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FUM3.2 - Fume cupboards and safety cabinets

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FUM3.2 - Fume cupboards and safety cabinets

Legal requirements

What are they?
Essentially, fume cupboards, carbon-filtered cabinets and Class I and II biological safety cabinets are open-fronted units. They work by sucking air from the laboratory, past the worker standing in front of the unit, over the working surface where the airborne contaminants are likely to be generated and pushing the air into the environment (back into the laboratory or outside the building depending on the type of unit).They are designed to remove airborne contaminants (be they gases, vapours, fumes, smells, dusts or aerosols) from the working environment before they can be breathed in by workers or their colleagues. They should also be capable of greatly reducing the concentration of any dangerous components before the air is vented to the environment.

They are not;
They are not capable of containing 100% of the material generated within the units themselves - some may escape through the open front. Only units where the exhaust air is filtered have the capability to remove (or significantly reduce the concentration of) the components before the air is vented to the environment.

Operator protection factors

• Open-fronted cabinets can never guarantee to prevent all airborne material from escaping into the laboratory. For general purpose work the minimum average air velocity should be 0.5 m/s at the working face.
• They should be capable of providing a minimum measurable performance under standardised conditions. With biological safety cabinets there is a test procedure in BS 5726 involving the release of a potassium iodide aerosol inside the cabinet and detectors positioned outside the front opening. The "KI-Discus" test should indicate that no more than 1 particle in every 100,000 released inside the cabinet escapes to be detected.
• Thus the minimum "operator protection factor" (OPF) for Class I and II biological safety cabinets is given as 10⁵. The test can be used in fume cupboards and carbon-filtered units but has not yet been adopted in their standards. There is a containment test involving sulphur hexafluoride for fume cupboards and results from this have been reported as comparable to those generated by the "KiDiscus" test.
• Operator protection will be reduced if the air flow falls below, or rises above, certain values. This can happen;
  • if the working surface is cluttered, 
  • if rapid movements are made across the open face of the cabinet, 
  • if large volumes of gases/vapours are released into the cabinet or 
  • if the units are not maintained adequately.

Checks and maintenance considerations

• Under the COSHH Regulations all facilities provided to control exposure to hazardous substances must be tested at least once every 14 months and maintained in good order.
• The University Estates department maintain the ducted fume cupboards.
• Our Department ensures the carbon-filtered units and biological safety cabinets are checked twice a year by an external contractor.
• Regular checks are made to ensure the average face velocity on these units is adequate.
• Carbon filters can be checked when necessary by using an electronic vapour detector.
• HEPA filters are checked by a smoke of vegetable oil (the test used to be performed with DOP (di-iso octyl phthalate)). This is performed by the contractor during the 6-monthly checks.
• Biological safety cabinets used for bio-containment should becontainment- tested at least once every 14 months (COSHH) by contractors using the KI-Discus test.

Air make-up

• With ducted units that withdraw volumes of air from laboratories the need to make up such volumes should have been addressed at the design stage of the building. In Building 4 South such considerations were made. In some buildings (but not 4S) the heat used to warm the lab air is reclaimed before the air is exhausted to the environment. Without such reclamation of heat the financial cost of lost energy is significant.
• When the fire alarm in Building 4S is activated the entire air-handling system is shut down (to reduce the spread of smoke and flames). As the fume cupboard air extract system is an integral part of the building’s air-handling arrangement, fume cupboards will stop functioning in the event of a fire alarm (but not a scheduled practice). When the alarms are reset the fume cupboards start up again.

Ducted units (fume cupboards and safety cabinets) must have the exhaust fans sited as far away from the cabinets as practicable. This is usually on the roof of the building. This ensures that if the ducting develops a leak then air will be sucked into the duct (under negative pressure). If the ducting was under positive pressure any air in the duct would leak out into the building along with possibly dangerous contaminants.

Scafftags
All fume cupboards, carbon-filtered cabinets and biological safety cabinets in the Department have a "Scafftag" holder stuck to them. These hold a green-coloured insert to indicate the unit can be used. If, for any reason, the green insert (or the holder) has been removed DO NOT USE THE UNIT. 

Ducted Fume Cupboards (refer also to UoB SM section 4.2.5).
They draw air from the laboratory and over the work being performed in the cupboard, carrying gases, vapours, fumes and dusts away from the work environment through a duct to the roof of the building. There a fan blows the air up through a chimney and discharges it at high velocity 3 metres above the roof. They are designed to eliminate or greatly reduce the risk from chemical and dust hazards generated by your work. All the 8 ducted fume cupboards (also called fume hoods) in Building 4 South are designed to conform to the standards detailed in the British Standard BS7258:1994.
Because fume cupboards are ducted they are fixed in position and, for all practical purposes, cannot be moved.

Scrubbers
All the 4S fume cupboards have water-spray scrubbers fitted. These can be effective at removing dusts, smoke, acid fumes and water-miscible organic solvent vapours. They are operated either by pressing the SCRUBBER START membrane key or switching on the SCRUBBER switch in the refurbished units (in 1.28, 1.37, 1.37A and 1.48). (New fume cupboards in building 5 West are equipped with carbon filters. In 1 South (the Chemistry research building) the exhaust from the fume cupboards is not filtered, nor is any heat reclaimed from it.)

