Multifunction Masks only in Japan
Before coming to Japan, in my hometown Colorado I've never used or seen a mask.
After coming to Japan I've seen many people using masks.
During the Fall season.
It's easy to catch a cold on a over packed train to work.
Many use masks to reduce inhaling the breath of the person standing next to you. Yuck!
During the Spring season
Pollen is a major issue at this time of year in Japan. Many people are troubled by cedar pollen allergy. You'll find many people using masks during this time of year.
During the Summer season
The numbers have reduced in the past years but you'll still find bike gangs running around town making load noise. They use masks to cover their faces.
Perhaps they should consider covering up their number plates as well.