HIMALAYAN MAP HOUSE SMOKY MOUNTAIN: A curtain of pollution haze from the Indo-Gangetic plains up to 3 km high covers the Kali Gandaki Valley and veils this spring view of Dhaulagiri from Poon Hill.

To the better-known threats to tourism in Nepal like global economic downturn or filthy cities, add a new one: the winter smog over the Indo-Gangetic plains that is obscuring mountain views in Nepal during the peak trekking seasons.

North India is one of the most-densely populated regions on earth and steadily getting more affluent, which means more cars burning fossil fuels and more coal-fired thermal power plants generating electricity. In winter, the soot is trapped in an inversion layer 3 km thick, wafted across the border and up to the mountains by prevailing westerly winds.

The United Nations Environment Programme (UNEP) has been investigating this Atmospheric Brown Cloud and found that most of it is from man-made emissions. It is particularly heavy and noticeable from November-April when rains that would otherwise scrub the dust is scant. The contaminants can move halfway around the world within a week.

A similar bloc of particulate air referred to as 'Asian Dust' is present over East Asia carrying smog from Chinese industry as well as fine sand particles from the Gobi Desert. The combined effect of these masses of airborne pollutants in Asia is huge. People in Nepal, India, Bangladesh, and Pakistan use smoky fuels like wood, charcoal, coal, and kerosene for domestic cooking. Waste, including plastics, are often incinerated outdoors. The practice of setting fire to fields after harvests, brick kilns with smoky stacks and forest fires are other sources of soot.

Haze from the Indo-Gangetic plains covers 10 million sq km and stretches out into the open Indian Ocean. This dark layer of smog has worsened winter fog over the plains, and the haze spoils views of Nepal's well-endowed, world-renowned scenery in winter and spring. The thawing of Himalayan glaciers is happening faster than in the polar regions, partly because of soot deposits melt ice and snow faster.

Nepal is one of the lowest-per-capita emitters of greenhouse gases, but it is being unfairly affected by the historical carbon emissions of the industrialised west, and now by the pollution from south of the border. There is a temptation to blame others and do nothing. Rather than looking at what others are contributing and waiting for worldwide consensus to mandate a plan, Nepal needs to pay attention to its own environment and each Nepali can adjust personal behaviour by limiting pollution.

One place to start could be to phase out the use of plastic bags and bottles as well as revive the holy Bagmati and Vishnumati rivers which have become dumpsites. The country also needs to reduce a growing reliance on imported diesel and petrol, and begin the switch to electric public transportation.

With abundant hydropower potential, this would make both environmental and economic sense. This may not significantly reduce the smog over Asia, but it would improve the quality of life within Nepal.

Alonzo Lyons, a Stanford epidemiologist, first came to Nepal in the mid-1990s and is the co-author of Trekking Nepal, Edition 8

HIMALAYAN MAP HOUSE SEE FOREVER: One of the rare November days when northwesterly winds wash off haze from India and pollution of Kathmandu Valley to reveal mountains as far away as Dhaulagiri, 300km away.

HIMALAYAN MAP HOUSE BLACK PYRAMID: Machapuchre is 6990m high, yet it is almost devoid of snow in winter. Deposit of pollution soot is accelerating global warming and the melting of permafrost in the Himalaya.

NASA EFFLUENCE OF AFFLUENCE: A NASA satellite image taken on 8 February 2010 showing the pollution haze over the India and Bangladesh which also creeps up the valleys in Nepal and Bhutan. The Tibetan plateau is in bright sunshine.

MIN RATNA BAJRACHARYA UP WHERE THE AIR IS CLEAR: The mountains of central Nepal are enveloped in blue haze blown in from the south, while the high Himalaya, including Gauri Shankhar (left) and Mt Everest rise above it all.