Relative positions of water spray scrubbers at rear of fume cupboards. 
(courtesy of Dynamic Plastics Ltd..)

Points to be aware of;

• The fans are on all the time and cannot be switched off by lab workers.
• They will be shut down in the event of a fire alarm (not a scheduled fire alarm practice).
• If the fume cupboard alarm sounds do not use it. It means the airflow has fallen below an acceptable level because either the fan has failed or the sash has been raised above its maximum recommended working height. If the alarm sounds when the sash is lowered please contact either Ray Dickson or Chris Davey.
• Most of the cupboards (i.e. all except those in 0.58, 0.76, 1.28 and 1.48) have a scrubber liquor sampling tap (green with RED and blue inner) on the left side of the fascia and a drain cup (on the left of the work surface) leading to the scrubber liquor tank. The tap does NOT supply cold water and the drain cup must NOT be used as a waste drain.

Carbon Filter cabinets

What they are not !!
They are not conventional fume cupboards and certainly not biological safety cabinets. (Even they are not necessarily guaranteed to be 100% effective at protecting the operator (or the work)). They are not laminar flow work stations. The accepted British standard for manufacture and performance of these units is BS7989:2001

What are they ?
Essentially mobile units that draw air from the lab over the work, through a dust filter, a carbon filter (and/or possibly a HEPA filter) and exhaust the air back into the laboratory. They are designed to DRAMATICALLY REDUCE the airborne concentration of a range of chemical vapours, fumes, smells and dusts in air to acceptable levels. They can mop up low levels of some vapours and fumes very efficiently, but they might not cope well with gross spillages or boiling off solvents or acids. They are cheaper than conventional ducted fume cupboards BUT THEY ARE NOT A SUBSTITUTE FOR THEM. There are prefilters on all the units to take out 99% of dusts, mists and aerosols in the range 0.5 micron to 2 micron (and hold them there when the fans are switched off). (The HEPA filter on the powder weighing unit (in lab 1.52) is designed to take out more than 99.997% of particles of 0.2 micron size.)

Carbon Filters.
Activated carbon will adsorb any organic chemical with a molecular weight above 30 and a boiling point above 60^'C. However, not all chemicals are absorbed with the same tenacity. The carbon filters can be specifically tailored to give several types for adsorbing particular substances more effectively. The 14Kg filter block can adsorb nearly half its own weight of some solvents and the filtration efficiency can approach 100%. The filter beds can be made with a combination of different types of carbon (e.g. ACI and SUL). Refer to this table for the optimum type of filter for a particular compound.To achieve optimum filtration efficiency and capacity, important design criteria must be met by the filtration equipment. The air must take at least 0.3 seconds ("dwell" time) to pass through the carbon.

How do they work ?

• Remember operator protection factors for fume cupboards and biological safety cabinets?
• Average face velocity required to capture most releases of low hazard material is generally regarded to be 0.5 metres/second.
• Prefilter for large dust particles. Works by strong electrostatic forces generated by the fibres of the "Filtrete" prefilter.
•  Main carbon filter dwell time is at least 0.3 seconds, the air flows through it at a rate of 0.3m/s
•  Filtration takes place in an active filter zone
•  Air is exhausted to lab (can also be ducted)
•  Carbon filter efficiency will fail rapidly

What you can do in them.
Essentially, work activities (other than simply smelly work) performed in these units must be assessed with the Department’s risk assessment scheme.

• Use of most "smelly" chemicals.
• General dispensing of most organic solvents and acids that you are likely to use.
• The powder weighing unit in SB 1.52 can be used for weighing of powders in still air followed by purging.

What you must not do in them.

• Do not boil or evaporate large volumes of organic solvents or acids.
• Do not work with highly toxic materials.
• Do not work with pathogenic micro-organisms or genetically modified organisms.
• Do not leave any spillages in the units.

How do you know they are working ?

• Green Scafftag insert should be displayed.
• Manometer (if fitted) should indicate a good seal around the filter, adequate air velocity across the working face and also when the prefilter is getting clogged with dust. The dust filter efficiency will fail rapidly.
• General absence of smell (in laboratory) of compounds in use in cabinet.

How is performance tested ?
Users can check the manometer on a daily basis. 6-monthlycheck on face velocity by Department staff. 6-monthly check of carbon filters (and HEPA filter in powder weighing unit) by contractor.

Table to determine optimum carbon filter type to be used to adsorb specific chemical vapours.
The range of materials capable of being filtered by carbon filters can be viewed on this web site; http://www.aaceurovent.co.uk/mediaindex.htm
Chemicals marked * are poorly adsorbed by all filters and should be used only in small quantities. However, their exhaust concentrations will be considerably lower than the input concentration due to retardation in the filter matrix (chromatography effect).
The information below was supplied by Astec-Microflow of Weston-Super-Mare.

