Fume cupboards


The Control of Substances Hazardous to Health (COSHH) 2002 regulations impose a duty on employers to manage the exposure of their employees to hazardous substances. Where it is not reasonably practicable to prevent exposure, control measures must be employed to reduce it to an appropriate level. In the laboratory, fume cupboards are a significant means of controlling exposure to hazardous substances, and the main type of local exhaust ventilation (LEV) used under the COSHH Regulations. By providing partial containment, they may provide protection for both users and co-workers from the hazardous effects of gases, vapours, aerosols and particulates. Because they only provide partial containment and because a number of factors affect their performance, there may be circumstances where a fume cupboard does not provide adequate protection. This policy aims to provide workers with the information to make the risk assessments needed to select a suitable fume cupboard for their work and to use it safely. It also describes the University’s policy for maintenance and testing.

2. Types of fume cupboard

a              Ducted fume cupboards

This is by far the most common type of fume cupboard and a wide range of ages and types may be found in the University. Some specialist types are available, e.g. walk-in fume cupboards, or those fitted with water wash-down, but they all function by drawing laboratory air into the fume cupboard, thus containing and diluting the contents before discharging them to the environment, usually without filtration, three metres above roof level.

There are several types of ducted fume cupboard:

i           Constant Air Volume (CAV)

These units are designed to maintain a constant air extract volume regardless of the sash position. Face velocities will vary depending on where the sash is positioned and will increase as the sash is lowered. Air bypass openings situated above the sash ensure that changes in face velocity are kept within a specified range.

ii           Variable Air Volume (VAV)

These units use sash positioning controls to vary the fume cupboard fan speeds and subsequently alter the air extract volume. The extract volume varies depending on where the sash is positioned and allows face velocities to remain constant, at a predetermined level. The fume cupboard extract may be linked to building ventilation systems to enable extract and supply air to be balanced.

iii          ‘Low flow’ fume cupboards

Many fume cupboard manufacturers now offer units that are ‘dynamic low flow’, that is the internal design provides full containment of hazardous substances at much lower face velocities than older conventional models. These units are increasingly attractive from an energy conservation perspective due to the need for reduced make-up air, reduced sizing of extract ducts and fans and therefore much lower running costs.

The majority of existing fume cupboard installations in the University are of the types described in (i) and (ii), above.

b              Recirculatory filtration fume cupboards

These may appear superficially attractive as alternatives to ducted fume cupboards, largely because of their significantly lower initial costs. They may also be considered where the installation of ductwork for conventional fume cupboards is impossible, or where insufficient make-up air is available to accommodate ducted fume cupboards. They operate by drawing air into the fume cupboard and exhausting it through a set of filters (usually some type of activated charcoal) back into the laboratory.

3. Selection

A suitable risk assessment must be made in order to select an appropriate type of fume cupboard.

a          Ducted fume cupboards

Ducted fume cupboards are suitable for most applications, though consideration may need to be given to design and to the materials of construction. For instance, where perchlorates are used, then there must be no wood or other absorbent surfaces present; water wash-down facilities may need to be provided in the ductwork. Where significant amounts of hydrofluoric acid are handled, then plastic sash windows should be fitted, to avoid the loss of visibility by etching which will occur if glass is used.

b          Recirculatory filtration fume cupboards

Recirculatory filtration fume cupboards are not generally suitable for use with hazardous materials and should only be used for the control of nuisance odours, for small quantities of solvents etc. The University Safety Office should be consulted whenever their use with hazardous materials is proposed.

There are a number of features of recirculatory filtration fume cupboards that make them unsuitable for general use.

i               The air flow rate must be high enough to provide containment, but in order to produce good filtration it must be low enough to provide adequate residence time on the filter(s). The cupboard must have a warning/alarm for low air flow.

ii              The correct choice of adsorbent in the filter(s) is essential; no single filter can cater for all uses. The cupboard should have a filter saturation warning/alarm.

iii             Adequate laboratory ventilation (8 - 12 air changes per hour) must be provided in order to prevent the accumulation of any contaminants which may not be fully retained by the filtration system.

iv             Some system of monitoring the quality of the filtered and exhausted air is necessary if they are used with hazardous materials.

v              Saturated filters will require changing without exposing users to hazardous substances and will require safe disposal as hazardous waste.

Recirculatory fume cupboards may be suitable for some low risk work (e.g. dispensing small quantities of solvents or weighing hazardous chemicals), but they are not a substitute for ducted fume cupboards.

In particular, they are not suitable for

i           work with carcinogens or suspected carcinogens

ii           work with substances not effectively trapped by the filter(s)

iii          work with gases or vapours which are odourless, or which have odours detectable only at concentrations greater than the Workplace Exposure Limit

iv         work involving large quantities of substance (e.g. boiling off solvents)

v          work where sources of heat must be used

vi         use in poorly ventilated laboratories.

