LEV Extraction Systems Explained: How They Work and What the Law Requires

Local exhaust ventilation is the engineering control UK employers reach for when they need to prevent workers from breathing in dust, fume, vapour, or mist. It captures the contaminant at the point it is generated — before it can spread into the workplace air and before it reaches the worker's breathing zone.

Understanding how an LEV extraction system works makes it easier to operate one correctly, identify when it is underperforming, and meet the legal duties that come with it.

The Basic Principle

An LEV system creates a zone of airflow around the source of contamination. The capture air velocity at that zone must be high enough to entrain the contaminant — pull it into the extraction stream — and carry it away from the worker before it has a chance to disperse.

Three conditions must all be met for that to happen:

1. The airflow must be at the right velocity at the right point. A capture hood that is too far from the source, or a system that has lost airflow through a clogged filter or leaking duct, may fail entirely even though air is visibly moving.
2. The capture hood must be designed for the source. The geometry, face velocity, and position all affect how effectively the hood draws contaminant in.
3. The system must be used correctly every time. An extraction arm left pointing at the ceiling, a hood blocked by a workpiece, or a system switched off to reduce noise — any of these means the worker is unprotected.

The Five Components

Every LEV extraction system has the same five elements, whether it is a small portable dust extractor or a large centralised extraction plant.

1. The Hood

The hood is where contaminant enters the extraction system. It captures contaminated air and draws it into the ductwork. The design of the hood matters enormously — the same fan and ductwork can perform very differently depending on whether the hood is the right type for the source.

Common hood types include:

• Enclosing hoods — the source is surrounded on most sides. A fume cupboard in a laboratory or a spray booth for vehicle painting are examples. Enclosing hoods are the most effective type because capture velocity is low and turbulence from cross-draughts has less effect.
• Receiving hoods — positioned to receive a stream of contaminated air that is already moving, such as a canopy above a hot process or an overhead hood above a grinder wheel. The natural movement of the contaminant assists capture.
• Capturing or induction hoods — positioned close to the source to create enough face velocity to capture the contaminant before it disperses. LEV arms for welding and cutting, push-pull systems over open tanks, and on-tool extraction shrouds are all capturing hoods.

The further a capturing hood is from the source, the more airflow it needs to achieve the same face velocity. Doubling the distance from source to hood reduces face velocity by roughly the square of the distance — a small movement of the hood can dramatically reduce capture effectiveness.

2. The Ductwork

Ductwork carries the contaminated air from the hood to the air-cleaning unit and then to the discharge point. Well-designed ductwork maintains sufficient transport velocity throughout to keep particulate in suspension and prevent it from settling out. Low-velocity sections in horizontal duct runs allow dust to accumulate, blocking the duct progressively and reducing airflow to the hood.

Ductwork also has to be leak-tight. Any leak between the hood and the fan reduces the negative pressure available at the capture point. Multiple small leaks in a long duct run add up to significant airflow loss.

Flexible connections — used where equipment moves or vibrates — are a common maintenance point. Flexible hose degrades with age and use, and cracks or tears allow leakage.

3. The Air-Cleaning Unit

Before contaminated air is discharged, it passes through an air-cleaning unit. The type depends on the contaminant:

• Fabric filters (bag filters, cartridge filters) — used for dry particulate. The filter medium captures dust while clean air passes through. Filter loading increases resistance to airflow over time; clogged filters reduce system performance. Filters must be replaced or cleaned before they reach the point at which resistance causes significant airflow loss.
• Cyclone separators — use centrifugal force to separate heavier particles from the airstream before they reach the filter, extending filter life. Common in woodworking extraction.
• Wet scrubbers — used for gases, vapours, and some sticky or hygroscopic dusts. Water contacts the contaminated air and absorbs the contaminant.
• Activated carbon beds — used for organic vapours and solvents. Carbon adsorbs the vapour molecules. Carbon beds have a finite capacity and must be replaced when saturated.
• HEPA filters — used for very fine or hazardous dusts, including some pharmaceutical and fine silica applications. High efficiency but high resistance; motor sizing must account for filter pressure drop.

