Frequently Asked Questions
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Frequently asked questions on local exhaust ventilation, LEV testing, workplace airflow assessment, extraction systems, indoor air quality and ventilation compliance in UK workplaces. Answers are written for health and safety managers, facilities teams, engineering leads and occupational hygienists who need a clear, defensible starting point before commissioning work on site.
How to use this FAQ
The questions below cover the topics that come up most often when UK employers review their workplace ventilation arrangements. They are general guidance and not a substitute for a site-specific assessment by a competent person. For complex or borderline situations — particularly anything involving carcinogens, sensitisers or substances with a Workplace Exposure Limit (WEL) — measured data from the actual process is what determines the right answer.
The FAQs are grouped loosely: LEV and statutory testing, airflow assessment, extraction systems, indoor air quality, and commercial questions about scoping work.
Frequently asked questions
What is local exhaust ventilation (LEV)?
Local exhaust ventilation is an engineering control that captures airborne contaminants — dust, fume, mist, vapour or gas — at or near the point where they are generated, transports them through ductwork, removes them via filtration where required, and discharges the cleaned air safely. LEV is the most common engineering control under COSHH for hazardous substances that cannot be eliminated or substituted.
How often does LEV need to be tested in the UK?
Under Regulation 9 of the Control of Substances Hazardous to Health Regulations 2002 (COSHH), most LEV systems must be thoroughly examined and tested at least once every 14 months. Some systems — for example those controlling exposure to certain dusts or chemicals — must be tested more frequently. Records of the examination must be kept available for inspection.
What does an HSG258 LEV thorough examination cover?
HSG258 — the HSE guidance on controlling airborne contaminants — describes a structured examination of the hood, ductwork, fan, filtration, controls and discharge, along with quantitative measurements such as face velocity, duct transport velocity, static pressure and (where relevant) capture performance. The report should classify the system, identify defects and recommend remedial action with target dates.
Who is a 'competent person' to carry out LEV testing?
A competent person has the necessary training, knowledge and experience to examine and test the specific system, interpret the results and judge whether the LEV is providing adequate control. Many practitioners hold qualifications such as BOHS P601 (Initial Appointed Person — Examination and Testing of LEV). Competence must be appropriate to the type of system, not just LEV in general.
What is the difference between LEV examination and LEV testing?
Examination is the visual and functional inspection of the system to identify wear, damage, blockage, leakage or deterioration. Testing is the quantitative measurement of performance — typically face velocity, duct velocity, static pressure and airflow — compared against the design intent or HSG258 benchmarks. Both are part of the statutory thorough examination and test.
What is workplace airflow testing?
Workplace airflow testing measures the supply, extract and transfer airflow within a space against the design intent and relevant HSE guidance. It is used to verify general ventilation performance, investigate complaints about draughts, stuffiness or odour, and to check that mechanical ventilation systems are delivering the outside air rates assumed in the building design.
When is dilution ventilation acceptable instead of LEV?
Dilution ventilation can be appropriate where the contaminant is of low toxicity, generated at a low and predictable rate, and the receptor is not close to the source. It is generally not acceptable as the primary control for carcinogens, sensitisers, substances with low WELs or high-generation processes — these usually require local capture at source. A competent assessment is the only way to confirm which strategy applies.
Do welding fumes really need LEV?
Yes. Welding fume is classified as carcinogenic to humans, and HSE has stated that all welding indoors requires effective engineering controls. Ambient extraction alone is rarely sufficient as the primary control. On-torch extraction, capture hoods, movable extraction arms or downdraught benches are typically required, supported by general ventilation and respiratory protection where residual risk remains.
What is a typical face velocity for an LEV hood?
Hood capture and face velocity requirements depend on the contaminant, the generation mechanism and the distance between source and hood. HSG258 sets out minimum capture velocities for different categories of contaminant — for example higher velocities for active sources such as grinding, and lower velocities for substances released into still air. Design and assessment should be against the relevant capture velocity for the specific process, not a single generic figure.
What is a ventilation risk assessment?
It is a structured review of where airborne contaminants are generated, how they reach occupants, what existing controls do, and whether the residual risk is adequately controlled. It draws on COSHH risk assessment, HSG258 principles and the hierarchy of control, and usually combines a walk-through, process review and measurement.
What is indoor air quality (IAQ) in a workplace context?
Indoor air quality describes the chemical, biological and physical condition of the air inside an occupied building. In an office context it typically focuses on CO₂, particulates, VOCs, thermal comfort and humidity. In industrial settings it overlaps with COSHH exposure assessment, because the same air may contain process contaminants from production activities.
How is CO₂ used as an indicator of ventilation?
CO₂ is widely used as a proxy for outside-air delivery per occupant, because occupants are the dominant CO₂ source in most offices. Sustained elevated readings — for example consistently above guidance values during occupied hours — usually indicate that the ventilation rate is insufficient for the actual occupancy. CO₂ alone does not measure contaminants from process sources.
What is the difference between IAQ and COSHH exposure assessment?
IAQ focuses on overall air quality and occupant comfort in normally occupied spaces. COSHH exposure assessment focuses on individual worker exposure to specific hazardous substances against Workplace Exposure Limits and other criteria. The two overlap in some workplaces — for example where general air quality is influenced by an industrial process — but the regulatory drivers and acceptance criteria are different.
How do you investigate persistent workplace odour complaints?
A structured odour investigation looks at sources, pathways and receptors. It identifies the most likely source — a process, a drain, a material, an external source — assesses how the odour reaches the affected area, and reviews existing ventilation, extraction and pressure relationships. The objective is to remove or capture the source where possible, rather than mask it with additional dilution.
Can ventilation testing be done without disrupting production?
Most LEV testing and airflow measurement is carried out during normal operations, because the system must be evaluated under realistic conditions. Some checks — for example smoke visualisation of capture, or measurements requiring access above ceilings — are scheduled to minimise disruption. A competent contractor will agree a method and schedule with the site before attending.
What records should an employer keep for LEV?
Records should include the original design intent and commissioning data where available, thorough examination and test reports for at least the last five years, maintenance and repair records, and evidence of remedial actions taken to address any defects identified. These records support COSHH compliance and provide the audit trail expected by enforcing authorities.
How long does an LEV test or ventilation assessment take?
Time on site varies with the size and complexity of the system. A small workshop with a handful of hoods may be examined and tested in a single day. A larger production area with multiple systems, or a building-wide airflow and IAQ assessment, can take several days. The reporting and analysis stage typically takes one to two weeks after the site visit.
How do I get a quote or arrange an assessment?
Contact our team with a short description of the site, the processes involved, the systems to be examined and the reason for the enquiry. Email info@workplaceventilation.co.uk or call 0800 433 7914. For complex projects we will usually arrange a brief scoping call before quoting, so the proposed scope and deliverables match what the site actually needs.
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