Here’s a link to the paper under discussion1: http://www.atsjournals.org/doi/abs/10.1164/rccm.200812-1911OC#.WBcwWvmLR7O
Pollution in our cities is a big problem. Cycling to work every morning through central London exposes me to a broad range of tiny carcinogenic particles. Google maps now labels high pollution areas for the conscientious cyclist to avoid, but sometimes this is impossible, and it is arguable how much distance you actually need to be away from a problem area for this to be effective.
Most face masks designed to protect the wearer from airborne particles have filters that are too large to be effective – most of the harmful stuff is pretty tiny. There is a notable exception in ‘totobobo’ masks, which provide OK protection. Warning – these do not look trendy, and they make it pretty hard to, um, breathe.