Abstract: A fail-safe air handling and safety system is described. The system is for use by workers during removal of hazardous substances, such as asbestos, lead, and toxic chemicals, from within a structure. The structure has a containment area with air exhaust openings for maintaining the containment area at negative pressure with respect to ambient. A decontamination unit sealingly connects to a doorway in the containment area and provides for an air locked entryway for the workers. For convenience of on-site erection, the decontamination unit is modular with the modules sealingly connected in a series with each module connected the one to the other. Unique air inlet diffusers provide inflowing makeup air, and the devices are interlocked so that, upon loss of negative pressure in the containment area, a standby air filter becomes operational.