Open flame use in a Class II Biological Safety Cabinet
A Class II Biological Safety Cabinet (BSC) provides user/operator, environmental and product/sample protection through the use of HEPA filtered, directed and laminar airflow. For the purpose of sterility, HEPA filters are typically rated at 99.99+% efficient for particles 0.3 micron in size. Typical microbiological procedures often utilize Bunsen burners or other open flames to sterilize and/or reduce cross contamination.
The use of such open flames inside of a BSC is not recommended for several reasons:
(1) The Class II BSC maintains product protection through delivery of laminar (air volumes traveling in a single direction at a constant speed - without turbulence) down over the work area of the cabinet, the thermal heating of the air by the open flame causes air to rise against the laminar downflow, creating turbulence; thus eliminating product protection.
(2) An open flame also has the capacity for melting the bonding agent that holds the HEPA filter media to its frame. This destroys the HEPA filters effectiveness, leading to loss of containment in the positive pressure plenum.
(3) Finally, if the flame was to go out, and the valve not shut, flammable gas would be introduced to the cabinet at a steady rate. In the case of an A2, where 70% of the air in the BSC is recirculated, concentrations of the flammable gas could reach explosive potential and pose a serious risk to not only the BSC, but to the user and the laboratory it is occupied in.
The Centers for Disease Control and Prevention (CDC) and National Institute of Health (NIH) have also addressed this in the publication:
Biosafety in Microbiological and Biomedical Laboratories, 5th edition, (BMBL 5th). Several measures can be taken to reduce the chance for cross-contamination of materials when working in a BSC. Opened tubes or bottles should not be held in a vertical position. Investigators working with Petri dishes and tissue culture plates should hold the lid above the open sterile surface to minimize direct impaction of downward air. Bottle or tube caps should not be placed on the toweling. Items should be recapped or covered as soon as possible. Open flames are not required in the near microbe-free environment of a biological safety cabinet. On an open bench, flaming the neck of a culture vessel will create an upward air current that prevents microorganisms from falling into the tube or flask. An open flame in a BSC, however, creates turbulence that disrupts the pattern of HEPA-filtered air being supplied to the work surface. When deemed absolutely necessary, touchplate micro burners equipped with a pilot light to provide a flame on demand may be used. Internal cabinet air disturbance and heat buildup will be minimized. The burner must be turned off when work is completed. Small electric “furnaces” are available for decontaminating bacteriological loops and needles and are preferable to an open flame inside the BSC. Disposable or recyclable sterile loops should be used whenever possible.
As indicated by the CDC, if a flame is deemed absolutely necessary, there are products on the market that can be used that are safer alternatives to the Bunsen burner. Some of these are low profile pedal operated flames, others detect motion. If there are questions consult with your biosafety officer, your Labconco Territory manager, or myself at firstname.lastname@example.org.
Above: a biosafety cabinet HEPA filter damaged from use of an open flame inside the cabinet.