Enormous brown clouds of pollution hanging over Asia are killing hundreds of thousands of people, melting glaciers, changing weather patterns and damaging crops, the United Nations said Thursday.

Car traffic, factory emissions and indoor cooking are among the culprits for the "Atmospheric Brown Clouds", which are up to three kilometres (1.8 miles) thick, according the UN's Environment Programme (UNEP).

Releasing a landmark report on the phenomenon, the UNEP said getting rid of the clouds could help ease many environmental problems in Asia.

"The Atmospheric Brown Cloud is both complex and in need of a great deal more attention," UNEP executive director Achim Steiner told reporters.

Unlike greenhouse gas emissions blamed for global warming, which take decades or longer to disperse, the clouds would disappear in a matter of weeks if the sources of the problem ceased to pollute, the report said.

Five Atmospheric Brown Cloud (ABC) hotspots have been detected around the world, three of which are in Asia.

Asia's dense population and heady development of recent decades are two of the main reasons for the clouds, with one vast expanse of soot hovering across the Arabian Peninsula through to China and the western Pacific Ocean.

The clouds are so prevalent that black soot has been detected at Mount Everest base camp at levels normally expected in urban areas, according to the report.

The soot that has fallen on the glaciers of the Himalayas and other mountainous regions of Asia have amplified the effects of climate change, because the black particles absorb more heat.

However the clouds can both magnify and mask climate change, because the pollutants also block sunlight.

But the report said many of the consequences of the clouds were indisputable, such as accelerating the melting of glaciers, which in turn has a long-term negative impact on water resources and crop yields across Asia.

The pollutants have also contributed to decreases in the Indian summer monsoons and a north-south shift in rainfall patterns in China.

"The human fatalities from indoor and outdoor exposures to ABC-relevant pollutants have also become a source of grave concern," the report said.

The UNEP estimated that as many as 340,000 people died each year in China and India alone from cardiovascular, respiratory and other diseases linked to exposure to the pollutants.