Home / LEV Testing / Guide
LEV testing - the dust lamp
The most dangerous dust is the dust you cannot see. The fine particles that reach deep into the lungs are invisible in ordinary light, so an LEV can look like it is capturing everything while a cloud of respirable dust escapes unnoticed. A dust lamp makes that invisible cloud visible - and reveals capture failures the eye alone would never catch.
The short answer
A dust lamp is a simple but powerful diagnostic technique - a recognised method known as MDHS 82/2 - that uses a strong beam of light and forward light scattering to make airborne dust visible. Its value is that the respirable particles smaller than about ten microns, the ones that reach deep into the lungs, are normally invisible in ordinary lighting - so an LEV can appear to be working while a cloud of fine dust escapes its capture unseen. The dust lamp lights up that cloud, letting you actually see whether the extraction is drawing the dust in or letting it escape, and pinpointing where capture is failing.
The dust you cannot see is the problem
There is a dangerous gap between what you can see and what actually matters when it comes to dust. The particles that do the most harm - the respirable fraction, small enough to travel deep into the lungs - are also the ones too fine to see under ordinary lighting. Larger, visible dust settles out and is less able to reach the deep lung; the truly hazardous particles are those you cannot see hanging in the air. So judging an LEV by whether the air looks clear is misleading: the air can look perfectly clear while carrying a substantial cloud of invisible respirable dust.
This is exactly why an LEV can appear to be working when it is not. If the visible dust is being captured, the extraction looks effective - but the invisible respirable cloud may be escaping into the breathing zone unseen. The eye simply cannot tell whether the fine, harmful fraction is being drawn in or is drifting past the hood into the room. So there is a real need for a way to see the invisible dust - to reveal whether the extraction is capturing the fraction that actually matters. The dust lamp is that tool.
How the dust lamp works
The dust lamp works on a simple optical principle. When a strong, focused beam of light passes through air containing fine particles, the particles scatter the light - and viewed correctly, against a dark background with the beam coming from the side or in front, that scattered light makes the particles visible as a glowing cloud, much as a shaft of sunlight reveals dust motes in a dim room. This forward light scattering brings out particles far too small to see in ordinary diffuse lighting, including the respirable fraction under about ten microns that is otherwise invisible.
The technique is a recognised method - published as MDHS 82/2, The Dust Lamp - precisely because it reliably reveals what ordinary observation misses. Set up properly, it shows the airborne dust cloud in real time: where the dust is being generated, which way it is moving, and crucially whether it is being drawn into the LEV hood or escaping past it. It is a qualitative technique - it shows you the dust rather than measuring a concentration - but as a way of seeing what the extraction is actually doing with the fine dust, it is remarkably direct and revealing. You watch the cloud and see the capture happen, or fail to.
What it reveals
What the dust lamp exposes is capture failure that would otherwise go unseen. With the cloud made visible, you can watch whether the dust generated at the process is drawn into the hood or drifts past it into the workroom air. A hood that is too far from the source, or positioned so the dust escapes around it, or fighting a cross-draught that carries the dust away, shows its failure plainly: the visible cloud escaping capture, streaming past the hood rather than into it. None of this is apparent in ordinary light, where the fine dust is invisible and the system looks fine.
This makes the dust lamp a powerful complement to measurement. Airflow and capture velocity readings tell you the numbers; the dust lamp lets you actually see the consequence - the contaminant being captured or escaping. It can reveal problems the numbers alone might not flag clearly, like a cross-draught defeating an otherwise adequate hood, or dust escaping from a point you had not considered. And it is compelling evidence: watching a cloud of fine dust stream past a hood into the air a worker breathes is far more persuasive than a number on a report. Seeing the invisible dust turns an abstract capture failure into something undeniable.
Where it fits in testing
The dust lamp fits into LEV assessment as a diagnostic technique that shows the airflow's effect on the actual contaminant. A thorough examination measures the system's performance quantitatively; the dust lamp adds a qualitative dimension, letting the examiner and the duty holder see the dust cloud and how the extraction handles it. It is particularly valuable for understanding why a system is or is not working - visualising the capture at the hood, spotting escapes, and checking that the extraction is drawing in the fine fraction and not just the visible dust.
Its real message, though, is a cautionary one that applies whether or not a lamp is used on a given day: you cannot judge an LEV by whether the air looks clear, because the dust that matters is invisible. The dust lamp proves this vividly by revealing the hidden cloud - and in doing so it reinforces why proper testing matters. A system that passes the eye can fail the lamp, and a system that passes the lamp can still need the measurements to confirm control. Seeing the invisible dust is a powerful reminder that with fine dust, looking fine and being fine are two very different things - which is the whole reason extraction has to be tested rather than trusted to appearances.
The takeaway
The most harmful dust - the respirable fraction that reaches deep into the lungs - is invisible in ordinary light, so an LEV can look like it is capturing everything while a cloud of fine dust escapes unseen. A dust lamp, the recognised MDHS 82/2 technique, uses a strong beam and forward light scattering to make that invisible cloud visible, letting you actually watch whether the extraction is drawing the dust in or letting it escape past the hood.
What it exposes is capture failure the eye would never catch - a hood too far from the source, a cross-draught carrying dust away, an escape from an unconsidered point - all plain to see once the cloud glows in the beam. As a qualitative complement to the airflow and capture-velocity measurements, it shows the consequence the numbers describe, and it drives home the core lesson: with fine dust, the air looking clear proves nothing. The dust that matters is the dust you cannot see, which is exactly why extraction has to be tested rather than trusted to appearances.
Questions
A diagnostic technique - the recognised method MDHS 82/2 - that uses a strong beam of light and forward light scattering to make airborne dust visible. Its value is revealing the fine respirable particles that are invisible in ordinary lighting, so you can see whether an LEV is capturing them or letting them escape.
Because the dust that matters most - the respirable fraction that reaches deep into the lungs - is too fine to see in ordinary light. The air can look perfectly clear while carrying a substantial cloud of invisible respirable dust, so judging an LEV by appearance is misleading.
When a strong, focused beam passes through air containing fine particles, the particles scatter the light. Viewed against a dark background, that scattered light makes the particles glow as a visible cloud - much as a shaft of sunlight reveals dust motes - including particles far too small to see in ordinary lighting.
Capture failures the eye would miss: whether dust generated at the process is drawn into the hood or drifts past it into the air. A hood too far from the source, a cross-draught carrying dust away, or an escape from an unconsidered point all show plainly as the visible cloud streaming past the hood.
No - it complements it. Airflow and capture-velocity readings give the numbers; the dust lamp lets you see the consequence, the contaminant being captured or escaping. It is qualitative rather than a concentration measurement, but it can reveal problems the numbers alone might not flag clearly.
That with fine dust, looking fine and being fine are two very different things. By revealing the hidden cloud, the dust lamp proves you cannot judge an LEV by whether the air looks clear - which is exactly why extraction has to be tested and measured rather than trusted to appearances.
Our thorough examination measures whether your extraction is capturing the fine dust that matters - the respirable fraction you cannot see in ordinary light, which is exactly why looking clear is never proof of control.
