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Understanding Vented Enclosures

August 3, 2016

Personal Protection

Chemical Fume Hood: One of the most common of all vented enclosures. A Chemical Fume Hood is an enclosed work space under negative pressure, with an adjustable sash opening for splash protection and velocity control. Chemical Fume Hoods are used to limit worker exposures to hazardous fumes, vapors and dusts by capturing them at the source of generation. An average inward face velocity of at least 100 feet per minute is required. Additionally, an audible or visual airflow meter must be installed to ensure air is flowing properly and that conditions are safe for the user.

(Also referred to as Chemical Fume Cabinet or Chemical Fume Closet)

Biological Safety Cabinet (Class I):

  • A Class I cabinet does not protect the product from contamination because dirty room air constantly enters the cabinet front to flow across the work surface.
  • As a partial containment unit, the Class I cabinet is suitable for work involving low to moderate risk agents (biosafety levels 1, 2 and 3) where there is a need for personal protection, but not for product protection.
  • Unlike conventional fume hoods, the HEPA filter in the Class I cabinet protects the environment by filtering air before it is exhausted.
  • Personnel protection is made possible by constant movement of air into the cabinet and away from the user.

(Also referred to as a Class 1 cabinet)

Product Protection

Clean Bench: Available in either horizontal or vertical configurations, the clean bench provides product protection from environmental conditions via a steady outward stream of HEPA filtered air. Due to the wide variety of design specifications, airflow requirements vary from unit to unit, however a laminar airflow of 75 feet per minute with a variance of no greater than 20% is generally required.

The clean bench provides product protection by ensuring that the work in the bench is exposed only to HEPA-filtered air.

  • The clean bench is recommended for work with non-hazardous materials where clean, particle-free air quality is required.
  • It does not provide protection to personnel or to the ambient environment.
  • It is not designed to contain aerosols generated by the procedure; the user is exposed to these aerosols.

(Also referred to as a Laminar Flow Hood or a Laminar Flow Bench)

Personal/Product Protection

Biological Safety Cabinet (Class II): The Class II Biological Safety Cabinet provides both personal and product protection by passing air through a pair of HEPA filters. A minimum inflow velocity of 100 feet per minute is required. The Class II cabinet is suitable for work involving low to moderate risk agents (biosafety levels 1, 2 and 3) where there is a need for both personal and product protection.

Cabinets previously classified as Class II, Type A/B3 are now classified as Class II, Type A2. These biosafety cabinets may be exhausted to the room, or connected to a facility exhaust system via a "canopy" connection (NSF recommends a canopy connection for cabinets exhausted to the outdoors).

Biological Safety Cabinet (Class III): The Class III Biological Safety Cabinet is generally only installed in maximum containment laboratories and is specifically designed for work with BSL-4 pathogenic agents, providing maximum protection. It is a gas-tight enclosure with a non-opening, completely sealed, viewing window. Gloves attached to the front of the unit prevent direct contact with hazardous materials.

Both supply and exhaust air are HEPA filtered. Exhaust air must pass through two HEPA filters, or a HEPA filter and an air incinerator, before discharge to the outdoors. Air Flow is maintained by a dedicated independent exhaust system exterior to the cabinet, which keeps the cabinet and all associated ducting under negative pressure (usually about 0.5 inches of water pressure).

(Also referred to as a Glove Box or an Isolator)