In brief
General ventilation dilutes contaminated air; local exhaust ventilation (LEV) captures the contaminant at source before it reaches the employee's breathing zone. It is not enough for an LEV system to be running; it must actually be capturing the contaminant, and the basic measure of this is the capture velocity at the hood. Annex III of the Work Equipment Regulation requires ventilation installations to be inspected once a year; in between, pressure readings, filter maintenance and simple smoke tests make the system monitorable.
The difference between general ventilation and local exhaust ventilation
In a plastics or chemicals plant, the sentence "there is ventilation" can mean two very different things. Roof extractors and open windows work by diluting contaminated air with clean air. An extraction hood at the mouth of a mixing tank, a movable arm on a welding table or the cover of a grinding machine, however, capture the contaminant at source before it spreads into the room. The second is called local exhaust ventilation (LEV), and it is the real protection against harmful dust, fumes, vapours and gases.
| General ventilation | Local exhaust ventilation | |
|---|---|---|
| Working principle | Dilutes the contaminant | Captures the contaminant at source |
| Where it is suitable | Low-toxicity, widespread and small amounts of contaminant; heat and comfort | Contaminant from a toxic, dusty or intense source |
| Effect on the breathing zone | The contaminant passes the employee first | The contaminant is removed before reaching the employee |
| Monitoring measure | Air change rate, workplace measurement | Hood capture velocity, duct velocity, pressure |
The legislation makes this distinction too. Annex 1 item 16 of the Regulation on Health and Safety Measures in Workplace Buildings and Annexes requires a mechanical ventilation system separate from the general ventilation system to be installed to remove suffocating, toxic or irritant gases, dust, mists and fumes from the environment. Art. 7 of the Regulation on Health and Safety Measures in Work with Chemical Substances lists collective protective measures such as adequate ventilation to prevent risk at source, where substitution is not possible, before PPE. Annex I item 2.5.1 of the Regulation on Health and Safety Requirements for the Use of Work Equipment requires work equipment that presents a hazard of emitting gas, vapour, liquid or dust to be fitted with suitable systems to contain or extract them at source. For the order of measures, see our article on the hierarchy of controls.
What is an LEV system made of, and where does it fail?
Every LEV system consists of five parts, and its performance is limited by whichever is the weakest link:
- Hood (extraction point): Captures the contaminant. Hoods fall into three types: enclosing (spray booth, glove box), capturing (welding arm, tank rim extraction) and receiving (grinding wheel guard, canopy over a hot process). The most effective is the one that encloses the source most.
- Ductwork: Carries the air. Crushed flexible hose, branch dampers left open and dust building up inside are the most common losses.
- Air cleaner: Bag or cartridge filter, cyclone, wet scrubber or activated carbon unit. A blocked filter reduces extraction; a holed filter sends dust out of the stack or back inside with the return air.
- Fan: Drives the system. A loose belt, a motor running backwards or worn blades quietly reduce performance.
- Stack: Discharges the cleaned air outside. A stack close to a fresh air intake allows contaminated air to return to the building.
The situation met most often in plants is that today's line is not the same as the line on the day the system was first installed. A branch is added to the existing duct for a new machine, the extraction point is pulled back because "it gets in the way of the work", and capacity grows without the fan being changed. So every change made to the system requires re-measurement under management of change.
Hood capture velocity: measurement and acceptance values
Three velocities are often confused in an LEV system:
- Capture velocity: The air velocity needed at the point where the contaminant is released to draw it towards the hood.
- Face velocity: The velocity measured at the face of the hood; in enclosing booths it is measured across the open face.
- Duct velocity: The velocity in the duct, needed in particular to carry dust without it settling.
The required capture velocity depends on how fast the contaminant is released and into how much air movement. The ACGIH Industrial Ventilation manual, used as a reference in industrial ventilation, gives the following typical ranges:
| How the contaminant is released | Example | Typical capture velocity |
|---|---|---|
| Into still air, with practically no velocity | Evaporation from tanks, degreasing | 0.25–0.5 m/s |
| Into fairly still air, at low velocity | Welding, plating baths, intermittent container filling | 0.5–1.0 m/s |
| Actively into moving air | Spraying in shallow booths, drum filling, conveyor loading, crushers | 1.0–2.5 m/s |
| At high velocity into very moving air | Grinding, abrasive blasting | 2.5–10 m/s |
The lower end of the range is for low-toxicity and intermittent work, the upper end for toxic and continuous work. The real problem is distance: the velocity in front of a capturing hood drops very quickly with distance from the opening. For a simple open hood, at a distance equal to the opening's diameter, the velocity falls to about one tenth of the velocity at the face. A welder who puts the welding arm 50 cm away instead of 15 cm has effectively switched the system off. We cover this specifically for welding fume in our article on welding fume and extraction systems.
