Biological safety cabinets:
Applications, selection and use

1.0 Introduction 

Surveys have shown that a significant number of laboratory-acquired infections are believed to be due to exposure to aerosols that are produced in common microbiology laboratory procedures (see AS2243.3, 3.1). Aerosol containment techniques and facilities appropriate to the microorganisms being handled should be provided. AS2647 defines biological safety cabinets as the primary barrier to protect laboratory personnel against exposure to airborne contamination in these applications.
 
2.0 Biological safety cabinets

2.1 General

Class I and II biological safety cabinets  are open-fronted, ventilated containment enclosures, intended for work with microorganisms classified in AS2243.3 as Risk Groups 2, 3 and 4, and which can be deactivated by the formaldehyde fumigation procedure described in AS2647, Appendix C. Cabinets incorporate HEPA filters and variable-speed fans. Class I and II cabinets are described in separate parts of AS2252. They are self-contained work-stations and operate independently of other air-handling systems. This is in contrast to overseas practice, where cabinet exhaust air is often ducted to atmosphere.

Class III cabinets are totally-enclosed, ventilated containment enclosures, intended for work with high-risk, possibly-contagious microorganisms in Risk Groups 3 and 4. Exhaust air is ducted directly to atmosphere, typically via HEPA filters. The limitations of laminar flow cabinets with respect to levels of protection are set out in 3.0 below.

2.2 Class I cabinets

Class I cabinets are 100%-exhaust enclosures with HEPA filtration of exhaust air and inward air velocity at the work opening of  0.5 - 0.8 m/s. An inward flow of room air through the work opening sweeps aerosols generated during the work process into a two-stage filter system. Air is directed into a prefilter and a HEPA filter before being exhausted to the room. Class I cabinets are described in AS2252.1.

They provide personnel and environment protection, but do not protect products (materials within the cabinet).

2.3 Class II cabinets

Class II cabinets are part-recirculating laminar air flow enclosures with HEPA filtration of exhaust air and an air barrier at the work opening. Separate fan/HEPA filter systems are provided for exhaust and laminar air flow. An inflow of room air into a full-width grille in the work opening creates an air barrier. A quantity of air, equal to that of the barrier air, is exhausted to the room via a HEPA filter. Vertical laminar airflow is recirculated within the work zone to provide product protection. 

Class II cabinets combine the levels of protection afforded by Class I cabinets and laminar flow work-stations for product protection, which are specified in AS1386.5. They are described in AS2252.2.

They provide personnel, environment and product protection.

2.4 Class III cabinets

Class III cabinets are totally-enclosed, 100%-exhaust enclosures with HEPA filtration of intake and exhaust air. In the UK, double-filtration of exhaust air is common. All work within the work zone is conducted through attached rubber gloves. 

A small number of Class III cabinets is in use in Australia, whereas they are in widespread use in Europe. It has been reported that their use has extended to the pharmaceutical industry, for sterility testing where enhanced assurance of sterility is required. An Australian Standard for Class III cabinets has not been published.

They provide personnel, environment and product protection.

3.0 Laminar flow cabinets

Laminar flow cabinets (clean benches) are self-contained Class 3.5 laminar flow clean work stations which are designed to provide a high degree of protection for process products and apparatus in laboratory and production facilities. Cabinets are produced with horizontal or vertical airflow in the work zone. These cabinets are intended for work involving the handling of non-hazardous materials in such applications.

They are not safety cabinets. They do not provide personnel or environment protection as aerosols from the work zone are directed towards the operator. 

4.0 Other applications and cabinet types

4.1 Work with radioactive materials

Work in safety cabinets involving radioactive materials requires careful consideration of the risks posed to personnel. The fitting of internal and/ or external lead shielding to cabinets may be required. The National Health and Medical Research Council has published recommendations for handling such materials, and relevant legislation exists in all States. AS2243.4 specifies safe practices and environments for work where sources of ionizing radiation are used.

4.2 Work with cytotoxic drugs

Biological safety cabinets as specified in Australian Standards are unsuitable for use with cytotoxic drugs and other toxic chemicals as these materials cannot be deactivated by fumigation. Aerosols of the materials, which are recirculated within the cabinet, contaminate fans and internal plenums. Therefore, if used for handling such materials, cabinets suffer permanent contamination of internal surfaces. This precludes safe internal maintenance.

Special cytotoxic drug safety cabinets are described in AS2567.

5.0 Fume cabinets

Special fume handling enclosures may be required for some corrosive, toxic or flammable chemicals. Ducted fume cupboards and recirculating fume cabinets are described in AS2243, Part 8 and Part 9, respectively. When working with such materials, suitable protective garments, gloves, safety glasses and respirators should be worn. 

6.0 Cabinet size

120cm-wide cabinets are normally selected for single-operator applications, and 180cm for two (2) operators. Concern regarding operator-produced air turbulence may limit the application of 180cm cabinets in work with high-risk organisms. 

