In simple terms, the APF (Assigned Protection Factor) is the ratio of hazardous substance outside the RPE to the amount inside the RPE. To help you, each RPE type and class is categorised by an assigned protection factor (APF). The APF is a number rating that indicates how much protection that RPE is capable of providing. For example, RPE with an APF of 10 will reduce the wearer’s exposure by at least a factor of 10 if used properly, or, to put it another way, the wearer will only breathe in one-tenth or less of the amount of substance present in the air. There are only a few number ratings used, so RPE APFs will be either: 4; 10; 20; 40; 200 or 2000.

According to the Health and Safety Executive, via their Practical Guide HSG53, you need to ensure that the RPE you select can protect the worker from the hazardous substance in the air around them. Your decision will depend on the amount in the air and its form (eg particles, vapour). There are various types of respirator and BA available. The protection they offer will be determined by a number of things, including the protection factor.