A great number of the mask users in Japan are allergy sufferers. The Forestry agency in Japan planted mono-culture trees, fast growing cedar. The pollen from these trees waft from the cedar farms in yellow clouds. Having a single predominate plant species putting out so much pollen triggers awful allergies for those people who are sensitive. And triggers normal-bad allergies in people who might not have become allergic without the large exposure to that mono-culture pollen.