When is it necessary to decontaminate a BSC?
There are 5 basic reasons why it may be fundamental to carry out a decontamination procedure in BSCs:
1. Before performing any service activity which requires access to potentially biologically contaminated areas of the BSC such as absolute filters or fan replacement.
2. Prior to relocation or movement of the biological safety cabinet in another room or building to avoid any biological contamination risk during the operation.
3. In the event of an accidental spill of a hazardous microorganism in the working area.
4. When working procedures are drastically changed and a different microorganism is used inside the BSC.
5. As part of a routine decontamination protocol when BSL-3 or BSL-4 organisms are
How often should a decontamination procedure be carried out?
In most research facilities and hospitals, decontamination will be performed once or twice during the life of a cabinet, especially in those BSC providing additional pre-filters that can extend HEPA filter life up to 7 years.
In pharmaceutical laboratories it could be the need to conduct decontamination cycles between batches and numerous experiments. This could be a weekly activity.
How do I decontaminate a BSC?
BSC manufactures must define a recommended decontamination procedure that has to be validated to proof the 6 log reduction. The validation is done by placing chemical and biological indicators in several places in the contaminated areas, and they are incubated to check if the BI has been sterilised. Validation is considered successful when at least 3 different tests results are the same. This procedure will be included in the user manual provided with the equipment.
Decontamination must be performed using a chemical disinfectant in a gas or vapour state. The main reason for this is because it must pass through HEPA filter media and decontaminate external and internal surfaces to avoid any contamination risk.
Most manufacturers offer accessories to facilitate the decontamination process such as covers and a connection for vapour hoses. However, any cabinet can be decontaminated using plastic film/bag and sealing tape.
Decontamination specialists will assure proper sealing to avoid any leak and will consider the permitted national exposure limits to the chemical according to local authorities.
The use of formaldehyde for BSC decontamination has been the most common procedure in the market because it can be effective reaching a 6 log reduction when using Bacillus Atrophaeus biological Indicators (BI). However, this chemical has been proven to be carcinogen and was forbidden in Europe in 2016. It is still accepted in other regions. Its toxicity makes critical the sealing of the cabinet during the decontamination, and it may be necessary to cover the cabinet with a full plastic bag sealed towards the floor with sealing tape.
Decontamination with formaldehyde gas is performed either by vaporising formalin solution, (37% water-based solution called formalin) or by depolymerisation of solid paraformaldehyde. It is necessary to circulate the gas in the BSC for up to 12 hours and afterwards neutralisation is usually achieved using ammonia gas. This process will leave residue on working area surfaces that has to be manually cleaned (which is difficult to accomplish in the plenums, fans, etc.), extending the total decontamination process up to 24 hours before its ready to be used once again.
Hydrogen peroxide works by producing destructive hydroxyl free radicals that can attack membrane lipids, DNA, and other essential cell components. Catalase, produced by aerobic organisms and facultative anaerobes that possess cytochrome systems, can protect cells from metabolically produced hydrogen peroxide by degrading hydrogen peroxide to water and oxygen. This defence is overwhelmed by the concentrations used for decontamination.
Hydrogen peroxide is active against a wide range of microorganisms, including bacteria, yeasts, fungi, viruses, and spores. It has proved to be effective reaching a 6 log reduction when using Geobacillus Stearothermophilus biological Indicators (BI). The high oxidation potential of the Vhp can superficially affect some materials (copper or POM) in high concentrations with long period of exposure, but it has proven not to affect most of the materials used in a BSC when using a low concentrated solution.
By having a higher exposure limit makes the sealing of the BSC enclosure a less critical aspect and the whole process is made easier, but an external sensor is recommended to detect the concentration (ppm) in the ambient.
Chlorine dioxide is an effective sterilising gas, so can easily pass through filters. This gas is non-carcinogenic, non-flammable, and effective at ambient temperatures, but is highly toxic and corrosive with some materials and it is not recommended when required decontamination frequency is high. The accepted exposition limits (TWA PEL) are just 0.1 ppm, much lower than when comparing with Vhp limits. It is necessary to evacuate personnel from the room and the BSC frame sealing becomes a critical process.
A typical chlorine dioxide sterilisation process uses a sequence of preconditioning, conditioning dwell period, charge, and exposure, followed by aeration. Aeration can be done directly when the cabinet is ducted to an external exhaust system, otherwise the chlorine dioxide must be removed by an activated carbon packed scrubber or a sodium thiosulfate counter-current scrubber. Like Vhp, it does not leave any residue after the process.
All the methods listed above can be effective reaching 6-log reduction, but BSC manufacturer recommendations must always be the main option. Of course, considering the strong and weak points of each process, some users may prefer to use another method, in which case it is their responsibility to perform a validation test to check the effectiveness If any doubts arise they must contact the BSC manufacturer for further clarification and assurance.