A risk assessment is essential before recirculatory filtration fume cupboards may be used. They are not suitable for use in situations where use cannot be effectively controlled, e.g. where multiple users frequently change procedures or where inexperienced workers operate. In cases where a recirculatory fume cupboard is used supervisors should take steps to ensure that adequate supervision and training is provided to prevent work for which the filters are unsuitable, or work where they may become saturated.

b              Other considerations

Note that neither type of fume cupboard is suitable for protection against microbiological hazards, in which case microbiological safety cabinets (MSCs) must be used. MSCs function by containing and capturing hazardous micro-organisms on high-efficiency particulate filters before discharging the filtered air to the environment or to the laboratory. Although the recirculatory type of fume cupboard may appear to be similar, it does not perform the same function. The construction, installation and maintenance of microbiological safety cabinets must conform to the current British Standard and further details may be found in University Policy Statement S5/09 (Biorisk Management).

Where protection must be provided against both chemical and microbiological hazards, then advice must be obtained from the University Safety Office before work commences. Advice on the selection of fume cupboards for work with radioactive materials must be obtained from the University Radiation Protection Officer.

4. Fume cupboard specification

a          Ducted fume cupboards

The design, construction, siting and commissioning of all new ducted fume cupboards should conform to the relevant standard, currently BS EN 14175:2004.

They should be:

  • type tested in accordance with BS EN 14175 - part 3
  • commissioned after installation in accordance with BS EN 14175 - part 4, with a full commissioning test report.

In addition, face velocity measurements should be undertaken during commissioning. The key attribute of a fume cupboard is its ability to provide good containment of hazardous materials. Although it was previously thought that there was a simple relationship between containment and face velocity, this is no longer accepted, especially for modern fume cupboards. However, once the fume cupboard has been installed, face velocity will be the only easily available relative measure of performance.

In certain circumstances the Safety Office may stipulate additional tests during the commissioning period, for example air flow visualisation, containment and robustness of containment tests.

Older conventional fume cupboards should normally operate with a face velocity of not less than 0.5 ms-1. Modern aerodynamic designs can successfully contain hazardous substances at face velocities less than 0.5 ms-1. These “low-flow” fume cupboards may be used in new installations provided they operate at a face velocity of not less than 0.4 ms-1. No reduction may be made to the design face velocity of existing fume cupboards without the approval of the Safety Office.

b          Recirculatory fume cupboards

Recirculatory fume cupboards must conform to BS 7989: 2001.

5. Installation

No new fume cupboard may be installed and no alterations to fume cupboard ductwork may be made without the approval of the University Safety Office and Estates Services. This will ensure that all relevant factors are properly considered, e.g. compatibility with existing plant or other installations, compliance with British Standards and circumstances that preclude the use of recirculatory fume cupboards or low face velocities.

Fume cupboards must be carefully situated to avoid disturbance from draughts. BS EN 14175 - part 5 contains recommendations on siting, including diagrams showing the setting-out distances from walls, doors, other fume cupboards, benching and circulation space. New installations must comply with this standard.


6. Services

Fume cupboards should be fitted with a water supply and a sink. Controls for services should be outside of the fume cupboard, as should electrical socket outlets. Adequate lighting must be provided, with fittings accessible from outside of the fume cupboard. Appropriate earthing must be provided for metal surfaces or fixtures.

7. Maintenance and testing

a          Ducted fume cupboards

Fume cupboards must be maintained within specified performance parameters which are confirmed at installation and recorded during commissioning.

The Estates Regulations set out the relative responsibilities of departments and the Buildings and Estates Sub-Committee (BESC) in respect of fume cupboards. Generally, departments are responsible for the provision and maintenance of the fume cupboard itself, including controllers, alarm systems, sash mechanisms and stops, and BESC is responsible for the provision and maintenance of the associated fans and ductwork.

Work on fans or ductwork may not start until the department has fully assessed the risk from harmful contaminants in the ductwork. Contamination is most likely where particulate or condensable materials have been used and in these circumstances the department should keep a record of the substances used and their quantities.

