Scientists who had thought diseases were mostly transmitted by people or animals now see dust clouds as possible transmitters of influenza, Sars and foot-and-mouth, and increasingly responsible for respiratory diseases. A rise in the number of cases of asthma in children on Caribbean islands has been linked to an increase in the dust blown across the Atlantic from Africa. The asthma rate in Barbados is 17 times greater than it was in 1973, when a major African drought began, according to one major study.

Researchers have also documented more hospital admissions when the dust storms are at their worst.

"We are just beginning to accumulate the evidence of airborne dust implications on health," said William Sprigg, a climate expert at Arizona University.

The scale and range of some recent dust storms has surprised scientists.

Japanese academics reported in July that a giant dust storm in China's Taklimakan desert in 2007 picked up nearly 800,000 tonnes of dust which winds then carried twice around the world.

Dust from the Gobi and Taklimakan deserts is often present over the western United States in the spring and can lead to disastrous air quality in Korean, Japanese and Russian cities. It frequently contributes to the smogs over Los Angeles. Britain and northern Europe are not immune from dust storms. Dust blown from the Sahara is commonly found in Spain, Italy and Greece and the WMO says that storms deposit Saharan dust north of the Alps about once a month. Last year Britain's Meteorological Office reported it in south Wales.

Some scientists sought to attribute the 2001 foot-and-mouth outbreak to a giant storm in North Africa that carried dust and possibly spores of the animal disease as far as northern Britain only a week before the first reported cases.

The scale and spread of the dust storms has also surprised researchers.

Satellite photographs have shown some of the clouds coming out of Africa to be as big as the whole land mass of the US, with a major storm able to whip more than a million tonnes of soil into the atmosphere. Sydney was covered by an estimated 5000 tonnes of dust last week but, says the WMO, Beijing was enveloped by more than 300,000 tonnes in one storm in 2006.

"The 2-3 billion tonnes of fine soil particles that leave Africa each year in dust storms are slowly draining the continent of its fertility and biological productivity," said Lester Brown, director of the Earth Policy Institute research group in Washington DC.

Brown and Chinese scientists say the increased number of major dust storms in China is directly linked to deforestation and the massive increase in numbers of sheep and goats since the 1980s, when restrictions on herders were removed. "Goats will strip vegetation," said Brown. "They ate everything and dust storms are now routine. If climate change leads to a reduction in rainfall, then the two trends reinforce themselves." China is now planting tens of millions of trees to act as a barrier to the advancing desert.

However, research increasingly suggests that the dust could be mitigating climate change, both by reflecting sunlight in the atmosphere and by fertilising the oceans with nutrients. Iron-rich dust blown from Australia and from the Gobi and Sahara deserts is largely deposited in oceans, where it has been observed to feed phytoplankton, the microscopic marine plants that are the first link in the oceanic food chain and that absorb large amounts of carbon dioxide. In addition, the upper layers of the rainforest in Brazil are thought to derive much of their nutrient supply from dust carried over the Atlantic from the Sahara.

Just as scientists struggle to understand how dust is affecting climate, evidence is growing that another airborne pollutant, soot, is potentially disastrous. Minute particles of carbon produced by diesel engines, forest fires and the inefficient burning of wood in stoves is being carried just like dust to the remotest regions of the world.

A study by the United Nations Environment Programme has just concluded that the pollutant has played a major part in shrinking the Himalayan glaciers and has helped to disrupt the south Asian monsoon.

"Soot - or black carbon - accounts for anywhere from 10 per cent to more than 45 per cent of the contribution to global warming," said Achim Steiner, director of the UN's environment programme.

"It is linked to accelerated losses of glaciers in Asia because the soot deposits darken ice, making it more vulnerable to melting."

- OBSERVER

By John Vidal