Why Is Kathmandu So Polluted? The Causes of Kathmandu Air Pollution

This guide explains why Kathmandu is so polluted: where the dirty air comes from, why the valley holds on to it, and why it is worst in the morning and evening. For today’s reading, check the live Kathmandu AQI.

The short answer

Kathmandu produces a lot of smoke and dust, and the valley is shaped like a bowl that keeps it in. Vehicles, brick kilns, burning rubbish and wood, and dust from roads and building sites are the main local sources. Polluted air from other parts of Nepal and from northern India adds to it. On cold nights a layer of warmer air sits over the valley like a lid, so the pollution stays close to the ground until the sun breaks it up.

The source shares come from a winter study of the valley’s air by Kim and others (2015). The outside-air findings come from Kafle and others (2026). Both are listed under Sources.

Where Kathmandu’s pollution comes from

Most of the smoke in the valley is made in the valley. The 2015 study took air samples through the winter and traced the soot and carbon back to their sources. Brick kilns on the edge of the valley and vehicles in the city came out on top, followed by fires that burn rubbish, wood and other biomass.

Open burning of rubbish is one of the dirtiest sources for its size. A 2015 field study in the valley and the Terai, called NAMaSTE, measured about 7 grams of PM2.5 for every kilogram of dry household rubbish burned. Damp rubbish gave up to 125 grams per kilogram, the highest of any fire the team tested. PM2.5 is the fine dust that goes deepest into the lungs; our guide to PM2.5 health effects in Kathmandu explains what it does to the body.

Dust is the other big local problem. Road works, building sites and unpaved roads throw dust into the air every dry day, and traffic keeps lifting it again. Homes near a busy road or a building site get far more of it than the city average. For homes with this problem, see our air purifier for dust guide.

Pollution that blows in from outside the valley

Not all of Kathmandu’s dirty air is made in Kathmandu. A study of PM2.5 readings from 2017 to 2020 at stations in Nepal and India traced polluted air into the valley along three routes: from the east, the west and the south. Eastern Nepal was the largest source area. Air from India came second.

The same study found that forest fires in Nepal drive the worst spring days, and crop burning in India loads the air near the border in winter and after the monsoon. Our post on forest fire smoke in Nepal covers the spring haze.

Kathmandu is not the dirtiest place in Nepal. A 2026 study of PM2.5 across the country found the eastern Terai the most polluted region, followed by the western Terai, which includes Butwal and Lumbini.

Why the valley traps pollution

Kathmandu Valley is a bowl ringed by hills. The Bagmati leaves it through one narrow gorge at Chobhar, and there are only a few low passes on the rim. When the wind is weak, air does not get out easily, so smoke and dust build up over the city.

The valley’s wind follows a daily routine. Panday and Prinn measured it at a pass on the western rim: air flows into the valley from late morning until late evening and flows out the rest of the time. In the afternoon that inflow brings cleaner air over the hills and pushes pollution away. At night the air goes still.

What a temperature inversion does

A temperature inversion is a layer of warm air sitting on top of cold air. Normally the air near the ground is the warmest, so it rises and carries smoke up and away. On clear winter nights the ground cools fast, the coldest air slides down the slopes and pools on the valley floor, and warmer air sits above it.

That warm layer works like a lid on a pot. Smoke from traffic, kitchens and fires cannot rise through it, so it spreads sideways and stays at the height where people breathe. This is the main reason winter air in Kathmandu is so much worse than monsoon air. Our post on winter air pollution in Kathmandu covers the cold months in more detail.

Why mornings and evenings are the worst

In the evening, the lid forms around sunset. That is the same time as the evening rush hour and cooking fires, so the new smoke has nowhere to go. Pollution climbs until late evening, then eases slowly through the night.

In the morning, the sun warms the hill slopes first and air starts to move up them. Panday and Prinn found that part of the morning peak is pollution from the night before, held in layers above the valley floor and mixed back down as the air starts moving. Add the morning traffic and the air gets worse, not better, after sunrise.

By early afternoon the valley floor has warmed, the lid breaks and the westerly wind blows through. A year of hourly readings by the Nepal Health Research Council shows the result: PM2.5 peaked at 87 µg/m³ at 8 to 9 am, fell to 31 µg/m³ at 2 to 3 pm and rose again to 59 µg/m³ at 8 to 9 pm. To plan a walk or run around this, read the best time for a morning walk in Kathmandu.

Who studied Kathmandu’s daily pollution cycle

The daily cycle was first measured in detail by Arnico Panday and Ronald Prinn at the Massachusetts Institute of Technology (MIT). They measured carbon monoxide, ozone, PM10 and the weather around the valley through the 2004 to 2005 dry season and published the results in the Journal of Geophysical Research in 2009. A second paper the same year, with Christoph Schär, modelled the valley’s winds to explain the pattern.

Today the Department of Environment runs air quality stations across Nepal, including Ratnapark and Shankapark in Kathmandu. Its readings are public on the government air quality portal.

What this means for the air in your home

You cannot change the valley’s air, but you can change the air in the room where you spend most of your time. Outdoor PM2.5 gets indoors through doors, windows and small gaps, so keep windows shut during the morning and evening peaks and air the house in the early afternoon. Our post on indoor air pollution covers the sources inside the home.

For the particles that still get in, a HEPA air purifier cleans the room air again and again. Blueair’s HEPASilent filtration captures 99.97% of particles down to 0.1 micron, which covers PM2.5, dust and smoke. See our guide to choosing an air purifier for home.