Three classification schemes appear across the filtration literature and they are not interchangeable. EN 779, with its G1 to G4, M5 to M6 and F7 to F9 classes, was fully withdrawn on 1 July 2018 after an 18 month parallel period and replaced by ISO 16890, which evaluates filters across a 0.3 to 10 micron particle spectrum and assigns a group where the filter reaches at least 50 percent efficiency against a mass fraction, reported as ePM1, ePM2.5, ePM10 or ISO Coarse. EN 779 rated its M5 to F9 classes on a single 0.4 micron particle size. High efficiency filters fall outside ISO 16890 and remain under EN 1822, which ISO 29463 has superseded in essential parts. ASHRAE 52.2, which produces MERV 1 to 16, is a separate North American scheme.
No exact conversion exists between EN 779 classes and ISO 16890 groups, because the test methods differ. Published cross reference tables are indicative only and disagree with one another. This has direct consequences for reading the research: the swine filtration literature specifies in MERV, including Dee 2005, the Osborne 85 farm review and Desrosiers and Cousin 2023, while the German laboratory work of Wenke and colleagues in 2017 specifies in EN 779 classes. Both appear in this library, and the scheme name must travel with any class designation quoted from either.
One feature of ISO 16890 is worth knowing for its own sake. The standard conditions filters in isopropanol vapour before rating them, which strips the electrostatic contribution from the measurement. That procedure exists because of the failure mode Raynor and Chae documented in 2004 and the Osborne review found in the field, where charged media rates well when new and loses efficiency in service without any change in pressure drop to signal it. A standard that removes charge before testing is reporting what the media does mechanically, which is what remains after the charge is gone.