Air pollution

I've taken to wearing a mask in big cities; the air pollution causes my nose to run. Initially I purchased the mask to ward off germs from sick people coughing and sneezing; however, it's also useful against air pollution. 

I first noticed my nose running while in Seoul; when I left the city my nose stopped running. It started again when I visited Kyoto and here in Hiroshima; when I move out of the cities, it stops.

Lots of people wear masks and while mine isn't comfortable and can cause my glasses to steam, the benefits outweigh these minor inconveniences.