Maintenance & systems
LEV and general ventilation are not the same thing, and one cannot do the other's job. Extraction captures a hazardous substance at its source; general ventilation dilutes what escapes and keeps the wider air breathable. Get the relationship right - including the make-up air LEV depends on - and both work; confuse them and neither does.
The short answer
LEV and general ventilation are not the same thing, and one cannot do the other's job. Extraction captures a hazardous substance at its source; general ventilation dilutes what escapes and keeps the wider air breathable. Get the relationship right - including the make-up air LEV depends on - and both work; confuse them and neither does.
The detail
It is easy to blur LEV and general ventilation together as 'the air handling', but they do fundamentally different jobs. Local exhaust ventilation is a targeted control: it captures a hazardous substance at or very near the point it is generated, before it disperses, and removes it from the workplace. General ventilation is a dilution measure: it moves and replaces the building's air to keep overall quality acceptable and to clear the low-level background that no capture system catches perfectly.
Because they do different jobs, they cannot substitute for each other. Turning up the general ventilation will not control a concentrated emission at a machine - it only dilutes what has already escaped into the room, by which point people nearby may already have breathed it. Equally, LEV alone does not manage the whole workspace atmosphere; it is deliberately local. Good air quality comes from the two working together: capture at source first, dilution of the residual second.
The hinge between them is make-up air. Every volume of air an LEV system pulls out has to be replaced, and that replacement air comes through the building's ventilation, whether by design or by whatever gaps it can find. If make-up air is not planned, the extraction fights a depression, capture velocity drops at the hoods, and the LEV underperforms - so the general ventilation is not just parallel to the LEV, it is part of what makes the LEV work at all.
Cross-draughts are the flip side of the same relationship. Ventilation that is poorly arranged - a strong supply blowing across a capture hood, say - can push contaminant out of the capture zone the LEV is trying to hold. The two systems have to be considered together, so that the general air movement supports capture rather than fighting it.
What it means for you
The workplaces that get air quality right treat LEV and ventilation as one problem. They control the hazardous substances at source with properly designed extraction, ensure enough make-up air to feed it, and use general ventilation to manage the residual and the wider comfort - rather than hoping one system will quietly cover the other's gaps.
This also clarifies where each duty sits. LEV is the COSHH control for the hazardous substance and carries the commissioning, testing and maintenance obligations that go with it. General workplace ventilation sits under wider workplace health and welfare requirements. Knowing which system answers which need stops a business over-relying on extraction for general air, or on general air for a hazard that needs capture.
The takeaway is simple: LEV protects people from the specific hazard at its source; ventilation keeps the shared air breathable and, through make-up air, keeps the LEV able to do its job. Neither is optional and neither replaces the other. Designed and maintained as a pair, they give a workspace that is both controlled at the dangerous points and comfortable across the whole floor.
The service behind the guide
Questions
No. LEV captures a hazardous substance at its source before it disperses; general ventilation dilutes and replaces the building's air. They do different jobs and cannot substitute for each other.
No. Dilution only spreads and thins an emission that has already escaped into the room. Controlling a concentrated hazard needs capture at source, which is what LEV provides.
It is the replacement air that supplies what the LEV extracts, coming through the building's ventilation. Without enough of it, the extraction fights a depression and capture velocity drops at the hoods.
Make-up air feeds the LEV, so poor ventilation starves it. Badly arranged supply can also create cross-draughts that push contaminant out of the capture zone, so the two must be designed together.
LEV controlling a hazardous substance sits under COSHH, with its commissioning, testing and maintenance duties. General workplace ventilation sits under wider workplace health and welfare requirements.
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