7.0 Cabinet options

Commonly-specified safety-cabinet options are:

additional power point
 UV lamp
 gas tap, with safety solenoid valve
 vacuum tap with membrane filter
 floor stand
 exhaust handing (no-cost variation for Clyde-Apac cabinets)
 fumigation adaptors

Although these options attract modest additional cost when ordered with a new cabinet, fitting of some items to installed cabinets can be very costly.

8.0 Installations

8.1 Location

Cabinets should be installed in accordance with AS2647, local regulations and specified user requirements. They should be located in a clean, draught-free area, not subject to air turbulence from air conditioning inlets, room exhausts, personnel traffic and other sources. All windows should be inoperable.

Temporary loss of air barrier containment can be caused by air turbulence in front of cabinets. Partitions to minimise effects from personnel traffic can be installed , and excessive velocities from room air inlets can be inhibited by regulating dampers and/or blanking or baffling. Dedicated locations should be considered for work with very hazardous microorganisms. 

8.2 Services

Electrical power and other reticulated services to meet the requirements of cabinets and their accessories should be provided at the specified locations.  Vacuum sources require in-line filtration to provide microbiological containment. Hoses and power leads should not be introduced into cabinets through the work opening. Compliance with local regulations regarding gas connections should be confirmed. 

8.3 Exhaust air discharge

Clearance in the direction of exhaust discharge should be at least 60cm, in order to allow free air flow and access for maintenance and testing of the exhaust HEPA filter installation. The placing of large items, such as work opening covers, on top-exhaust cabinets can restrict airflow and reduce containment. 

Work with odoriferous materials may demand treatment of cabinet exhaust air by installation of an adsorbent filter, or by ducting to atmosphere. Adsorbent filters should only be fitted to installed cabinets after it has been determined that cabinet fans are able to maintain design airflow, with an acceptable reserve capacity to cope with dust loading of HEPA filters. AS2252 specifies that cabinets should have a fan-reserve capacity of not less than 125Pa.

If ducting of cabinet exhaust to atmosphere is contemplated, consideration should be given to the effect on the room ventilation system, and to Appendix A of AS2647 dealing with recommended exhaust arrangements. Any such exhaust system should be via an indirect connection, with the room exhaust air quantity greater than that of the cabinet. The exhaust system should not influence cabinet airflow.

9.0 HEPA filters

HEPA filters are the physical containment barrier in safety cabinets and arrest sub-micron particles. They incorporate a very fragile filter medium which is easily damaged by physical contact and which may suffer degradation if splashed with liquid.  These filters cannot be cleaned, and are normally replaced when their increased resistance to airflow impairs cabinet performance, when excessive leak repair is necessary, or when heavy surface contamination occurs.

Replacement filters should be suitable for use in critical applications and should be individually tested and certified in a NATA-registered laboratory. Arrestance efficiency should be > 99.995% to the Sodium Flame test, or > 99.99% to the Hot DOP test. Additionally, filters should be certified for integrity (freedom from pin-hole leaks) in accordance with AS1807.6. Some filters, although labelled as meeting these requirements, provide no means of traceability of factory test results, and their use cannot be recommended. Determination of the in-situ integrity of HEPA filters and their installations is the most important testing procedure for cabinets.

10.0 UV lamps

Most biological safety cabinets are fitted with germicidal ultraviolet (UV) lamps in the work zone. UV can be a useful adjunct to surface cleaning procedures, but should not be seen as a panacea that replaces good cleaning technique. The intended use and occupational health and safety aspects of UV should be understood by laboratory managers and cabinet users, i.e: UV lamps should be used for 20 to 30 minutes at the beginning and end of work programmes. They should not be left on for extended periods, although this practice is common. 
Personnel exposed to UV radiation may suffer eye damage and erythema. Work opening covers should be in place whenever UV lamps are in use. 
UV radiation degrades nitrile, plastics and rubber products and organic coatings, such as those used in typical cabinet construction.
UV is ineffective in dynamic air streams, on dried organic matter and is not penetrating. Radiation intensity reduces over time due to degradation and external staining of lamps. Where the use of UV is a significant element of surface decontamination procedure, regular testing of lamp intensity should be specified. 

11.0 Testing and certification

11.1 General

All testing procedures should be conducted in accordance with AS1807  by a NATA -accredited laboratory, using calibrated apparatus. The nominated cabinet user representative should be provided with a report, together with relevant worksheets, detailing the test results. A test label, being an extract of the report, should be affixed to the cabinet. Unsafe cabinets must be clearly marked.

Laboratory managers and cabinet users should understand the significance of the test results which are reported. 

11.2 Frequency

Cabinets are tested in the factory and further testing is necessary as follows:
On site, prior to use. 
After any electrical or mechanical maintenance.
After filter replacement.
After re-location.
At least annually.

In special circumstances, such as a significant change in the work programme, or where unsafe cabinet operation is suspected.

11.3 Clyde-Apac services

Refer to our testing and certification information.