Image caption Experts say the smog problem is becoming increasingly serious

A rapid rise in air pollution from fossil fuels and biomass burning has worsened winter smog and extended its duration in many parts of South Asia, scientists and officials have said.

In Bangladesh, India and Nepal the temperature has plummeted and clouds of fog and smoke hang in the sky blocking sunlight for several days.

Normal lives have been affected with many flights diverted and suspended and trains delayed because of low visibility.

Experts say they have noticed that the intensity of smog has grown in the Indo-Gangetic plains in the last few years, leading to increased impacts.

"Since 1990 onwards, there has been increase in the number of [smog-affected] days in northern India," says BP Yadav, director of the Indian Meteorological Department.

"It is not a linear trend showing an increase every year. There are, of course, year-to-year fluctuations.

"But there are more years that have seen dense fogs."

Nepal's Department of Hydrology and Meteorology director-general Keshav Prasad Sharma agrees the issue of smog is becoming increasingly serious in the plains in southern Nepal bordering India.

"Until 10 years ago, we did not have such dense fog for long durations like we have these days," he says.

"Although the 10-year period is too short for statistical trends, it is indeed being seen as a major issue now."

At times, all means of transport come to a complete halt because of zero visibility Iqbal Habib,, Bangladesh Environment Movement

Some are also investigating whether the conditions can be linked to health problems in parts of the region. Although widely reported as the direct effect of a cold wave, medical professionals say deaths and illnesses are often related to respiratory diseases.

"None of our patients died of hypothermia," says senior consultant physician Gaurang Mishra of a regional referral hospital in south-eastern Nepal where dozens of people have been reported to have died during the last three weeks that saw many smoggy days.

"They mostly suffer from chronic pulmonary obstructive disease that is caused by burning of wood and cow-dung cake and pollution from industries and vehicles, mainly during winter season."

The number of such patients, particularly children and elderly people, is also in the rise in Bangladesh.

"But it is not just about people's health in our country," says Iqbal Habib of the Bangladesh Environment Movement (BAPA). "At times, all means of transport come to a complete halt because of zero visibility and all walks of lives are affected.

"The working hours come down to as little as four hours a day."

Image caption Vehicles are regular sources of pollution, but there are many others

Experts say besides regular sources like vehicles, industrial factories, power plants and dust from gravelled roads, air pollution in some areas in Bangladesh is getting worse because of fast increasing numbers of brick kilns.

Some studies have shown that they account for around 40% of air pollution in and around the capital Dhaka.

"Since we have a sustainable economic growth rate, we need more bricks and the number of brick kilns is going up day by day," admits Monowar Islam, director general of Bangladesh's Department of Environment.

"We know the situation is becoming serious but it is not alarming.

"We have been demolishing unauthorised brick kilns and have been implementing the World Bank-supported clean air and sustainable environment project through which we patronise new technologies that reduce air pollution."

Just like in Bangladesh, India also sees lots of constructions during winter as this is the dry season before the region gets monsoon rainfall preventing such works.

"Construction works too are major contributors for the smog in this season as they lead to more pollution in the air," says the Indian Meteorological Department's BP Yadav.

That is in addition to pollutants from energy sources.

Energy demand

In its World Energy Outlook 2010, the International Energy Agency said: "India is the second-largest contributor to the increase in global energy demand to 2035, accounting for 18% of the rise."

Scientists say pollutants and aerosols in the air enhance condensation of water in the atmosphere causing dense smog.

"The more pollutants in the air, the denser the smog," says Keshav Prasad Sharma at Nepal's Department of Hydrology and Meteorology. "In some Nepal-India bordering areas, smog blankets can be seen from early evening."

When such blankets of smog block sunlight, sending temperatures down, people make fire from wood, cow-dung cake and hay to warm themselves and that creates more air pollution which leads to denser smog.

Scientists say the real trouble is that smog during winter cannot escape to the upper atmosphere as it can during other seasons, because of meteorological conditions.

"During winter, the cold air that blows towards the southwest from the northeast tends to push the boundary layer (the layer of atmosphere closest to the Earth surface) low," William Lau, deputy director for atmospheres at Nasa's Goddard Space Flight Center told BBC News.

"As a result, all the pollutants get trapped in the boundary layer that is pushed down to as low as one kilometre from the Earth's surface while it is more than five kilometres away during other seasons.

"The cold wave becomes severe because of this local trapping of the aerosols and other pollution that block off the solar radiation and create very unhealthy air in this part of the world."