Local Exhaust Ventilation (LEV)
Services · Services overview
Local exhaust ventilation (LEV) is an engineering control that captures airborne contaminants — dust, fume, mist, vapour or gas — at or close to the point they are generated, before they can spread into the wider workplace and be breathed in. A well-designed LEV system is one of the most effective ways UK employers meet their duties under COSHH to control exposure at source rather than relying on respiratory protective equipment alone.
What local exhaust ventilation is
Local exhaust ventilation is a powered extraction system that draws contaminated air away from a defined source — a welding arc, a sanding belt, a chemical vessel, a soldering bench — and removes it from the working environment. The principle is straightforward: capture the contaminant where it is produced, transport it cleanly through ductwork, separate or filter it, and discharge the cleaned air to a safe place.
Unlike general or dilution ventilation, which lowers room-average concentrations by mixing fresh air through the whole space, LEV targets the source directly. That makes it the preferred control for processes that release significant quantities of harmful substances, or substances with low workplace exposure limits, because dilution alone cannot reliably keep individual operator exposures below the limit.
In COSHH terms, LEV sits high in the hierarchy of control — above administrative measures and PPE — and is normally specified whenever elimination, substitution or process enclosure is not reasonably practicable.
Where LEV is used
Local exhaust ventilation is used across almost every UK industrial and laboratory sector. Typical applications include welding bays, foundries, woodworking shops, metalworking and machining, stone and ceramic processing, paint spraying, chemical handling, electronics assembly, pharmaceutical weighing and dispensing, and food and bakery operations.
The same engineering principles apply whether the system is a single on-torch fume extractor on a MIG gun, a downdraft bench in a body shop, a ducted hood network serving multiple grinding stations, or a fume cupboard in a research laboratory.
The main components of an LEV system
Every LEV system is built from five functional parts. Each one affects overall performance, and a failure in any single part can compromise the whole system even if every other component is in good order.
- Hood or capture point — the inlet that draws contaminated air in. Hoods range from simple receiving hoods over hot processes, through capturing hoods close to the source, to fully enclosing booths and partial enclosures.
- Ductwork — the network that transports captured air to the air-cleaning stage. Duct sizing, transport velocity, run length and bend geometry all influence whether contaminants stay airborne or settle out.
- Air cleaner — filters, cyclones, scrubbers or electrostatic precipitators that remove contaminants from the airstream before discharge.
- Fan — the powered air mover that creates the airflow. Fan selection and condition determine the system's capacity and resilience to filter loading.
- Discharge — the exhaust point where cleaned air is released. The discharge location must avoid re-entrainment back into the workplace through doors, windows or fresh-air intakes.
Capture effectiveness and airflow
An LEV system only protects the operator if the hood draws contaminated air in faster than the contaminant can escape into the breathing zone. This is described by capture velocity: the air speed at the point of release that is sufficient to overcome ambient air currents and pull the contaminant into the hood.
Capture velocity falls off rapidly with distance from a hood. A plain unflanged hood often loses most of its capture performance within one hood diameter of the face — which is why hood design and operator working position are as important as fan capacity. Adding flanges, partial enclosures or push-pull airflow can dramatically improve capture for the same extracted volume.
Air volume, transport velocity in ductwork, static pressure across the fan and filter, and discharge airflow are all measured during a performance assessment and compared with the original commissioning data, where available.
Common LEV performance problems
Most LEV systems we encounter in service do not fail catastrophically. They drift — quietly losing capture performance over months and years until they no longer adequately control the exposure they were installed to manage.
- Hoods relocated or removed by operators to improve access, reducing capture distance to zero or moving the source outside the capture envelope.
- Ductwork that has been extended, re-routed or partially blocked by accumulated dust, reducing transport velocity and changing system balance.
- Filters left unchanged beyond their service life, raising resistance and starving the system of airflow.
- Fans operating off their original duty point because of belt slippage, impeller wear, motor degradation or unintended throttling.
- Discharge points placed where prevailing wind or proximity to fresh-air intakes allows re-entry of contaminated air.
- Operator practice that defeats the system — working outside the booth, leaning past the hood, or running the process with extraction switched off.
Inspection, testing and maintenance
LEV systems should be subject to a formal Thorough Examination and Test (TExT) at least every 14 months under Regulation 9 of COSHH, and more frequently for some substances. The methodology is set out in HSE guidance HSG258, which describes how to measure performance, judge capture effectiveness and report findings.
Between statutory examinations, employers are expected to maintain the system: visual checks by operators, planned filter changes, monitoring of pressure indicators where fitted, and prompt repair of damage or modification. A documented user manual and maintenance log makes both day-to-day management and the next thorough examination considerably easier.
Relationship with COSHH and HSG258
COSHH places duties on employers to assess the risks from hazardous substances and to control exposure. Where LEV is the chosen control, COSHH Regulations 7 and 9 require that the system is suitable, kept in efficient working order, properly maintained, and examined and tested at suitable intervals.
HSG258 — Controlling airborne contaminants at work: a guide to local exhaust ventilation — is the HSE's principal practical guide and the reference point for competent LEV examiners. It describes design intent, performance benchmarks, test methods and the structure of an examination report.
When to request a review
An independent review of your LEV provision is sensible whenever the process changes, when the system has been altered or extended, when operators report symptoms or visible escape of dust or fume, when statutory test dates are approaching, or when an HSE inspection or insurance audit is anticipated.
A review is not the same as the statutory test — it is a broader engineering and exposure-control judgement that helps decide whether the existing system is the right answer for the current process at all.
Frequently asked questions
Is local exhaust ventilation a legal requirement?
LEV is not specifically mandated by name, but where COSHH risk assessment identifies that exposure to a hazardous substance must be controlled and engineering control is reasonably practicable, LEV is normally the chosen solution. Where it is installed, COSHH Regulation 9 requires that it is examined and tested at suitable intervals — at least every 14 months for most applications.
How is LEV different from general workplace ventilation?
General or dilution ventilation reduces the average concentration of contaminants across a whole room by introducing fresh air. LEV captures the contaminant at the point of release before it disperses, which is far more effective for hazardous substances and is what COSHH expects where reasonably practicable.
Who is responsible for LEV in a workplace?
The employer is responsible under COSHH for ensuring that any LEV system controlling exposure is suitable, maintained and examined. Day-to-day operation typically sits with line management and operators; statutory testing should be carried out by a competent examiner.
Can an existing LEV system be improved without full replacement?
Often, yes. Many performance problems are caused by hood design, ductwork modifications, filter condition or fan settings rather than the fundamental system being undersized. A diagnostic assessment will identify whether targeted upgrades — better hoods, re-balanced ductwork, new filters, push-pull flow — can restore performance, or whether replacement is the proportionate option.
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