Measurements are made with a vane or hot-wire anemometer at the hood face and at the point where the contaminant is released; in ducts, with a pitot tube and static pressure measurement. Values measured when the system is commissioned should be recorded as reference values; every later measurement is compared with them.
Ventilation periodic inspection: what does the legislation require?
Three separate obligations overlap for ventilation systems:
| Legal basis | Provision |
|---|---|
| Workplace Buildings and Annexes Reg. Annex 1 item 17 | Mechanical ventilation is kept in working order at all times; if failure would be dangerous, a control system signalling failure is installed; maintenance, repairs and filter changes are carried out annually by authorised persons |
| Workplace Buildings and Annexes Reg. Art. 5 | Ventilation systems are cleaned regularly for hygiene and records are kept |
| Work Equipment Reg. Annex III Table 3 | Periodic inspection of ventilation and air conditioning installations once a year for compliance with the criteria in the design |
Under Annex III item 2.3.6, the installation design is required at the periodic inspection; having an installation without a design designed is the employer's obligation. This provision has applied since 31.12.2023. The inspection is carried out by authorised persons from the professional groups listed in Annex III, such as mechanical engineers, technical teachers, technicians or senior technicians registered on EKİPNET (the Ministry's register of authorised inspectors). For the full list of equipment see our article on periodic inspection intervals for work equipment, and for how to assess the report our article on how to read a periodic inspection report. The 2026 basic administrative fine for failing to have the necessary inspections and measurements carried out is 33,326 TL.
Dust collection system inspection and filter maintenance
Bag and cartridge dust collectors are the backbone of plants with dry dusts such as plastic granules, pigments, fillers and wood dust. Inspection of the dust collection system should not be limited to the annual periodic inspection, but tied to a cycle such as the following:
- Every shift: Are the extraction points open and in place; can extraction be seen with smoke or a paper strip; is the filter's differential pressure gauge (manometer) in the normal range?
- Every week: Is the dust discharge hopper full; is there dust leakage at duct joints; is the pulse-jet cleaning working properly?
- Every month: Fan belt and vibration, build-up inside ducts, damper positions, visible dust at the stack outlet.
- When changing filters: The job is done wearing a dust mask and disposable coveralls, the removed filter is bagged before being taken out, and the seal of the new filter is checked.
A sudden drop in differential pressure indicates that the filter is holed; a steady rise indicates that it is blocked. In systems where cleaned air is returned to the building, a filter failure carries dust directly into the workplace; continuous monitoring is even more important in these designs. With combustible dusts, the dust collector is also a source of explosion risk; explosion vents, spark prevention and earthing are assessed under dust explosion and the explosion protection document.
How effectiveness is monitored: measurement, health surveillance and records
The real measure of an LEV system's success is the air the employee breathes. So the system's performance is monitored from four sources together:
- Commissioning measurement: Design values and the first measured capture velocities.
- Annual periodic inspection: Compliance with the design and comparison of measured values with the reference.
- Workplace measurements: The level of dust, solvent vapour or metal fume in the breathing zone. We plan these measurements with authorised laboratories and enter the results in the risk assessment.
- Health surveillance: Trends emerging from lung function tests and other tests are interpreted by the workplace physician together with ventilation performance.
For each system, keep the following in a single file: system diagram and design, list of extraction points, reference and latest measured velocities, differential pressure records, filter change dates, faults and repairs, and periodic inspection reports. In chemicals and plastics plants, mixing tanks, dosing stations and hot extrusion heads are the most critical LEV points; for the sector's general risk picture see our chemicals and plastics sector page, for inspection planning our periodic inspection page, and track inspection dates with the OHS calendar. For the specific case of solvent vapour, read our article on solvent vapour exposure.
Frequently Asked Questions
How often is the periodic inspection of a ventilation system carried out?
Annex III of the Work Equipment Regulation requires ventilation and air conditioning installations to be inspected once a year for compliance with their design. In addition, the Regulation on Workplace Buildings and Annexes provides for maintenance, repairs and filter changes to be carried out annually by authorised persons.
Is a capture velocity for local exhaust systems set in the legislation?
No, there is no general numerical capture velocity in Turkish legislation. The value is set at the design stage according to how the contaminant is released; guidance such as ACGIH gives typical ranges. What matters is that the reference value from commissioning is maintained and that workplace measurements stay below the limit values.
We have an old ventilation system without a design; what should we do?
Under Annex III, the installation design is required at the periodic inspection, and having an installation without a design designed is the employer's obligation. First a survey and measurement of the existing system is carried out, and the design is prepared on that basis.
If the fan is running, does that mean the system is working?
No. With a blocked filter, a crushed duct or an extraction point pulled back, the contaminant may not be captured even while the fan is running. Verify that extraction is happening at the source with a smoke test and a velocity measurement.
Equipment inventory and an inspection calendar under the new Annex III, with inspections carried out through our solution partners, plus report and defect follow-up.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.