Approved to EN12941 class TH2 or TH3 dependant on headtop; Assigned Protection Factor (APF) of 40 with 3M™ Versaflo™ S-Series, M-300 ; M-400 headtops, APF of 20 with all others. Versaflo™ BT-40 Heavy Duty Breathing Tube. This heavy-duty breathing tube is designed for use with the Versaflo™ TR-300 Powered Air Respirator. This breathing tube is highly durable and robust, designed specifically for use in heavy industrial environments or where hot particles may be present.