How Class I Biological Safety Cabinets Work

The Class I Biological Safety Cabinet (BSC) provides personnel and environmental protection, but no product protection. It is similar in air movement to a chemical fume hood, but usually has a limited fixed work access opening and the exhaust air must be HEPA filtered, to protect the environment. The Class I BSC is designed for general microbiological research with low- and moderate-risk agents. In addition, the work zone, which is typically much smaller than a chemical fume hood, is geared to working with biologicals whose process/procedures dictates the use of moderate amounts of volatile liquids, that are not suitable for recirculating cabinets.

The Class I BSC is similar to a standard chemical fume hood, in that unfiltered room air is drawn through an access opening, which provides personnel protection, then across the work surface, through an exhaust plenum and out the top. To be classified as a Class I BSC, however, the inward flow of air MUST maintained at a minimum inflow velocity of 75 linear feet per minute (FPM) through the front access opening. The design of the access opening should be in accordance with accepted design procedures for fume hoods, such as airfoils, exterior fluorescent lighting and properly designed interior baffles to minimize interior roll, etc.

Without an internal blower, the Class I BSC must hard-ducted to the building exhaust system, which must provide the negative pressure necessary to draw adequate room air into the cabinet. Typically a minimum of 0.5-inches water gage negative. Cabinet air is drawn through a HEPA filter located at the top of the work zone before it enters the exhaust system. A second HEPA filter may be installed at the terminal end of the exhaust system, but it is not required. Also note that use of two filters increases the static pressure on the fan.

Some Class I BSCs are equipped with an integral exhaust blower. This cabinet's blower must be interlocked with the building exhaust fan. In the event that the building exhaust fan fails, the cabinet's exhaust blower must be turned off so that the exhaust ducts are not pressurized. HEPA Filter(s) should be installed on the outlet side of the fan as shown in order to easily check the integrity of the filter. It is good design practice to surround all contaminated positive pressure plenums with a vacuum, to preclude plenum leaks from escaping the cabinet into the laboratory. If the ducts are pressurized and the HEPA filter develops a leak, contaminated air could be discharged into other parts of the building or the environment.

A steel panel, also known as a glove port panel, with arm holes to allow access to the work surface can be attached to the front opening of the Class I cabinet. The restricted opening results in increased inward air velocity, thereby increasing worker protection. For added safety, arm-length gloves can be attached to the panel. Makeup air may be drawn through an auxiliary air supply opening (which may contain a filter) and/or around a loose-fitting front panel.

With the product protection provided by the Class II Biological Safety Cabinets, however, general usage of the Class I BSC has significantly declined. In many cases, however, Class I BSCs are used specifically to enclose equipment (e.g., centrifuges, harvesting equipment, necropsy [NU-807], balances [NuAire's Labgard 813 or Labgard 819], or small fermenters), or procedures (e.g. cage dumping [Allergard 607], aerating cultures or homogenizing tissues) with a potential to generate aerosols.