CONTAINMENT STANDARDS FOR VETERINARY FACILITIES

4. FUME HOODS

Fume hoods should be routinely used to contain hazardous gases, vapours, mists, aerosols and particulates generated during the manipulation of chemical substances. To be effective, a fume hood must confine contaminants within the hood, remove them through the ductwork, and filter or disperse them so that they do not return to the building through air intake systems. All laboratory fume hoods can be described as one of three types:

• Conventional - basic enclosure with a movable front sash; as sash is opened the air volume entering the hood remains constant, but the air velocity decreases due to reduced flow constriction

• By-pass - operates at a constant exhaust air volume, regardless of sash position; accomplished with openings above the hood sash through which air passes when the sash is closed

• Auxilliary-air - equipped with a supply air system separate from the room supply system; discouraged by the ASHRAE Handbook, Heating, Ventilating, and Air-conditioning Applications (1995) due to difficulties and installation criteria associated with the supply of auxilliary air

Note: recirculating ductless fume hoods have limited use in the laboratory because of the wide variety of chemicals used. Inappropriate filter selection, inappropriate chemicals and/or chemical concentrations, and inadequate monitoring systems for filter replacement can lead to contaminated air being recirculated into the work environment.

Materials exhausted from fume hoods are often disposed of by dilution into the atmostphere. The inclusion of exhaust air treatment devices (e.g. activated carbon filters) must be consistent with applicable local regulations. Where required, filters must be selected according to the type of contaminant to be removed, the efficiency required to meet occupational and/or environmental exposure limits and the required residence time. Filters must be located upstream of the exhaust fan and must permit replacement without contaminating the surrounding environment. Filters present an initial high cost and are associated with high maintenance associated. Carbon filters also present present a pollution source during disposal.

Fume hoods located inside containment level 3 and 4 laboratories must comply with the requirements for HEPA filtration (see Section 4). The installation of a charcoal filter prior to the HEPA filter may be considered as a measure to protect the HEPA filter from deleterious effects from chemical vapours and to protect the individual performing maintenance and certification testing of the HEPA filter.

Fume hoods and associated exhaust systems must comply with design and installation requirements outlined in the CSA standard Z316.5-94, Fume Hoods and Associated Exhaust Systems (1994). Fume hoods should be located as follows:

away from normal traffic patterns and interfering room air currents

• the front sash should be 1.5 m away from room air exhaust diffuser and 1.5 m away from air supply diffusers (this distance can be reduced when using velocity controlled diffusers, i.e. maximum throw velocity of 15 m/m @ 1 m from diffuser or ½ face velocity of fume hood, and when proven that fume hood performance is not affected)

• the front sash should be 1.5 m away from doorways and,, 2.0 m away from opposing walls or other obstructions,

• the side of the fume hood should be 0.3 m away from walls or other obstructions projecting beyond the plane of the sash and 1 m away from doorways

• the hood should not be located directly opposite seated work stations, other fume hoods or biological safety cabinets

The CSA Z316.5-94 standard for fume hoods requires the performance of tests conducted on all fume hoods upon initial installation and periodically thereafter to maintain them in good operating condition. All requirements for test conditions, equipment accuracy and calibration, and outlined in the CSA standard must be followed. This includes the performance of tests in accordance with ANSI/ASHRAE 110, Method of Testing Performance of Laboratory Hoods, (1995).

5. BIOLOGICAL SAFETY CABINETS

When properly maintained and used in conjunction with good laboratory techniques, biological safety cabinets (BSCs) provide effective primary containment for work with zoonotic pathogens. In AP containment level 2 facilities, BSCs are used for procedures with potential for producing infectious aerosols and with high concentrations or large volumes of infectious material. In AP containment level 3, all open vessel activities with zoonotic materials are conducted in a biological safety cabinet. In AP containment level 4, all activities with microorganisms are conducted in a BSC in order to minimize contamination of the lab. Every employee working in a BSC must be trained in its correct use and have a good understanding of the different types of cabinets and how they work.

Detailed information on the selection, function and use of BSCs can be found in the Centers for Disease Control/National Institutes of Health Primary Containment for Biohazards: Selection, Installation and Use of Biological Safety Cabinets (1995).

5.1 CLASSES OF BIOLOGICAL SAFETY CABINETS

There are three classes of biological safety cabinets: Class I, Class II and Class III. Class I cabinets have unrecirculated air flow away from the operator that is discharged through a HEPA filter. Class I cabinets provide good operator protection but do not protect the material within the cabinet (the product) from contamination. Class II cabinets have inward air flow for personnel protection, downward HEPA-filtered air for product protection and HEPA-filtered exhaust air for environmental protection. They are divided into two types (A and B) based on construction, air flow velocities and patterns, and exhaust systems. Class III cabinets are totally enclosed and gas-tight with HEPA filtered supply and exhaust air. Work is performed with attached long-sleeved gloves. Class III cabinets protect the worker and the product.

