Protecting your digital camera is important during the trek as during the trekking season (I was trekking in April) the air and soil on and along the trekking trails is very dry, polluted with animals' excrements. Dust was everywhere and it took me long time to clean my SLR.
KTM is one of the most polluted cities in the world, similarly to Mexico City. In my opinion face masks when walking on the streets in the traffic peak is highly recommended.
Face mask is essential during the trek as during the trekking season (I was trekking in April) the air and soil on and along the trekking trails is very dry, polluted with animals' excrements so it's better to use the mask rather than breath without it.
Every member of my trek group experienced infections mostly because of nasty things they inhaled.
Traffic on the roads are often chaotic in Nepal. To make matters worse, there are also animals such as cows walking on the road, plus there are many pot-holes. Therefore driving in Nepal can be an adventure in itself !
City, i.e. Kathmandu/Patan, pollution levels are horrendous. The burning of pollutant fossil fuels fills the air with thick gray smog. Most of this is caused from old car exhausts. Some traffic police wear masks to avoid breathing in all the smog. City pollution isn't strictly air pollution. Littering is a huge problem in Nepal, as many places don't even have trashcans. Pollution in Nepal, as in every nation, is sickening. Try not to add to the pollution. Take electric taxis and do not to litter.
In Kathmandu, the air pollution was quite serious. The exhaust gases generated by vehicles and dust filled in the air. Each time we came back to hotel after walking around the city, we found that our skins and clothes were covered by a black layer. If you have any respiratory problem, please take extra care.