Millions of tons of dandruff are circling the Earth, blocking out sunlight, causing rain and spreading disease, startling new research shows.
Flaky as it may seem, the research - partly funded by the German government - may provide the solution to one of the world's most enduring pollution mysteries: the origin of much of the vast clouds of fine dust in the atmosphere. It suggests that more than half of the dust is a rich soup of organic detritus, including particles of decaying leaves, animal hair, dead skin and dandruff.
The research is published in the April issue of Science magazine by Dr Ruprecht Jaenicke of Mainz University - who has been leading the study for the past 15 years - and says "it has got to be taken very seriously".
The discovery has intrigued scientists, who have long known that countless billions of tiny particles - some 8,000 times smaller than the thickness of a human hair - are wafted around the globe, affecting the climate and reducing the amount of sunlight reaching the earth by one 10th, with incalculable consequences for agriculture.
It is known that about 60 per cent of them come from pollution, soot, dust, ash, desert sand and sea salt, but the origin of the rest has remained a mystery.
Now after taking air samples from the Amazon to Greenland, and Germany to Russia's Lake Baikal, Dr Jaenicke calculates that there are a billion tons of organic detritus in the atmosphere.
"Whenever you brush your hair or take off a sweater, you release a cloud of biological dust," he says. "When a bird flies through the air it leaves behind particles from its feathers, and when the wind blows through a tree it releases dust from the leaves."
He has, however, not yet done the work to break down the constituents of the dust clouds, so that he can tell precisely how much is dandruff.
He says that the particles take up water in the atmosphere, and that ice forms around them, increasing rainfall. But their effect on global warming is unclear, as they would be involved in different processes to increase or modify it.
Dr Gene Shinn of the US Geological Survey believes that the discovery could help to explain the epidemic of asthma around the world. He says that levels of the disease have soared in the Caribbean after increases in dust borne across the Atlantic Ocean.