PM 2.5 is particulate matter (in the air) with diameter of less than 2.5 micrometers. I only know this because the Tokyo weather reports on TV include the PM 2.

PM 2.5 is particulate matter (in the air) with diameter of less than 2.5 micrometers. I only know this because the Tokyo weather reports on TV include the PM 2.5 forecast for tomorrow as well (it seems to mostly blow over from China, although there must be domestic sources as well). Very heavy PM 2.5 can cause disruptions like schoolkids being kept indoors... from this, I've deduced it is probably harmful to health, or at least presumed to be so. Finally, masks that can filter out PM 2.5 can be easily purchased from webstores, but none of them are comfortable to ride a bicycle or walk around all day in. Now you know as much as I do. :)