the first time the extent of organochloride pollution in mountain regions.

Analysis of snow samples from mountains in western Canada showed that organochloride accumulation increased drastically between altitudes of 770m and 3,100m.

Writing in Nature the researchers said: "There is reason to believe that levels of organochlorides in snow would continue to increase at higher elevations.

"Cities like Denver and Mexico City derive their water supply from snow melted on mountains over 3,000m high. They are also much closer to industrial and agricultural sources of contaminants."