According to ENN ie Environmental News Network, the research is part of a report known as the 'Atmospheric Brown Clouds' ,ABC report, by the United Nations Environment Programme, the UNEP.
The report indicates that the dirty brown haze, sometimes three-kilometers thick, stretches from the Arabian Peninsula to China and the western Pacific Ocean.
The clouds have blocked up to 25 percent of sunlight in many cities, with Guangzhou, in southern China, among several cities to record more than 20 percent reduction in sunlight since the 1970s.
Other megacities threatened by brown clouds include Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran.
The clouds are also found in parts of Europe and on the Eastern seaboard of the United States but are less dangerous because winter rains and snow wash them away.
The giant brown haze comes from a mix of ozone, black carbon and soot particles released by coal-fired power plants, wood-burning stoves, burning fields and vehicles on the road.
It contains a variety of aerosols, carcinogens and tiny particles that have been linked to respiratory diseases and cardio-vascular problems.
According to the UN report, the toxic material could kill 340,000 people n China and India every year. The toxic clouds also threaten the massive Hindu Kush-Himalaya-Tibetan glaciers.
"If the current rate of retreat continues unabated, these glaciers and snow packs are expected to shrink by as much as 75 percent before the year 2050," said the report.
China's glaciers have already shrunk five percent since the 1950s. Some in India retreat at a rate of 10 to 25 meters each year.
The glaciers' disappearance could threaten water supplies to millions of people who depend on rivers to drink and irrigate their crops.
In fact, the brown clouds may alter the traditional climate change scenario.
On the one hand, they are filled with black carbon and soot particles that absorb sunlight and heat the air and other gases. On the other hand, they contain other particles, such as sulfates, that reflect sunlight and cool the earth's surface.
In effect, the clouds may be dampening the rise in global temperatures by 20 to 80 percent, according to the scientists.