Replacement or replaceable filters consist of filter material or media. There are particle filters, gas filters and combined filters. Replaceable filters' increasing filtration efficiency Particle filters should conform to the EN 149:2001 standard and are classified as P1, P2 and P3 in order of filtration efficiency. There are two types of these respirators. One is intended to be used for a maximum of a single shift (marked with the letters NR for Not Reusable). The others are intended to be used for more than a single shift (marked with the letter R for Reusable). Disposable, self-contained units have the medium encased in a polypropylene or plastic housing. These can be connected to perform a multitude of filtration, venting and clarification applications. Filter media are available in several pore sizes.