Air in Kathmandu is polluted. It doesn't require any test from any scientists. A person who goes to the city in the morning and comes back home in the evening can feel the effect of air pollution.
This photo taken recently from airplane shows the effect of air pollution in Kathmandu's air. The reduction of visibility is due to polluted air. The first one on the left side is taken from above the polluted air. You can see clear snowy mountain peaks and down there is layer of polluted air. The second picture on the right side was taken from below the polluted air and you barely see the beautiful mountains because the polluted layer of air blocks and visibility is very poor.
The pollution in Kathmandu is mainly due to vehicular emission, poor quality rood, fuel etc and is adverse in winter due to bowl shaped topography of the valley which doesn't let the air circulation and due to 'temperature inversion'.
Kathmandu valley is otherwise very beautiful, rich in culture. Many foreigners come to see this place. If the pollution level is not controlled it will effect the tourist industry as well. On the other hand, there is effect on the health of people which will cost much more to treat.
You wanna know more about air pollution of Kathmandu? Here are some resources for you.
http://www.most.gov.np/pollution/pollution.php Daily data of PM10 in KTM from Nepal Government.
http://oregonstate.edu/~girib/air_pollution_of_ktm%20published%20in%20science%20and%20future.pdf My article in Science and Future, a science magazine published from KTM
State of the Environment of Nepal by UNEP