A philtre that is at least 99.97% efficient in removing monodisperse particles of 0.3 micrometres in diameter. The equivalent niosh 42 cfr 84 particulate philtres are the n100, r100, and p100 filters. (source: niosh respirator selection logic, 2004. Dhhs(niosh) publication no. 2005-100. And assigned protection factors for the revised respiratory protection standard, 2009. Osha u.s. department of labour publication no. 3352-02 2009.)