What is a fume cupboard?
Laboratory fume cupboards serve to control exposure to toxic, offensive or flammable vapors, gases and aerosols. Fume hoods are the primary method of exposure control in the laboratory. The laboratory fume cupboard is a type of local exhaust ventilation system (engineering control). A typical fume cupboard is a cabinet with a moveable front sash (window) made out of safety glass. A properly used and properly functioning fume cupboard exhausts hazardous gases, dusts, mists, and vapors from a confined location and helps protect workers from inhalation exposure.
A typical fume hood has a box like structure with a moveable sash window. Experimental procedures are performed within the hood which is consistently and safely ventilated, usually by means of an extract blower and ductwork.
Fume cupboards prevent exposure to hazardous substances in laboratories, when they are correctly maintained. Safety and maintenance of fume cupboards is incredibly important, and needs to be fully observed at all times.
IMPORTANT COMPONENTS OF A FUME HOOD
- Hood Body: The visible part of the fume hood that serves to contain hazardous gases and vapors.
- Baffles: Moveable partitions used to create slotted openings along the back of the hood body. Baffles keep the airflow uniform across the hood opening, thus eliminating dead spots and optimizing capture efficiency.
- Sash: The sliding “door” to the hood. By using the sash to adjust the front opening, airflow across the hood can be adjusted to the point where capture of contaminants is maximized. Each hood has its optimum sash configuration. The sash should be held in this position when working in the hood and closed completely when the hood is not in use. The sash may be opened completely to set up equipment, but must be returned to the optimum setting prior to generating contaminants inside the hood. Hoods are equipped with one of three types of sashes:
- Vertical: Vertical sashes provide the greatest overall access and are the most common. 2 3 1 4 6 5 7 5
- Horizontal: Horizontal sashes provide the best access to the top interior of the hood, but do not provide access to the entire width at the same time. However, they can be positioned as a body shield to protect the operator, while enabling access through the sides. Z
- Combination vertical and horizontal: Combination sashes combine horizontal sash panes, in a vertical rising frame. As such, they offer the advantages of both vertical and horizontal sashes.
- Airfoil: Located along the bottom and side edges the airfoil streamlines airflow into the hood, preventing the creation of turbulent eddies that can carry vapors out of the hood. The space below the bottom airfoil provides source of room air for the hood to exhaust when the sash is fully closed. Removing the airfoil can cause turbulence and loss of containment.
- Work surface: Generally a laboratory bench top, or the floor in the case of a floor-mounted hood, this is the area where the work is conducted.
- Exhaust plenum: The exhaust plenum helps distribute airflow evenly across the hood face. Materials such as paper towels drawn into the plenum can create turbulence in this part of the hood, resulting in areas of poor airflow and uneven performance.
- Face: The imaginary plane between the bottom of the sash and the work surface. Hood face velocity is measured across this plane.
CORRECT USAGE OF FUME CUPBOARD
It is vital that fume cupboards are used only for experimental work. A fume cupboard should never be used as a storage cupboard for pieces of science equipment. Filling the cupboard can compromise the air flow and increase the chance that harmful substances may leak into the laboratory. The presence of stored chemicals in the cupboard could increase the risks if an accident were to occur.
Likewise, it is important to avoid unnecessary clutter such as safety screens, ovens and trays. Again, this will cause turbulence in the air drawn across the base of the cupboard. The effect of turbulence can be lessened by raising all large objects off the base of the cupboard by placing them on blocks.
Do not set up equipment close to the front edge of the fume cupboard – as this can increase the likelihood of creating turbulent air flow with the stream being drawn towards to front of the cupboard. As a rough guide – all pieces of equipment should be placed at least 150mm from the front of the glass.
It is important to avoid rapid movements in front of the fume cupboard and within the installation. Again, this can disrupt and disturb the air flow – potentially allowing harmful substances to escape.
Furthermore, fume cupboards are not designed for work with micro-organisms. This work and research should be reserved for use with microbiological safety cabinets.
RISKS INVOLVED WITH THE IMPROPER USE OF FUME HOODS
A fume hood is a piece of safety equipment that can be misused to the extent that they can be less effective than expected. Injury from misuse can arise from two causes:
- From the fume hood not providing adequate flow rates for the work required (i.e. sash being left open or from excess clutter that reduces the containment of noxious substances).
- From the hood itself (i.e. if the fan belts are slipping, the exhaust duct has blockage due to paper towels being sucked into the duct, the duct damper is restricted). Always realize the most likely person to be injured is the hood user. Escaping noxious material into the laboratory can also affect all laboratory occupants.
- Power Outages: In case of a power outage, the hood sash should be lowered within an inch to maintain a chimney effect to keep some air flowing into the hood.
- Exhaust: Care should be taken with use of paper products, aluminum foil, and other lightweight materials within a hood. For example, a single paper towel or chemical wipe can potentially decrease the airflow into the hood if it restricts exhaust flow.
It is a legal requirement that all fume cupboards should be maintained and tested at least once every 14 months in accordance with the British Standard. Records of inspections and maintenance must be kept on file, alongside certificates provided by maintenance contractors. A fume cupboard which is not inspected on schedule, or fails the inspection, will be immediately taken out of use.
The contractors will complete the following checks and tests:
- Check the conditions of the services to the fume cupboard and test the functioning of any alarms and all controls
- Complete a face velocity test
- Check the condition of the fan
- Check the duct work, and clean where necessary
- Ensure the make-up air pumped into the laboratory is of a satisfactory standard.
The face velocities (the speed at which air is drawn through the front) of the fume cupboards must also be clearly marked on the exterior of the cupboard.
OBSERVE THE LEGISLATION
There are a number of pieces of legislation which relate to the use of laboratory fume cupboards. These are:
This act explains that it is a legal requirement to ensure the systems of work are safe, providing no risks to health. It continues to state that full training, instruction and information to ensure health and safety are all provided, and that all potential risks are removed.
These regulations have been stated as a control measure to prevent exposure to hazardous substances. It makes it a legal requirement to control exposure to such substances within acceptable limits.
Suitable and sufficient risk assessments must be completed prior to work activity being undertaken, and all subsequent arrangements must be made.
All pieces of work equipment, including fume cupboards, must be maintained to a safe working standard. Sufficient protection must be given to all users of the fume cupboards.
It is vital that all electronics within, and supporting, the fume cupboard are regularly checked and maintained to ensure nothing has corroded.
Laboratory fume hoods only protect users when they are used properly and are working correctly. A fume hood is designed to protect the user and room occupants from exposure to vapors, aerosols, toxic materials, odorous, and other harmful substances.
Fume hoods should be certified, at least annually, to ensure they are operating safely. Typical tests include face velocity measurements, smoke tests and tracer gas containment. Tracer gas containment tests are especially crucial, as studies have shown that face velocity is not a good predictor of fume hood leakage.
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Written by Gabriel Oweh