Each year workers in the United Kingdom contract lung disease or asthma because they have breathed in too much dust, fume or other airborne contaminants at work (for example flour dust in bakeries, mist from paint spraying, fumes from welding or solvents from painting).

A properly designed, maintained and operated local exhaust ventilation (LEV) system can remove airborne contaminants before people breathe them in and will protect workers' health.

What is local exhaust ventilation (LEV)?

Local exhaust ventilation is an extract ventilation system that takes airborne contaminants such as dusts, mists, gases, vapour or fumes out of the workplace air so that they can't be breathed in.

Properly designed LEV will:

  • collect the air that contains the contaminants
  • make sure they are contained and taken away from people
  • clean the air (if necessary) and get rid of the contaminants safely

For more information on local exhaust ventilation systems watch this useful video that explains how LEV can protect people's health:

Why you need local exhaust ventilation

Every year over 2000 people die in Northern Ireland due to lung disease caused by airborne contaminants they have breathed in at work.

If your work produces dust (e.g. flour dust in bakeries), mist (e.g. paint mist from spraying), fume (e.g. from welding), gas (e.g. carbon monoxide from furnaces) or vapour (e.g. solvents from painting), there may be a risk to health.

Employers must prevent or adequately control exposures to hazardous substances, a requirement of the Control of Substances Hazardous to Health Regulations (Northern Ireland) 2003.

Installing LEV may help you to do this. However, you should first consider other options where it's reasonably practicable to do so, such as:

  • change your method of work so exposure to hazardous substances can no longer occur
  • substitute the material being used to something safer
  • reduce the amount of the contaminant released
  • modify the process to reduce the duration or frequency that the contaminant is released
  • reduce the number of employees involved with a process
  • apply simple controls, e.g. fitting lids to equipment

Many employers buy LEV to protect workers' health but find that it doesn't work. This may be because it's the wrong type or because it's not properly installed or maintained.

Sourcing a suitable local exhaust ventilation system

The employer is responsible for selecting a supplier that is competent to define, design and install a suitable LEV system. To help with this, employers may wish to:

  • invite more than one tender
  • provide a drawing of the area and the processes to be controlled
  • ask potential contractors to visit the site to see the processes
  • ask what are their professional qualifications, experience, memberships and whether they can provide case studies and references
  • review tenders and quotes against the Health and Safety Executive's (HSE) guidance, HSG258 Controlling Airborne Contaminates at Work

A key part in ensuring that the LEV system is designed correctly is for the employer to draw up a specification for the supplier.

In the first instance an employer should establish the following as part of the LEV specification:

  • the process (the way airborne contaminants are generated, for example in woodworking, processes may include cutting, shaping and sanding)
  • the contaminants (the airborne material that is trying to be captured i.e. dust, mist, fume, vapour, aerosol) and their hazards (i.e. how are they dangerous, e.g. isocyanate containing paints can cause asthma and welding fume can cause lung cancer)

Where the contaminant to be controlled is generated by a process, this is called the source. It is crucial that the LEV system designer understands how the source behaves in its location at the specific workplace. Depending on the process there could be single or multiple sources.

The specification should also require the supplier to:

  • fit indicators, such as pressure differential guages, that show that the system is working properly
  • ensure the LEV is easy to use, check, maintain and clean
  • provide training in how to use, check and maintain the LEV system
  • provide a user manual that describes and explains the LEV system, how to use, check, maintain and test it, along with performance benchmarks and schedules for replacement of parts
  • provide a logbook for the system to record the results of checks and maintenance

Controlling Airborne Contaminates at Work: A guide to local exhaust ventilation contains the roles and responsibilities of all parties involved in an LEV lifecycle including employers, employees, designers, installers and examiners. It is important for an employer to understand what to expect from each stakeholder as well as their own responsibilities.

The design characteristics of different LEV systems including the velocities required to control hazardous substances from different processes, how duct work should be designed and different filter and fan options is contained within HSG258. Designs and quotes from equipment suppliers should be compared to this criteria to ensure proposed systems will be effective and compliant.

The design of the part of the system that captures or contains the airborne contaminant (the hood) is critical to the performance of an LEV system and must match the process, the source and how the operator carries out the process.

Successful LEV systems contain, capture or receive the contaminant cloud within the LEV hood and conduct it away. The greater the degree of enclosure of the source, the more likely it is that control will be successful.

One of the most common types of LEV is the capture hood, this is where process, source and contaminant are outside of the hood. A capturing hood has to generate sufficient airflow at and around the source to 'capture' and draw in the contaminant-laden air. These systems require the employee to work in a particular location or to position the captor hood to ensure it can capture the contaminant before it can be breathed in. It is therefore important that the employer and employee know how to position the work and what the maximum distance is that they can work away from the hood.

What needs to be done once an LEV system is installed?

Once an LEV system has been installed at your premises there are a number of steps that need to be undertaken to ensure the ongoing effectiveness of control.


After installation the system must be commissioned to prove it is working correctly and capable of providing protection to your employees. The results of the commissioning should be used as a benchmark against the future performance of the system and a copy kept with the system until it is decommissioned and removed.


Staff should be trained in how to use the system, how to check it is operating correctly, any limitations of the system and how they should carry out the work activity to ensure maximum control of the contaminants.

Maintenance and servicing

The LEV system should be maintained in line with the manufacturer's recommendations. The user manual should specify daily, weekly or monthly checks on the performance and condition of the system to help ensure its effective control. These may include checks on the following:

  • hoods - including airflow indicators, physical damage and blockages
  • ducts - including damage, wear and partial blockage
  • dampers - position
  • filters - including damage, static pressure across the cleaner, and failure alarms

All user checks, maintenance and servicing should be recorded in the system's log book.

Thorough examination and test

Every LEV system requires a statutory thorough examination and test by a competent person, at least every 14 months. A thorough examination and test is a detailed and systematic examination to make sure that the LEV can continue to perform as intended and will contribute to the adequate control of exposure. The employer should keep a copy of the report for at least five years.

A detailed breakdown of what a thorough examination and test consists of can be found in HSG258 Controlling Airborne Contaminants at Work: A guide to local exhaust ventilation and Regulation 9 of the Control of Substances Hazardous to Health Regulations (Northern Ireland) 2003.


Key legislation

Please note that this link is to the original legislation, visitors should verify for themselves whether legislation is in force or whether it has been amended or repealed by subsequent legislation.