The European standard for air filters (EN779:2012) classifies air filters based on their lowest filtration efficiency, also referred to as minimum efficiency (ME). This ensures that filters can be compared after their initial filtration efficiency.
Fine filters previously rated as F5 or F6 to EN779:2002 are not required to meet an ME value in
the new situation. To clearly differentiate these from those that do, filter classes F5 and F6 have been renamed to M5 and M6 as part of a new medium filter category.