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All Class II biosafety cabinets must provide personnel, product, and environmental protection from particulate contaminants.


In some cases, personnel and environmental protection from non-microbial chemicals, vapors, and gases are also required.

As discussed in How HEPA Filters Work in Clean Benches and Biosafety Cabinets, HEPA filters trap particulates only.  They are completely porous to vapors or gases.  Each type of class II biosafety cabinets handles gas differently.

It’s essential for a user to understand what happens to volatile chemicals inside different types of Class II cabinets.

How Different Types of Class II Biosafety Cabinets Perform when Gas is Present


Distinctions between Type A2, Type B1 and Type B2 biosafety cabinets are most dramatic when all types are subjected to tests for the presence of chemical vapors.  In the Baker research laboratory, each type of cabinet was testing using Toluene, a clear, water-soluble, aromatic liquid that is typically used as a solvent.  Tests were conducted under strict experimental conditions using vapor concentrations higher than found in typical practice.  All cabinets were vented to the outside.

Test Protocol for Vapor Handling in Biosafety Cabinets


  • Liquid toluene was metered into each cabinet at a constant rate and dropped into a heated Petri dish held in a slanted position to flash evaporate the liquid to vapor.
  • The toluene vapor generator was moved from front to rear along the center line of the cabinet work surface.
  • Five replicate samples of vapor concentration in the downflow air were taken for each vapor generator position during each of four replicate runs on each cabinet.
  • All air samples were taken at a single location 8″ below the diffuser at the center of the work area.
  • Evaporation of 1.8 ml/mm of liquid toluene at the work surface generated a continuous release of 1558 mg/mm of toluene vapor into each cabinet.

Results for Vapor Handling in Biosafety Cabinets


Toluene vapor test results illustrate distinctly different vapor return patterns for each cabinet type.

Class II Type A2 Biosafety Cabinet

Vapor concentrations in the downflow air reached a constant value which did not significantly change as the vapor generator was moved from front to rear of the work surface.

Class II Type B1 Biosafety Cabinet

Significant changes occurred as the vapor generator was moved from front to rear of the work surface. With the generator placed in front, the amount of returned vapor paralleled the performance of the Type A2 cabinet. With the generator placed in the rear, returned vapor paralleled performance of the 100% total exhaust cabinet (Class II, Type B2). The downward curve illustrates the variance in vapor concentration related to location from front to rear.

Class II Type B2 Biosafety Cabinet

In the 100% total exhaust cabinet, no air is recirculated and all vapor was removed.

Which Biosafety Cabinet Should You Choose When Vapor is Present?


The ability of a biosafety cabinet to handle vapor is ultimately determined by what distinguishes the different types from one another.  That is airflow patterns and velocities, HEPA air filter positions, ventilation rates and exhaust methods.

Class II Type A2 cabinets are recirculating systems and can be vented either directly to the lab or to the outdoors.  A greater percentage of air is recirculated through the cabinet when compared to the volume of air exhausted, and therefore is not well suited for work with hazardous vapors.  As long as vapors are not hazardous and will not interfere with the work when recirculated, it is acceptable to use an A2 cabinet with a small amount of volatile chemicals when the cabinet is exhausted to the outdoors for removal of gases.  It is important to note that if vapors are used in a cabinet not directly vented to the outside, vapors will pass freely through the exhaust filter into the laboratory.

Class II Type B1 cabinets are partially-recirculating systems.  About half of the air is recirculated and the other half is exhausted outside.  The recirculated air is pulled through the vents in the front of the work area, and the exhausted air is pulled from the vents at the back of the work area.  Therefore it is recommended that if vessels containing hazardous materials which may vaporize are used in a B1 cabinet, that the container should be placed toward the rear third of the work surface, but not so far back as to block the flow of air through the rear grille.

Class II Type B2 cabinets provide no recirculation, and 100% of air is exhausted outside.  For this reason, when even minute amounts of hazardous vapors are present, it is recommended that the Type B2, 100% total exhaust cabinet be used.

Learn more about Class II Biosafety Cabinets


Review the protection levels and key features of Class II Biosafety Cabinets in this free guide that includes an easy-to-read chart explaining how the types of Class II cabinets differ and graphics illustrating the supply, exhaust and recirculation airflow patterns that classify Type A1, A2, B1 and B2 cabinets. 

Download the free guide

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