P921 is ABEK type class 1 filter and it protects from organic and inorganic acid gases and vapours sulphur dioxide ammonia and its derivatives where the boiling point is higher than 65°C. The special Portwest bayonet system ensures a safer connection between the mask and the filter while the twin filters offer improved comfort and balance. This filter is compatible with the P500 Full Face masks and the P420 P430 Half Masks.