Originally Posted by
cyccommute
The problem with both of these articles is that he doesn't give the range of particulate that is being filtered. A screen door can have a 99% filtration rate if the particles being filtered are 20mm wide. If the particles are 2mm wide, the screen door becomes less effective and if the particles are 0.2mm wide, the efficiency of filtration is zero.
This is the case of the masks in this "test". The bulk of the particulate being filtered is below the range of the filter so the "efficiency" for sub 0.3µ particles is effectively zero.
Even worse, these filters have absolutely no effect on the permanent gases that could actually harm a human. They don't filter CO, NOX, unburned hydrocarbons or even sulfuric acid droplets (which can happen with high sulfur fuels). Thankfully, those gases aren't really much of a problem in the US, Canada or Europe since our vehicles have to have devices to reduce all those materials.
Thank you for noticing the limits of these studies.
Those masks at least deal with particles in the PM2,5 range which aren't harmless.
http://www.who.int/mediacentre/factsheets/fs313/en/
The most health-damaging particles are those with a diameter of 10 microns or less, (≤ PM[SUB]10[/SUB]), which can penetrate and lodge deep inside the lungs.
NO2 is the main source of nitrate aerosols, which form an important fraction of PM[SUB]2.5[/SUB] and, in the presence of ultraviolet light, of ozone.