Note: Horizontal clean benches which direct air towards the operator are not biological safety cabinets and must not be used for handling infectious, toxic or sensitizing materials.

Selection of the proper class of BSC requires careful evaluation of the work involved. Class II (i.e. Type A and Type B) cabinets are designed for work involving microorganisms in AP containment level 2, 3 and 4 facilities. Cabinet air from Type A cabinets may be recirculated back into the laboratory in level 2 and 3 facilities. Ducting a Type A cabinet out of the building is possible, providing the method of ducting uses a "thimble" connection (i.e. a small opening around the cabinet exhaust filter housing) and the balance of the cabinet exhaust system is not disturbed. The thimble must be removeable or be designed to allow for proper certification of the cabinet (i.e. bubble tight damper to seal off the cabinet for decontamination, access port to allow scan testing of the HEPA filter).

Class II, Type B cabinets can be used when manipulating small quantities of chemicals as an adjunct to work with microorganisms in AP containment level 2, 3 and 4 laboratories. Type B1 cabinets recirculate 30% of the air within the cabinet and are suitable for work with minute amounts of chemicals and radionuclides. These cabinets must be "hard-ducted" (i.e. direct connection), preferably to their own dedicated exhaust system. The exhaust canopy must allow for proper BSC certification. Type B2 cabinets are total-exhaust cabinets with no air recirculation within them and are suitable for work with small amounts of volatile chemicals and radionuclides. These cabinets are also hard-ducted. The Type B3 cabinet is similar to a ducted Type A cabinet (also referred to as Type A/B3) with negative air pressure plenums. Type B3 cabinets are not recommended for work with volatile chemicals as recirculation of 70% of air can cause a build-up of chemical vapours in the cabinet.

Class III cabinets are designed for work with HC level 4 pathogens and have traditionally been installed in maximum containment laboratories. Cabinet lines consisting of several class III cabinets (e.g. for centrifuges, incubators, refrigerators) and transfer devices joined together must be custom built. Caution should be excercised for their use in level 4 laboratories as they require specialized laboratory support systems (e.g. transfer devices, HVAC, emergency power, controlled access). Class III cabinets may also have a role in field applications where full supporting structures are not necessarily needed.

Selection of Class II Biological Safety Cabinets

Note: Only cabinets which meet the National Sanitation Foundation (NSF) Standard No. 49 Class II Biohazard Cabinetry (1992) (design, materials and construction specifications for BSCs) and bear an NSF 49 seal are to be purchased.

5.2 INSTALLATION AND CERTIFICATION

The air curtain at the front of the cabinet is fragile and can easily be disrupted by people walking parallel to it, by open windows, air supply registers, or laboratory equipment that creates air movement (e.g. vacuum pumps, centrifuges). The following outline recommendations for BSC placement:

• BSCs to be located away from high traffic areas, doors and air supply/exhaust ducts that may interrupt air flow patterns (generally, the same principles outlined for fume hood locations should apply)

• minimum clearance of 30cm to be provided between exhaust outlet on top of cabinet and any overhead obstructions

• whenever possible, a 30cm clearance to be provided on each side of the cabinet to allow for access

• for ducted cabinets, blowers on the exhaust system should be located at the terminal end of the ductwork; failure of exhaust flow should signal an alarm to the user; to prevent pressurization of the cabinet, an interlock system should be installed to prevent the cabinet blower from operating whenever the exhaust flow is insufficient (e.g. flow/electrical control); an anti-backflow device to prevent reverse airflow through the HEPA filter may be required

Continuous operation of BSCs helps to control dust levels and other airborne particulates in the laboratory. Operating BSCs only when needed to conserve energy must consider the balancing of laboratory room air. In some cases, room exhaust is balanced to include the air exhausted through ducted BSCs and these cabinets must not be turned off.

The provision of propane gas to BSCs is not generally recommended. Open flames in the BSC create turbulence, disrupt air flow patterns and can damage the HEPA filter. When suitable alternatives (e.g. disposable sterile loops, micro-incinerators) are not possible, touch-plate microburners with a pilot light to provide a flame on demand may be used.

The correct operation of BSCs must be verified before they are used, after any repairs or relocation. Moving a cabinet can cause damage to the HEPA filter and it's seals. Testing must be performed by qualified individuals and it is recommended that only NSF accredited field certifiers be used. The following outline requirements for BSC certification:

• BSCs to be certified by NSF accredited field certifiers in accordance with the CSA Standard Z316.3-95 Biological Containment Cabinets: Installation and Field Testing (1995) at least annually and whenever the cabinet has been moved or serviced

• a copy of the certification report must be provided to the user and kept on file (ideally, a pocket containing the report should be affixed to the cabinet exterior)

• a label indicating the date of certification, the date of the next certification, to what standard the tests were performed and the name of the certifier must be affixed to the exterior of the cabinet