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LEV testing - technique
You cannot see whether a hood is capturing fume - air is invisible, and so is most of what it carries. Smoke testing solves that by making the airflow visible, so you can watch capture happen, or watch it fail.
The short answer
Smoke testing releases a controlled plume of smoke near a capture point so the otherwise invisible airflow can be watched directly. It shows whether contaminant is actually drawn into the hood at source, or whether it drifts past into the room. Used alongside the quantitative measurements of a thorough examination and test, it turns abstract airflow readings into something you can see working - or see failing.
Why you need to see the air
A measured face velocity tells you how fast air is moving into a hood, but not the shape of the airflow or whether it actually reaches the point where the contaminant is released. A hood can pull a healthy velocity at its face and still fail to capture fume generated a little too far away, or be defeated by a cross-draught from an open door or a nearby fan.
Smoke makes that visible. Released at the point where the real contaminant would be produced, it shows whether the airflow reaches out and draws the plume in, or whether the plume escapes into the operator's breathing zone. It is the difference between knowing a number and seeing the outcome that number is supposed to deliver.
What it reveals
Watching the smoke shows several things at once. Whether the capture zone actually extends to where the work happens. Whether the operator's own position and movements break the capture. Whether cross-draughts from doors, windows, heating or other extraction pull the plume away from the hood. And whether the contaminant, once captured, stays captured rather than rolling back out.
These are exactly the failures a velocity reading alone can miss. A system can measure adequately at the hood face yet still let contaminant escape because of where the source sits or how the room air moves. Smoke exposes that gap between the reading and the reality.
Where it earns its keep
Smoke visualisation is especially valuable on spray booths and enclosures, where clearance matters as much as capture: a smoke clearance test shows how long it takes for the booth to clear after spraying, which is directly relevant to how soon it is safe to enter. It is equally useful on open benches, downdraught tables and on-tool extraction, where the source can sit at the edge of a hood's reach and small changes in position make a large difference.
In these settings the visible test often catches problems that the numbers alone would pass - a booth that clears too slowly, a bench hood that captures at the centre but not the corners. Seeing it is what makes the fix obvious.
What it is not
Smoke testing is a qualitative technique - it shows the behaviour of the airflow, not a measured figure - so it does not replace the quantitative measurements of a thorough examination and test. The statutory examination still needs its face-velocity and capture readings, its condition checks and its assessment of control. Smoke sits alongside those, making them meaningful.
Used together, the two answer different halves of the same question. The measurements say how much air is moving and how fast; the smoke shows whether that air is doing the job where it matters. A good examination uses both, so the report reflects not just the numbers but the visible reality of capture.
Using the result
The practical value of a smoke test is that it points straight at the cause of a problem. If the plume escapes, you can see why - the hood is too far from the source, a cross-draught is defeating it, the operator is standing in the wrong place, or the extraction is simply too weak. That makes the remedy concrete: move the hood, block the draught, retrain the position, or increase the capture.
It also makes the case for a fix visible to the people who need convincing. A manager or an operator who watches contaminant drift past a hood understands the risk in a way a velocity figure rarely conveys. Seeing the air do its job - or fail to - is often what turns a recommendation into action.
Questions
Releasing a controlled plume of smoke near a capture point so the invisible airflow can be watched directly - showing whether contaminant is actually drawn into the hood at source or drifts past into the room.
No. It is a qualitative technique that shows how the airflow behaves. The statutory thorough examination and test still needs its measured readings, condition checks and assessment of control. Smoke sits alongside those.
Whether capture reaches the actual source, whether cross-draughts or the operator's position defeat the hood, and whether captured contaminant stays captured - failures a face-velocity reading alone can pass.
Because clearance matters as much as capture. A smoke clearance test shows how long a booth takes to clear after spraying, which relates directly to when it is safe to enter.
To find the cause of a problem and fix it - move the hood, block a cross-draught, correct the operator's position, or increase capture - and to show managers and operators the risk in a way a number does not.
It can be. A good thorough examination and test uses smoke visualisation where it adds insight, alongside the quantitative measurements the examination requires.
We pair measured readings with smoke visualisation where it helps, so you can see the system working - or catch it failing.
