The BBC's Clare Arthurs: The cloud contains soot, chemicals and carbon monoxide

The cloud, a mixture of pollutants from vehicle and industrial emissions, could wreak havoc on the region's climate and marine life.

Scientists sponsored by the US National Science Foundation spent six weeks earlier this year tracking the cloud with aircraft, ships, balloons and satellites.

The INDOEX project has mapped the extent of the cloud

The Indian Ocean Experiment, or INDOEX, revealed that the affected area includes most of the northern Indian Ocean, the Arabian Sea, and much of the Bay of Bengal - nearly 10m square km in total.

The cloud rises three kilometres into the Earth's atmosphere, and consists of tiny particles of soot, chemicals and carbon monoxide - by-products of burning fossil fuels.

During the winter monsoons, the cloud blows out from Asia over the ocean, and during the summer wet season it reverses direction to move back over the land.

Levels of sunlight reduced

Pollution affects many Asian cities

Scientists from the Scripps Institution of Oceanography in San Diego, which coordinated the project, say the pollution is unprecedented in the region's history, and will get worse as the population increases.

Tiny particles in the cloud reflect the sun's rays, cutting its heat by up to 10%.

In the ocean, less sunlight could threaten the survival of water-plants and plankton - key organisms in the world's food chain. Reduced sunlight also means that less water evaporates from the ocean - leading to lower rainfall and increasing the risk of drought.