Class III BSCs are often custom designed for specialized uses and range in size and engineering design from being portable (movement of animals) to stand-alone bench top to Class III BSC line configurations that span a room and have dedicated building ventilation systems.  Design and construction of the Class III BSC is based on a risk assessment, job hazard analysis, project requirements and its integration into the facility.  These and other factors lead to the generation of a basis of design document.

They may be designed to protect against biological hazards at BSL-3 or BSL-4; for handling powder, diagnostic or unknown samples, or conducting aerosol tests; or for working with mixed biological-chemical hazards (All Hazard Receipt Facilities, Public Health Laboratories).

There are some features that are standard across all Class III BSC.  The information in the following paragraphs is derived mainly from NSF 49 , CDC/NIH Biosafety in Microbiological and Biomedical Laboratories 5th edition (BMBL)  and the PHAC Canadian Laboratory Biosafety Manual (LBM) .  The Class III BSC was designed for work with highly infectious microbiological agents and for conducting highly hazardous procedures (i.e. aerosol studies, work with infectious or toxin powders).   Compared to Class I BSC and Class II BSC the Class III BSC provides the maximum degree of protection for the environment and the worker.

It is a gas-tight enclosure (no leak greater than 1x10-7 cc/sec with 1% test gas at 3 inches pressure wg), and is often custom manufactured for an institutes project requirements from stainless steel with a viewing window that cannot be opened without the use of tools.   Prior to conducting work, large pieces of equipment and instrumentation can be introduced through the viewing window or other removable panels.  Typically materials are introduced and removed in one of several ways to include via a (a) pass through chamber, (b) integral autoclave, (c) dunk tank, or (d) rapid transfer port or docking station.  Materials are typically manipulated by using arm length gloves that are attached to glove ports in the BSC in an airtight manner.  

Air is exhausted through two HEPA filters, or a HEPA filter followed by incineration prior to exhaust.    The supply air is also HEPA filtered.  Unlike the Class II BSC which provides laminar airflow, the airflow in a Class III BSC is described as turbulent.  This makes it all the more important to correctly define inward airflow rates to maintain safety while not creating adverse conditions while working with samples (i.e. powders).  The Class III BSC is maintained under negative pressure (minimum of 0.5 inches of water gauge.)  

Class III BSC design and operational requirements are influenced by several prominent industry accepted standards for performance testing of Class III BSC in North America and Europe. 

Subsequent posts will provide more information on Class III BSC design variation based on user requirements, technology advances, and performance testing criteria.

NSF 49 Class II (Laminar Flow) Biosafety Cabinetry, NSF/ANSI 49-2008, Edition: 11th