A K1 filter is a type of gas filter for protective masks and half masks, whose properties are defined by the European standard EN 14387. The letter 'K' denotes protection against ammonia (also known as spirits of ammonia or aqueous ammonia) and its organic derivatives, which are compounds derived from ammonia. The numeral '1' specifies a low filtration capacity, and therefore the filter is intended for environments with a low concentration of these gases, specifically up to a limit of 1000 ppm, which corresponds to 0.1% by volume. It often appears as part of combined filters, such as 'ABEK1', which protect against multiple types of hazardous substances simultaneously.