Move ahead, smoke behind

This is an ordinary street in Kathmandu Valley motorbikes, taxis waiting at the signal.

And then this old blue public bus pulls away, trailing a cloud of black smoke thick enough to blur the road behind it. It's a common sight here.

Kathmandu's public transport fleet is aging, and many buses and microbuses run well past their reasonable service life, with poorly maintained or outdated diesel engines.

These vehicles burn fuel incompletely, releasing black carbon, nitrogen oxides (NOx), sulfur dioxide (SO₂), carbon monoxide, and fine particulate matter (PM2.5) directly at street level exactly where pedestrians, shopkeepers, and bikers are standing. Vehicle emissions are estimated to contribute roughly a third of Kathmandu Valley's air pollution, and studies have repeatedly flagged old diesel public vehicles as a great challenge, often emitting several times more particulate matter than newer, properly serviced engines.

Black carbon especially is dangerous which penetrates deep into the lungs and is linked to bronchitis, asthma, and cardiovascular disease, and also acts as a short-lived climate pollutant, accelerating Himalayan glacier melt when it settles on snow. The smoke clears in a few seconds for the camera.

For the people standing on that sidewalk, it doesn't clear nearly as fa