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Keeping a Clean Atmosphere

This article attempts to provide a checklist against which the choice of chemical or biological containment cabinets may be measured. However, the benefit of an expert evaluation of the hazards and risks to be considered before any equipment is selec

by Simon Kear
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Protecting workers from exposure to harmful materials or practices is a key part of any manager’s duties.

While the Control of Substances Hazardous to Health (COSHH) regulations lay down guidelines on when protection against a certain chemical or particulate is required, they do not provide guidance about which type of containment or extraction system is the most suitable. 

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Therefore, this article attempts to provide a checklist against which the choice of chemical or biological containment cabinets may be measured. However, the benefit of an expert evaluation of the hazards and risks to be considered before any equipment is selected should not be underestimated.

Critical components of many laboratories’ environmental controls, chemical or biological containment cabinets are designed specifically to minimize employee exposure to chemicals and particulates – though rarely at the same time. 

Chemical hazards
Let’s first deal with the main groups of products designed to protect against chemical hazards:

• Ducted fume cupboards
• Recirculating carbon-filtration fume cabinets

Ducted fume cupboards
Traditionally, containing noxious fumes has been the domain of the fume cupboard and extraction system. These employ a very simple but, from the laboratory's perspective, highly effective approach. Waste fumes and gases are pumped straight out of the laboratory and vented into the atmosphere and replaced by uncontaminated, ambient air drawn in from the surrounding laboratory. This constant, inward flow of air through the front working aperture ensures that chemical fumes cannot overcome the operator or make their way into the surrounding laboratory atmosphere.

However, there are two fundamental disadvantages to consider. Although personnel inside the laboratory are safeguarded, expelling untreated air may expose everyone in the area to the fumes unless great care is taken with the design of the flue stack. Additionally, every liter of air pumped out of the laboratory and into the atmosphere must be replaced with air drawn from the laboratory itself. Since this air is heated, possibly even air-conditioned, this will inevitably increase the energy bill.

Recirculating or filtration fume cabinet
The filtration (or recirculating) fume cabinet operates in the same way as a ducted version, pulling in air through the front aperture and away from the user, and offers the same level of operator protection. However, instead of venting air to the atmosphere, the exhaust air is filtered and returned to the laboratory via a charcoal filter.

The recirculation principle allows great flexibility. Should the nature of the chemical hazard change, the carbon filters may be quickly and inexpensively changed to suit. And, because there is no physical attachment to a fixed ducting system, installation costs are eliminated as the unit can be simply placed on an existing bench, plugged in and used. New ducting, associated building work, and changes to the laboratory's heating and ventilation systems are eradicated and, if changes are necessary to the laboratory’s layout, the unit can be re-positioned easily.

Down-flow Benches
Evolved from horizontal flow cabinets, down-flow benches are designed to draw air down and away from the operators face without the need for an enclosure.

 Just like their forerunners, down-flow benches may be ducted to the atmosphere or be designed with filters to remove potentially hazardous material before the exhaust air is returned to the laboratory. Therefore, the advantages and disadvantages of each type are shared.

Biological Hazards

Before choosing which type of biological safety cabinet is most suitable for the handling of potentially pathogenic material, consideration must be given to the advice from the UK’s Advisory Committee on Dangerous Pathogens, part of the Health and Safety Executive. These criteria are reproduced below:

• Group 1 - A biological agent unlikely to cause human disease.

• Group 2 - A biological agent that can cause human disease and may be hazardous to employees; it is unlikely to spread to the community and there is usually effective prophylaxis or effective treatment available.

• Group 3 - A biological agent that can cause severe human disease and presents a serious hazard to employees; it may present a risk of spreading to the community, but there is usually effective prophylaxis or treatment available.

• Group 4 - A biological agent that causes severe human disease and is a serious hazard to employees; it is likely to spread to the community and there is usually no effective prophylaxis or treatment available.

To deal with these different threat levels, three classes of Microbiological Safety Cabinets (MSC's) have been developed.

Class I and Class III MSC's
Like a traditional fume cupboard or carbon-filter fume cabinet, a Class I MSC sucks air into the cabinet and away from the operator in order to generate a very high level of protection. However, unlike the ducted fume cupboard, all the exhaust air must pass through a filter before it is released to the atmosphere.

High Efficiency Particulate Air (HEPA) filters are used for this purpose and are usually fitted immediately above the working area in modern cabinets. Suitable for material in groups 1 – 3 above, the Class I cabinet would be used typically in laboratories where patient specimens are handled prior to the identification of any disease.

Class II MSC
Also suitable for Group 1 and 2 materials, and Group 3 material at the discretion of your Safety Officer, the Class II biological safety cabinet offers a high degree of operator protection through the mechanism of drawing laboratory air quickly around and past the operator. However, the Class II MSC also attempts to protect the integrity of the material under investigation.

Class III
Although we have said that the Class I and Class II MSC’s generate a high measure of protection for their operators, this is not absolute. Therefore, when working with Group 4 materials, a safety cabinet guaranteed to provide maximum security for the operator and the laboratory must be used.  This is the Class III MSC.

Very similar to a Class I except that there is no front aperture, operators access the work surface and samples via sealed glove ports. All materials must be transferred in and out of the cabinet through a transfer hatch or hatches, which resemble miniature airlocks. Exhaust air is pulled through one or more HEPA filters and dumped to the atmosphere via a ducted exhaust fan and make-up air is drawn into the cabinet via a HEPA filter. Finally, these cabinets are also usually sited in a purpose built, high biological security laboratory. In this way, every avenue is covered in attempting to make sure that the material under investigation cannot escape to harm the operators or the outside world.

Hopefully, you will now have a simple guide to the types of chemical and biological containment cabinets available to help you build a safe working environment and an awareness of the strengths and weaknesses of each type. But, knowing how to apply these principles in your own working environment may not be so straightforward.  Responsible manufacturers have experts that work daily to achieve the safest possible workplaces.  For your own safety, don’t hesitate to call them in.