Final answer:
Closed-Circuit SCBA is a type of respiratory protection that recirculates exhaled air after carbon dioxide removal and oxygen replenishment, used in hazardous environments. It is different from BSCs, which are containment units designed to protect from varying levels of biological agents, with Class III offering the highest protection.
Step-by-step explanation:
Closed-Circuit Self-Contained Breathing Apparatus (SCBA) refers to a type of respiratory protection equipment frequently used in environments with potentially dangerous or oxygen-deficient atmospheres, such as those found in firefighting and industrial applications. Unlike open-circuit SCBA which vents exhaled air into the environment, a closed-circuit SCBA recirculates the exhaled air after removing carbon dioxide and replenishing oxygen, allowing for longer duration of use in contaminated environments. Class I Biological Safety Cabinets (BSCs) protect against low to moderate risk biological agents, Class II cabinets offer additional protection by using directional airflow and partial barriers, and Class III cabinets offer the highest level of protection suitable for the most dangerous agents used in BSL-4 labs. These cabinets have stringent safety features including air being filtered by HEPA filters and, in the case of Class III, a double-door pass-through system for decontaminating materials and direct exhaust of air to the outdoors after filtration and incineration.