The definition of a local exhaust ventilation (LEV) system is: an engineering control system to reduce exposures to airborne contaminants such as dust, mist, fume, vapour or gas in the workplace. Simply put it is something that sucks an airborne contaminant out of the workplace.
It is vital that the correct LEV is chosen for a particular task and kept correctly maintained. If a process or activity with which the LEV is associated is changed, then the suitability and specification of the LEV system must be re-assessed.
Most systems consist of the following:
- Hood – where the contaminant enters the LEV
- Ducting ‐ to transport the contaminant and air
- Fan – To power the system
- Discharge – To release extracted air to a safe place
- Air cleaner or arrestor – to filter or clean the extracted air (not all systems have this type)
Types of LEV System:
- Total Enclosure – the process is totally enclosed, and the air extracted from the enclosure e.g. glove boxes/blasting cabinets/CNC machines
- Partial Enclosure – the process is not totally enclosed, and the operator can access the process. Air is pulled passed the operator and into the enclosure e.g. spray booths and milling machines
- Capture Hoods – the process is not enclosed by the system; the contaminant is pulled into the system e.g. ventilated bench, down draft table, welding extract, solder tip extraction, low level room extraction for liquid nitrogen areas or solvent stores, integrated extraction on equipment such as saws and sanders
- Receiving Hoods – the process is not enclosed by the system; the process provides the energy to deliver the contaminant to the hood e.g. canopy hoods over furnace or oven.
Advantages/Disadvantages of LEV:
- Properly positioned LEV and/or well-designed units will capture emissions at source and so protect the employee from exposure.
- The general supply/exhaust ventilation air volume can be reduced as it is not relied upon to dilute contaminants.
- If the LEV is incorrectly placed, contaminants can be drawn into either an employees breathing zone or the process itself.
- Emissions drawn into the system must be disposed of safely and without any adverse effect on the environment.
- It is an additional system to operate and maintain; otherwise it could become an exposure and/or fire hazard.
- Employees must be properly trained in the system’s correct use, its effectiveness and maintenance needs.
When using LEV to control exposure, companies must thoroughly assess the hazards to be controlled and be satisfied with the following:
- the system is fit for purpose
- trained employees are used to maintain the system
- the system is regularly maintained
- records are kept demonstrating that the system is both effective and ongoing.
Having a good understanding of what hazards need to be controlled is crucial to ensure that the initial design can achieve adequate control.
Useful tools include the HSE LEV calculator.
The Control of Substances Hazardous to Health (COSHH) Regulations requires local exhaust ventilation systems to be maintained to a standard suitable for their purpose. It is recommended that the system be checked at least every 14 months, or more frequently if the manufacturer recommends it. Also, simple routine checks can be carried out when the system is in use.
If routine maintenance is neglected extract efficiency will deteriorate and mechanical parts could be liable to fail. As a minimum, the manufacturer or supplier’s recommendations should be used as a guide to the maintenance regime.
Plants should draw up maintenance procedures to cover a full range of activities, from simple visual checks for defects to preventative maintenance and remediation. In addition, they must ensure that there are suitable arrangements in place for the disposal of material collected by filters or other air cleaning devices.
Most filters used in local exhaust ventilation systems will, due to their very nature, require particular handling and care, with disposal being via the appropriate hazardous waste disposal route.
With any maintenance plan, suitable records must be maintained by a named responsible person. Any maintenance records should be held in the vicinity of the LEV system or should be made available for inspection by users or other personnel who may wish to inspect or carry out work on the system.