A Class II Biological Safety Cabinet (BSC) uses HEPA filtered, laminar airflow to provide operator, environmental and sample protection. For the purpose of sterility, HEPA filters are typically rated at 99.99+% efficiency 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; however, the use of such open flames inside of a BSC is not recommended for several reasons:
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, touch-plate 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 above by the CDC, if a flame is deemed absolutely necessary, there are types of equipment widely available that are safer alternatives to the Bunsen burner. Some of these employ low profile, pedal attenuated flames; others detect motion.
If you still have questions about using flames in your process, please feel free to contact me.
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