Biological Safety Cabinets (refer also to UoB SM section 4.2.7)
For a useful review "Appraisal of the practical effectiveness of biosafety controls in biotechnology with special reference to safety cabinets" by Ray Clark refer to https://tspace.library.utoronto.ca/retrieve/273/by95003.pdf (BioSafety Volume 1, Paper 3 (BY95003), May 5th 1995).
Biological safety cabinets are designed to conform to British Standard BS5726 and are validated to BS EN 12469:2000 (including containment using KI-discus equipment).

Bio-exclusion or bio-containment?
For work involving hazardous biological agents these safety cabinets are used for bio-containment - they should contain the hazard from escaping into the environment. Cabinets can be tested for containment using the "KI-Discus" test. The minimum acceptable containment standard is to provide an operator protection factor of 10E5.
For non-hazardous tissue culture work they are used for bio-exclusion - to minimise the chance of contaminating the work in progress by excluding viable organisms from the laboratory air.
For either mode, HEPA filters and carefully controlled laminar air flows are utilised to effect both.

Recirculating biological safety cabinets use HEPA filters to clean air of aerosols and other fine particles but will dump any smells generated by the work into the laboratory.

HEPA (High Efficiency Particulate Air) filters can be rendered useless if they become wet or if they are physically damaged. The only way to test their integrity is to challenge them with an oil smoke and test the effluent air with a sensitive spectrophotometer. The Department does not have the equipment to do this.
HEPA filters should be protected from gross dust blockage by incorporating a prefilter. This should be cleaned or replaced on a regular basis.
They draw air over the work being performed in the cabinet, through a HEPA filter to be discharged either back into the laboratory or outside the building.
They are designed to eliminate or greatly reduce the risk from microbiological hazards generated by your work.
If safety cabinets are being used with pathogenic micro-organisms then arrangements must be made to decontaminate the units before any servicing or repair work is carried out and before they are moved. Formaldehyde fumigating units can be provided by manufacturers. Such units are in place on the cabinets in labs 0.47b and 1.14.
If a gas supply is required in safety cabinets then an electrically-operated solenoid valve should be fitted as on the cabinets in labs 0.43b, 1.14, 1.28b, 1.37b (MDH) and 1.45a.
Class I safety cabinets protect workers and the work environment from microbiological hazards (OPF>10E⁵). They do not protect the work from contamination. If the exhaust is double-HEPA filtered then the exhaust can be vented back into the laboratory. There is a double-HEPA filtered Class I safety cabinet in lab 1.52.
Class II safety cabinets protect both the work, workers (OPF>10E⁵) and the environment. The work area is bathed with a vertical flow of HEPA-filtered air. Air is also drawn, from the lab, downwards through the front opening of the cabinet. Underneath the perforated work surface the air from the lab is mixed with air passing over the work. That is then drawn through a dust pre-filter and a fan and about 20% of the air is HEPA filtered and vented either outside the building or back into the lab (if double-HEPA filtered). Older Class II cabinets (such as most of ours) may vent air back into the lab after only a single HEPA filtration. The remaining 80% is used to bathe the work area. There are Class II safety cabinets in labs 0.43b, 0.47a, 0.70 (2), 0.71, 1.14, 1.37b and 1.45a although not all are used for containment of pathogenic organisms.
Class III safety cabinets are totally enclosed and workers perform their work using glove ports. They are used for work with highly pathogenic organisms. The chamber is maintained at a negative pressure, air enters via a HEPA filter, materials can only enter the cabinet through an air-lock and the exhaust air is HEPA filtered before discharging outside the building (never back into the lab).
We do not have any Class III cabinets in the Department.

Microbiological safety cabinets (from Dr R.P. Clark and Science Reviews Ltd.)

Performance is tested by;

• Checking face velocity of Class I and II cabinets.
• Challenging HEPA filters with oil smoke.
• Checking operator protection factor in Class I and II cabinets used for bio-containment using the "KI-Discus" test.
• Checking integrity of ducting at least once every 14 months.

KI-Discus test being performed on a Class II biological safety cabinet (this one failed)

Tissue culture cabinets
These may be designed along the lines of, or may be identical to, Class II biological safety cabinets. However, if they are

•  not to be used with hazardous micro-organisms and are
• solely to be used for non-GM tissue culture workthen they should be;
• derogated and classified and labelled, as "Tissue culture cabinets. Not for use with hazardous micro-organisms or GMOs"
• and any biohazard warning sign removed.

In this way there should be no confusion about conducting hazardous work in them.Two such cabinets are in 1.28b and 1.37b.

Laminar Flow Work Stations

Never confuse these laminar flow work stations (LFWS) with cabinets designed to protect the worker from hazardous airborne contaminants. LFWS operate by blowing HEPA filtered air over the work surface. Horizontal flow cabinets blow the air onto the worker and into the lab, thus if any hazards are generated in the cabinet they will be deposited onto the worker and into the lab air! They can be used for aseptic manipulations and for working on non-sporulating and non-hazardous biological materials.

Laminar flow work stations (not safety cabinets). (These blow air over the work, the operator and into the laboratory).

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