Local exhaust ventilation systems

1. Introduction

The Control of Substances Hazardous to Health (COSHH) Regulations 2002 state that exposure to hazardous substances must be avoided completely, or adequately controlled where this is not practicable. The Regulations require control measures to follow a strict hierarchy and engineering solutions must be applied for the control of exposure before the use of personal protective equipment is considered.

Local Exhaust Ventilation (LEV) systems can provide a very effective means of exposure control.

In the laboratory the main types of LEV used to control exposure to chemicals and biological agents are fume cupboards and microbiological safety cabinets, respectively. Further information relating to them may be found in University Policy Statements S3/14 and S5/09.

Other forms of LEV are used in workshops, for example, where systems will vary according to the work being undertaken. However, many systems perform badly, often because of poor design specification, inappropriate additions, or lack of maintenance. This Policy Statement provides guidance for the design of such LEV systems and sets out requirements for their maintenance and testing.

2. Principles of LEV design

(a)  System components

LEV systems capture airborne contaminants at the point of generation, preventing inhalation by workers in the vicinity. They vary in complexity, ranging from a simple single-point extract serving a single machine or process to complex multi-point installations serving several machines. However the basic elements are:

(i)  a hood or collector to capture the contaminant close to the point of generation

(ii)  ductwork to convey the contaminant away from the source

(iii)  a filter or air cleaning device to remove contaminant from the extracted airstream

(iv)  a fan or air moving device to provide the necessary air flow

(v)  ductwork to discharge cleaned air to the external atmosphere at a suitable point.

(b)  Process considerations

To effectively remove contaminants from the workplace each element of the LEV system must function properly. Poor design of any one component will undermine overall performance and the ability of the system to control exposure. Efficient LEV systems are those designed to meet the requirements of a specific job or process and should take into account the following:

(i)  how the job is done and any specific requirements of the worker

(ii)  how, where and when the contaminant is being generated

(iii)  physical state of the contaminant (dust, mist, fume gas or vapour)

(iv)  relevant physical properties (boiling points or flash points)

(v)  toxicity

(vi)  relevant workplace exposure limits.

These factors will determine the specification of each system component, described in Section 2(a), and the air extract velocities required for efficient capture of the process contaminant and its transport through the cleaning device to the atmosphere.

(c)  Aerodynamic considerations

While the processes and applications will vary there are some general principles that may be applied to the design:

(i)  Moving parts of machinery should be enclosed as far as possible to minimise stray air currents.

(ii)  Hoods and associated connections should be designed to take advantage of the natural air movement of the machine. Turbulence should be avoided since this diminishes the extraction efficiency and creates noise. (Machine noise is also subject to University Policy Statement S1/06.)

(iii)  Captor hoods should be as close as possible to the contaminant source. The greater the degree of enclosure the greater the control afforded. The capture velocity at the hood must be sufficient to remove the contaminant efficiently, and is a function of design and distance from the source.

(iv)  The minimum number of hoods or inlets necessary to maintain the required transport velocity through the system must be identified and steps taken to ensure that at least this number are open whenever the system is in use.

(v)  Systems with more than one inlet must be balanced, with each branch extracting the optimum amount of air through the inlet it serves. Airflow in each branch is determined by the resistance of the inlet, the length, diameter and flow resistance of the branch duct, and the flow conditions at the junction with the main duct.

(vi)  Ductwork must be as simple as possible and the number of directional changes minimised to avoid airflow resistance. Junctions in ductwork should be at small angles and internal flanges avoided. Bends in the system should have large radii. Runs of ducting should have access holes to facilitate cleaning and access panels should be easily removable. Long runs of flexible ducting should be avoided, where possible, and restricted to sections that are attached to moving parts of machinery or where access to the process is required.

(vii)  Careful consideration must be given to materials of construction in LEV ductwork. Plastic ducting is not suitable for the movement of particles (e.g. wood dust or shavings) that require higher transport velocities to keep them airborne. The movement of wood dust may also generate electrostatic charges and ductwork must be manufactured from a conductive material and earthed to dissipate static charge. Plastic ductwork is usually designed for lower transport velocities and in the case of some fumes it may be preferable to metal, to avoid corrosion.

(viii)  Where possible, air cleaners / collection units should be situated externally and away from traffic routes. Where necessary (e.g. for wood dust collectors), units should be enclosed and fitted with adequate explosion relief, venting to a safe place.

(ix)  Selection criteria for fans or air moving devices include the required total airflow, overall resistance of the system, anticipated noise levels, limitations of space and physical properties of the contaminant. Fans and air movers should, ideally, be sited outside the workroom and must be regularly maintained to reduce their impact on workplace noise (UPS S1/06).

(x)  Cleaned air must be discharged to atmosphere at a suitable point, avoiding circulation back to the workplace.

3. Commissioning, inspection and maintenance

(a)  Commissioning

This is the formal procedure to bring the LEV system into service after installation, and proving it to be capable of meeting the design specification. Appropriate details of airflow velocities and pressures must be recorded to provide standard performance data for future comparison.