Departments should ensure that a face velocity measurement is made as part of the commissioning procedure for all new fume cupboards, as described in 4(a), so it may be used in future tests to check whether any deterioration in performance may have occurred.

i            For work with hazardous substances in the majority of the University’s fume cupboards, the tested face velocity with the sash set to 500mm height should be 0.5 ms-1 ± 10%. Any test result indicating a greater than 10% reduction in face velocity (i.e. < 0.45 ms-1) will require further investigation and remediation. In such cases supervisors must ensure that the relevant risk assessments are reviewed to exclude higher risk activities where exposure is possible, due to the reduced containment.

ii           Where the fume cupboard is solely used for the storage of chemicals a face velocity of 0.25 ms-1 is considered acceptable. However, a reduction in the face velocity greater than 10% will also require further action.

iii          Where fume cupboards are designed to operate at face velocities of less than 0.5 ms-1 then the tested face velocity should be 0.4 ms-1 ± 10% with the sash set to 500mm height. Any test result indicating a greater than 10% reduction in face velocity will require further investigation and remediation. In such cases supervisors must ensure that the relevant risk assessments are reviewed to exclude higher risk activities where exposure is possible, due to the reduced containment.

iv         Work with radioactive materials should be performed within a designated, conventional, ducted fume cupboard that has a minimum face velocity of 0.5 ms-1. In this case a 10% reduction in performance will require a cessation of work until full investigation is undertaken and further advice is obtained from the University’s Radiation Protection Officer.

Estates Services will arrange for specialist contractors to carry out annual measurements of the face velocities of ducted fume cupboards, but the head of department is responsible for ensuring that these tests have been performed on all the relevant installations. These tests ensure that the department complies with the requirement of the COSHH Regulations for annual testing of LEV systems. The head of each department is responsible for ensuring that Estates Services’ information is up to date, i.e. that they are notified of all new ducted fume cupboard installations or removals.

Estates Services sends the test results to the department and to the University Safety Office; under the COSHH Regulations they must be kept for a minimum of five years. For the users’ information, test results are also displayed on a label affixed to the fume cupboard, along with a label denoting the sash height at which the fume cupboard meets its performance specification. Risk assessment should show whether certain specialised uses require a higher face velocity. This is acceptable provided that it can be achieved without causing turbulence, which may reduce containment.

b          Recirculatory filtration fume cupboards

Departments must make their own arrangements with a competent contractor for testing the performance of recirculatory filtration fume cupboards. Departments must ensure that a maintenance agreement with a competent engineering company is in place that includes 14 monthly inspection and test, and a test of filter integrity.

When these devices are used as a control measure under COSHH, then a record of the performance tests must be kept for a minimum of five years. Filters must be changed when blocked or saturated (as indicated by any built-in alarm systems), or at any other interval specified by the filter supplier. Filters contaminated with hazardous materials must be regarded as chemical waste and disposed of via the University Safety Office.

8. Use

Before any work is started, a COSHH assessment should have determined whether a fume cupboard is appropriate for the task. For work with highly toxic material, better containment (e.g. a glove box or other fully enclosed apparatus) may be necessary.

The performance of any fume cupboard can be severely degraded by incorrect use, in particular by anything that disturbs the laminar flow of air into the enclosure. BS EN 14175 - part 5 contains detailed information on installation arrangements which will avoid such disturbance. All new installations and refurbishments must comply with this standard.

Any of the following can cause interference to airflow and cause fumes from within the enclosure to enter the worker’s breathing zone:

a          External draughts (caused by the user’s sudden movements, by people walking quickly past the front of the fume cupboard, by opening doors situated too close to it, by air conditioning units or by other fans).

b          The use of naked flames, hot air guns, ovens, hotplates, fans or centrifuges, all of which may cause turbulence.

c          Large items (e.g equipment, Winchesters or waste containers) covering much of the available workspace, or  placed too close to the front opening or too close to the back baffle.

a              The use of screens for protection against ionising radiation or explosion.

All users should note the following points:

a              Most fume cupboards are designed for use by only one person at a time. Before starting work, users should check that their fume cupboard is fully operational (i.e. that the sash works properly and that airflow is present).

b              Fume-generating apparatus should be placed at least 150mm behind the sash to ensure proper entrainment of fumes; but it should not be so far back that the user has to lean into the fume cupboard. If the use of large items or screens is unavoidable, raising them about 50mm on blocks so that the air can flow underneath will considerably improve airflow.

c              The back baffle should be kept clear of obstructions (e.g. bottles, equipment, or tissues) which could obstruct the airflow.

d              The sash should be kept closed as much as possible in order to maximise containment. In any event, during the experiment it should not be raised above the position at which the face velocity was measured (indicated by a label affixed to the fume cupboard). Before raising the sash at the end of an experiment, the fan should be allowed to run for a while to clear any fumes.

e              Fume cupboards are working areas, so those used for experimental work should not also be used for the storage of chemicals or apparatus. Besides the effect on airflow described above, there is the possibility that a minor incident could involve these stored materials and escalate into a more serious one.

f               Care should be taken in disposing of chemical wastes via fume cupboard sinks. Their drains connect with the normal building drains and nothing unsuitable for disposal via ordinary laboratory sinks may be put down fume cupboard sinks.

g              Particular care should be taken in the event of spillages inside fume cupboards. If the material spilt is unsuitable for drain disposal via laboratory sinks then it must not be washed down a fume cupboard sink.



October 2013 

J Black