The lives of millions of people are at risk, both from the toxic1 haze2 and the weather change it brings, the study shows

In the biggest-ever study of the phenomenon, 200 scientists warned that the cloud, estimated to be three kilometers thick, is responsible for hundreds of thousands of deaths a year from respiratory disease.

By slashing⁶ the sunlight that reaches the ground by 10 to 15 percent, the choking smog has also altered the region's climate, cooling the ground while heating the atmosphere, scientists said.

The potent⁷ haze lying over the entire Indian subcontinent -- from Sri Lanka to Afghanistan -- has led to some erratic⁸ weather, sparking flooding in Bangladesh, Nepal and northeastern India, but drought in Pakistan and northwestern India.

"There are also global implications, not least because a pollution parcel like this, which stretches three kilometers high, can travel half way round the globe in a week, " U.N. Environment Program chief Klaus Toepfer told a news conference in London on Sunday.

The U.N.'s preliminary report comes three weeks before the Earth Summit in Johannesburg, which opens on August 26, where all eyes will be on how not to overburden the planet.

Global threat
While haze hovers⁹ over other parts of the world, including America and Europe, what surprised scientists was just how far the cloud extended, and how much black carbon was in it, according to India's National Physical Laboratory.

A cocktail¹⁰ of aerosols¹², ash, soot¹³ and other particles, the haze's reach extends far beyond the study zone of the Indian subcontinent, and towards East and Southeast Asia.

While many scientists once thought that only lighter¹⁴ greenhouse gases, such as carbon dioxide, could travel across the Earth, they now say that aerosol¹¹ clouds can too.

"Biomass burning" from forest fires, vegetation clearing and fossil fuel was just as much to blame for the shrouding¹⁵ haze as dirty industries from Asia's great cities, the study found.

A large part of the aerosol cloud comes from inefficient¹⁶ cookers, where fuels such as cow dung and kerosene¹⁷ are used to cook food in many parts of Asia.

Acid rain
Using data from ships, planes and satellites to study Asia's haze during the northern winter months of 1995 to 2000, scientists discovered not only that the smog cut sunlight, heating the atmosphere, but also that it created acid rain, a serious threat to crops and trees, as well as contaminating oceans and hurting agriculture.

The report suggested the pollution could be cutting India's winter rice harvest by as much as 10 percent.

The report calculated that the cloud could cut rainfall over northwest Pakistan, Afghanistan, western China and western central Asia by up to 40 percent.

While scientists say they still need more scientific data, they suggest the regional and global impact of the haze will intensify¹⁸ over the next 30 years.

In the next phase of the project, scientists will collect data from the entire Asian region, over more seasons with more observation sites and refine their techniques.

But because the lifetime of pollutants¹⁹ is short and they can be rained out, scientists are hopeful that if Asians use more efficient ways of burning fuel, such as better stoves, and cleaner sources of energy, time has not run out.