Spread by the wind, like blowing pollen, they could be designed to replicate swiftly and reduce life on earth to dust in a matter of days, according to Eric Drexler, one of the nation's principal advocates for nanotechnology.

'It's no good one neighbour trying to grow seedless if next door his neighbour has seeded hops blowing pollen over the fence.'

Food makers around the world are finding traces of gene-altered crops in foods that were not supposed to be made with them; Midwestern farmers are complaining that wind is blowing pollen from gene-altered crops into neighboring fields planted with conventional corn.

Like other ragweeds, wind blown pollen from burro-weed may be highly allergenic.

The air was thick with blowing pollen, released by the thousands of wild flowers.

- Sea breezes blow pollen inland, so limit outdoor trips to rural areas and go to the sea instead.

The lowest counts occur in coastal areas and mountains, where wind tends to blow pollen away, although pollen has been detected as far as 400 miles out to sea and two miles up.

Mungo raised himself up to an im-probable height and blew exquisite pollen from his left nostril.

Tough omnivorous "bacteria" could out-compete real bacteria: They could spread like blowing pollen, replicate swiftly, and reduce the biosphere to dust in a matter of days.