Fume cupboards


1. Introduction

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: 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.  New installations must comply with the current British Standard, presently BS 7258:1994: Laboratory Fume Cupboards; and also with the University’s own standards and specifications for fume cupboards.  The latter contain certain design and installation requirements which in some cases exceed the British Standard (for information, these documents are available in the Safety Office library).  In order to ensure compliance with these standards and to identify any other relevant issues (e.g. planning concerns or potential conflicts with other projects), new fume cupboard installations must have the approval of the University Safety Office and the University Surveyor’s Office.

(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.

There is no British Standard for recirculatory filtration fume cupboards, although a draft for comment was issued early in 2000.  Consequently, fume cupboards from different manufacturers may differ significantly in performance.  Prospective buyers should make enquiries to ensure that any new purchase will meet the requirements of the proposed British Standard.

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 with face velocities of at least 0.5 m.s-1 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).

(ii)  The correct choice of adsorbent in the filter(s) is essential; no single filter can cater for all uses.

(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.

They are suitable for some low risk work (e.g. dispensing small quantities of solvents, 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 Occupational Exposure Standard

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

(v)  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.

(c)  Other considerations

Note that neither type of fume cupboard is suitable for protection against microbiological hazards, where microbiological safety cabinets must be used.  The latter 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 is required to conform to a British Standard and further details may be found in UPS S5/09 (Biological Health and Safety).

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.

(d)  Containment and face velocity

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.

University policy requires that fume cupboard face velocities must be a minimum of 0.5 m.s-1 if the fume cupboard is to be used for work with hazardous materials.  A lower standard of 0.25 m.s-1 is acceptable if it is to be used solely for the storage of hazardous materials.  (BS 7258 does not specify a face velocity figure for adequate performance, though it suggests that face velocities of less than 0.3 m.s-1 are unlikely to give satisfactory containment and that 0.5 m.s-1 or above may be needed).

4. 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.

Provision of services is fully covered in the University’s specifications for fume cupboards, referred to in 2 (a) above.

5. Maintenance and testing

(a)  Ducted fume cupboards

The Grey Book (Standing Orders for Works in University Departments) sets out the responsibilities of departments and the Buildings and Estates Sub-Committee (BESC) in respect of fume cupboards.  Generally, heads are responsible for the provision and maintenance of the fume cupboard itself 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 new fume cupboards, so it may be used in future tests to check whether any deterioration in performance may have occurred.  Once the fume cupboard has been installed, face velocity will be the only easily available relative measure of performance.  The University Surveyor’s staff carry out annual measurements of the face velocities of ducted fume cupboards, but the head of department is responsible for ensuring that these tests are performed on all the relevant installations.  The head of department is responsible for ensuring that the Surveyor’s information is up to date, so arrangements must be made to ensure that his office is notified of new ducted fume cupboard installations.  These tests ensure that the department complies with the requirement of the COSHH Regulations for annual testing of LEV systems.

The Surveyor 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, they 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.  For general use, a fume cupboard must have a face velocity of at least 0.5 m.s-1 with a sash opening of 0.5m.  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 and reducing containment.  Where a fume cupboard is used for storage only, a standard of 0.25 m.s-1 will suffice.

(b)  Recirculatory filtration fume cupboards

Departments must make their own arrangements (usually with the supplier or manufacturer) for testing the performance of recirculatory filtration fume cupboards.  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.

6. 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 7258: Part 2: 1994 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 could 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 placed too close to the front opening, or too close to the back baffle

(d)  the use of screens for protection against ionising radiation or explosion

The following points should be noted:

(a)  Fume cupboards must be carefully situated to avoid disturbance from draughts.  BS 7258: Part 2: 1994 contains detailed 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.

(b)  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).

(c)  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.

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

(e)  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.

(f)  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.

(g)  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.

(h)  Particular care should be taken in the event of spillages inside fume cupboards.  If the material spilt is unsuitable for drain disposal (as defined in (g) above), then it must not be simply washed down the fume cupboard sink.

THIS STATEMENT FORMS PART OF THE UNIVERSITY SAFETY POLICY.  UNIVERSITY GUIDANCE NOTE S8/86 IS WITHDRAWN.  PLEASE AMEND THE INDEX.

May 2001