A B2 filter is a type of protective filter designed for use with respirators and half-masks. It provides protection against specific inorganic gases and vapours, such as chlorine, hydrogen sulphide, or hydrogen cyanide. The letter 'B' in the name denotes this particular group of substances against which the filter is effective. The numeral '2' specifies the capacity class of the filter, which in this case is medium. This means that the filter can absorb a medium amount of harmful substances before it becomes saturated and replacement is necessary. For easy identification, this type of filter is marked with a grey colour according to the European standard.