4. The Fan

The fan provides the motive force — it creates the negative pressure that drives airflow through the hood, along the ductwork, and through the air-cleaning unit. Fan selection is matched to the system's resistance (pressure drop across ductwork and filters) and the required airflow volume.

As the system's resistance increases — through filter loading, duct accumulation, or added hoods — the fan's performance moves along its characteristic curve. A centrifugal fan that was correctly specified when the system was new may deliver lower airflow as resistance builds. Monitoring motor current is a practical way to detect changes in system resistance between formal tests.

5. The Discharge Point

Cleaned air must be discharged in a way that prevents re-entry into the workplace. The discharge point needs to be positioned considering wind direction, nearby air intakes, and the proximity of occupied areas. Recirculation of air back through an air intake or window can negate the benefit of extraction entirely.

Discharge directly back into the workplace — recirculation — is only permissible for certain low-toxicity dusts and requires specific technical justification. For most hazardous substance applications, the extracted air must be discharged to the external atmosphere.

What COSHH Regulation 9 Requires

COSHH Regulation 9 creates three distinct duties for LEV systems:

Maintenance. Under Regulation 9(1), LEV must be "maintained in an efficient state, in efficient working order, in good repair and in a clean condition." This is a continuous duty — not a periodic one. The system must perform as required at all times between tests, not just on the day of the TExT inspection.

Thorough examination and test. Under Regulation 9(2), LEV must be thoroughly examined and tested at least once every 14 months. For certain processes listed in Schedule 4 of the Regulations, shorter intervals apply. Metal casting blasting operations and jute cloth manufacture require examination at 1-month intervals. Non-ferrous metal casting dust or fume processes require 6-month intervals. Dry grinding or polishing of metal articles (other than gold, platinum or iridium) using mechanical power requires 6-month intervals — but only where the process is carried out in a room for more than 12 hours in any week and by dry (not wet) methods. Operations below that weekly-hours threshold remain on the 14-month baseline.

Records. Under Regulation 9(4), records of every examination, test, and repair must be kept for at least five years from the date they were made.

These three requirements are connected but separate. Meeting the TExT obligation does not satisfy the maintenance obligation. Keeping records is required even if all tests pass.

The Hierarchy of Controls

COSHH Regulation 7 sets out the hierarchy employers must follow when controlling exposure to hazardous substances. Prevention (eliminating the hazard entirely) is preferred. Where prevention is not reasonably practicable, "adequately controlled" exposure is required, with engineering controls — of which LEV is the primary type — ranked above personal protective equipment.

An LEV system is chosen because the employer has assessed that the process cannot be enclosed completely or that substitution with a safer substance is not reasonably practicable. LEV is a positive engineering control — it does not rely on the worker doing anything correctly to maintain protection, unlike PPE. But that advantage only holds as long as the system is performing correctly. A poorly maintained system that no longer achieves design capture velocities has ceased to be an effective control measure, and the employer is in breach of both Regulation 7 and Regulation 9.

Performance Indicators to Watch Between Tests

You do not need to wait for a formal TExT to notice that an LEV system is losing performance. Some practical indicators:

• Visible dust or fume beyond the capture zone during work. If the worker can see or smell what the LEV should be capturing, capture velocity has dropped.
• Reduced airflow at the hood face. Many systems have manometers, magnehelic gauges, or indicator smoke dispensers (smoke tubes) that allow a quick face-velocity check. A consistent reading below the system's design value indicates deterioration.
• Higher-than-normal motor noise. Fan belts slipping or bearings failing produce distinctive sounds before they fail completely.
• Filter pressure drop warning lights. Many modern dust extraction units have differential pressure gauges that alert when the filter is loaded beyond a service threshold.
• Increased dust accumulation at benches, floors, and horizontal surfaces. If housekeeping effort is increasing, extraction effectiveness may be decreasing.

Our LEV compliance checklist generator produces a customised set of checks for your systems. The LEV testing due date calculator tracks when your next formal TExT is due.

For more detail on the duties that apply to your systems, our guides on COSHH and LEV legal obligations and how often LEV must be tested cover the underlying legal framework in full.

---

This guide is for information only and does not constitute legal advice. Consult a competent person or the HSE's LEV guidance for advice on your specific systems.