I bought 4 buffs in Kathmandu and so happy I did!
During the day, it keeps the dust out as we walk the dusty windy trail.
At night it warms up the dry altitude air and helps prevent coughing (the famous Khumbu cough)
I wash them every two days and the color of the water is proof enough that it works!
Everything that keeps my lungs clear from now on, will help on the climb.