Full commissioning by a competent contractor[1] must be arranged as part of the design and installation brief, and the costs met by the project or the department, depending on who actions the work.           

A copy of the LEV commissioning data must be sent to the Safety Office for any newly installed system. The Safety Office will notify the University Insurance Section so that the new asset may be added to the annual statutory test programme.

(b)  Statutory examination and testing

The COSHH Regulations specify that LEV systems must be subject to a thorough inspection and test at prescribed intervals, usually not exceeding fourteen months (in practice this is usually an annual test). However certain LEV systems may require testing more frequently (relevant processes are listed in appendix 1). Examination and testing is co-ordinated by the Safety Office and the costs of the requisite tests by a competent contractor are met centrally. Upon receipt of the test results, the Safety Office forwards copies of the relevant reports to the departments, where the records should be kept for at least five years.

The annual inspection and test will include a visual inspection and measurement of airflows that are compared to previous test results or commissioning data. It will assess whether or not performance has deteriorated significantly and whether or not the system is adequately controlling exposure to substances hazardous to health. It will not reveal the precise cause of any unsatisfactory performance and does not, therefore, substitute for proper maintenance. (However, the test report will often include recommendations, which departments are advised to follow within a reasonable time frame.)

Where a process has been suspended indefinitely and the LEV equipment is not required (and therefore not tested as part of the statutory programme) then the LEV must be isolated to prevent inadvertent use. If at a later date the equipment is to be brought back into service the equipment must be examined and tested to ensure that exposure to contaminants is still adequately controlled (a departmental cost) and the Safety Office notified so that the information can be updated for the next annual statutory test (costs met centrally).

(c)  Ongoing inspection

In addition to the annual test the COSHH Approved Code of Practice (ACoP) requires that LEV systems be given a visual check at least once a week. This regular checking ensures that the system is working properly and identifies potential problems before the LEV performance deteriorates, so is important for maintenance purposes.

The scope of these weekly checks will depend on the complexity of the system but may include a check of:

(i)  the hood or inlet for a change of position or signs of damage

(ii)  the condition of the ductwork for leaks and the dampers for function

(iii)  signs of damage (or partial disconnection) in areas of flexible ductwork

(iv)  noise levels (increases may indicate turbulence, damage to mechanical bearings)

(v)  work area for signs of control failure (e.g. dust deposits, odours)

(vi)  pressure gauges or airflow indicators

(vii)  cleanliness of the filter or filter sock, filter bins not over filled

(viii)  motorised filter shaking devices operational

(ix)  correct water levels on wet scrubbers.

Records are also required for the outcome of these weekly visual checks, as well as information relating to repairs or remedial work carried out as a result of them. The records may be kept electronically, in a logbook, or in a weekly checklist.

(d)  Maintenance

COSHH requires LEV systems to be maintained to a standard suitable for their purpose. If routine maintenance is neglected extract efficiency will deteriorate and mechanical parts are liable to fail. As a minimum, the manufacturer’s recommendations should be used as a guide to the maintenance regime. Departments should draw up maintenance procedures to cover a full range of activities, from simple visual checks for defects to preventative maintenance and remediation.

The table in appendix 2 outlines some typical causes of failure due to inadequate maintenance.

4. Modifications to existing LEV systems

Additional extraction points must not be added unless the system is known to have the necessary extra capacity. The availability of make up air must also be taken into consideration, since insufficient air will reduce performance. Such evaluations must be made by a competent contractor. Significant modifications, such as the addition of an extract inlet, will require the system to be re-commissioned and baseline data recorded for reference during future tests. A copy of the re-commissioning data should be sent to the Safety Office.

5. Waste disposal

Suitable arrangements must be made for the disposal of material collected by filters or other air cleaning devices. In the case of wet scrubbers, for example, where collection media may be contaminated with solvents or corrosive materials, waste may not be discharged into drains, but should be disposed of via the hazardous waste disposal route (UPS S5/11).

6. Training

Supervisors must ensure that adequate training is given to new process operatives, to include proper use of the LEV system, any set parameters for use (e.g. the number of open outlets required for optimal performance) and any routine performance checks to be undertaken prior to work starting.

7. Summary of departmental action

Departments are required to ensure the following:

(a)  Notification to the Safety Office of all existing LEV systems (other than those encompassed by UPS S5/09 and S3/14), by completion of an annual return.

(b)  Notification to the Safety Office of any newly installed LEV system, along with a copy of the commissioning data.

(c)  Implementation of an appropriate maintenance plan for all LEV systems, with maintenance of suitable records by a named responsible person.

(d)  Drafting of a weekly inspection checklist, or similar system, to ensure that routine system performance is monitored and recorded, and any defect reported for remedial action.

(e)  Process operatives have received appropriate training in the use and monitoring of LEV systems.


[1] In this context a ‘competent contractor’ is a specialist ventilation engineer who is accredited for the appraisal, examination and testing of LEV systems (e.g. via the British Occupational Hygiene Society (BOHS) or the National Certification Scheme for In-Service Inspection Bodies (NCSIIB)).

January 2008