Mention the smog-laden skies above Kathmandu to Bhushan Tuladhar and he pulls no punches. “There may still be time to save Kathmandu, but we’ll soon reach the point where the damage done by pollution will be irreversible,” he says.

Tuladhar, the South Asia regional technical adviser for the UN’s Human Settlements Programme, was born in Kathmandu some 50 years ago, so he knows what he is talking about. And there is good cause for alarm. The name of Nepal’s capital – which means “wooden temple” in Nepalese – conjures up images of medieval architecture, but the urban sprawl of the past three decades has turned part of the valley into something akin to a huge gasworks.

The main ingredients of impending disaster are modernisation, administrative muddling, soaring population and road traffic. The city (population 2 million) is also hampered by its geographical situation at an elevation of 1,350 metres. The hollow it occupies is surrounded by mountains. Not only does the terrain stop air circulating, the experts explain, but the problem is exacerbated in winter by thermal inversion – cold air from the nearby Himalayas is trapped on the valley floor by a warmer layer of air above, which prevents pollutants from dispersing.

Anobha Gurung, a Nepalese graduate student at Yale University, carried out a survey monitoring the effects of pollution on people working in the city centre, both indoors and out. She observed “really, really high” exposure to airborne PM2.5 particles, which are particularly harmful. According to other studies the concentration of PM10 particles is often three times higher than World Health Organisation guidelines.

In 10 years the number of vehicles has tripled and there are now nearly 700,000 cars, buses, motorbikes and rickshaws on Kathmandu’s roads. New car registrations are rising by 12% a year. Two-thirds of the dreaded particles are caused by vehicle emissions and dust. A previous government launched an ambitious scheme to build new roads and widen existing ones, but the jams worsened.

Facebook Twitter Pinterest Kathmandu’s traffic police, who attempt to control the city’s jams, are among the most vulnerable to the effects of highly carcinogenic PM10 particles Photograph: Sunil Sharma/Xinhua/Corbis

“There is nothing surprising about what’s happening in Kathmandu and the surrounding valley,” says Kanak Dixit, founder Himal Southasian, a magazine that specialises in environmental issues in the subcontinent. “The title of our first issue, in 1987, was ‘The city is stifling’.” But the authorities failed to heed warnings. “Population growth is due to the migratory pressure from country people drawn to the city. As an impoverished migrant your obsession is survival, not pollution,” Dixit adds.

In 2014 the Yale Environmental Performance Index ranked Nepal as the second most polluted country in the world, after Bangladesh. “The city goes on spreading without any coherent plan,” Tuladhar says. “The Bagmati and Bishnumati rivers are open sewers and the supply of running water is totally inadequate. Demand exceeds supply more than three times over, with 90m litres flowing in daily, whereas what’s needed is 320m.”

With bureaucracy and recurrent political instability since the return of parliamentary democracy in 1990, the authorities have done little to keep Kathmandu’s expansion in check. The country is still recovering from a Maoist insurrection that claimed almost 18,000 lives between 1996 and 2006. Then in 2008 the monarchy was ousted. With rival parties in the new republic remain locked in a power struggle, and unable to agree on a new constitution, time may not be on Kathmandu’s side.

This article appeared in Guardian Weekly, which incorporates material from Le Monde