City Filter Twin Pack contains two replacement filters for use in the Respro City Mask to effectively filter out Primary pollutants associated with vehicle exhaust emissions.City Filter uses Dynamic Activated Charcoal Cloth (DACC) a 95% pure charcoal weave, originally developed by the U.K. Ministry of Defence for use in the protection against chemical and bacteria warfare situations.The DACC™ is laminated with medical grade non-woven polypropylene material which filters particulates from the air and helps prevent them from entering the respiratory system.