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LEV testing - how it is assessed
A proper LEV test is not just a set of numbers, and not just a visual once-over. It is both - measured readings that quantify how the system performs, and observed checks of whether the contaminant is actually being captured. Each answers a question the other cannot.
The short answer
Quantitative LEV assessment is the measurement side - taking readings of airflow, air velocity, pressures and capture velocities and comparing them against what the system needs to achieve. Qualitative assessment is the observational side - using techniques like smoke or dust-lamp visualisation to see whether the contaminant is actually being drawn into the hood rather than escaping into the worker's breathing zone. A thorough LEV test uses both, because the numbers and the observed reality each reveal things the other misses.
The two questions
An LEV system exists to capture a contaminant at source before a worker breathes it, and testing whether it does that well breaks into two questions. One is quantitative: is the system moving air at the velocities and volumes it needs to? That is answered with measurement - instruments taking readings of the airflow and pressures at various points. The other is qualitative: is the contaminant actually being captured, in reality, at the point it is generated? That is answered by observation - watching what the air is doing where it matters.
These are genuinely different questions, and a system can pass one while failing the other. A hood can be moving plenty of air on the meter yet still let contaminant escape because it is the wrong shape, badly positioned, or defeated by the way the work is done. Conversely, understanding whether observed capture is adequate needs the numbers to know if it is robust and within design. That is why a proper assessment does not choose between quantitative and qualitative - it uses both to answer both questions.
The quantitative side
Quantitative assessment is the measurement half. Using calibrated instruments, the tester takes readings across the system - the face or capture velocity at the hood, the air velocity in the ducts, static pressures at key points, and the overall airflow the system is moving. These figures are then compared against the benchmark the system needs to meet: the capture velocity required to control that particular contaminant, and the airflow the system was designed to move. The numbers show whether the system is performing to specification.
The value of the quantitative side is that it is objective and comparable. A measured capture velocity either meets the figure needed for that contaminant or it does not, and readings taken now can be compared with the design and with previous tests to show whether performance has drifted. Measurement is what turns a vague sense that a system seems to be working into evidence about whether it actually is - and it is what identifies, for instance, that a fan has weakened or a duct has partially blocked, by the readings falling below where they should be.
The qualitative side
Qualitative assessment is the observational half, and it answers the question numbers alone cannot: is the contaminant actually being drawn in where it is released? Techniques like smoke tubes or a dust lamp make the air movement and the contaminant visible, so the tester can see whether the plume of dust, fume or mist is being captured into the hood or is escaping around it into the air the worker breathes. It reveals how the system behaves in the real geometry of the work, not just what the meters read.
This matters because capture depends on things measurement at a point does not fully show - the position and shape of the hood relative to the source, how the operator works, draughts and cross-currents in the room, and whether the contaminant is generated with enough energy to escape the airflow. A hood with good measured airflow can still fail to capture if it is too far from the source or the work throws contaminant past it. Qualitative observation catches that, showing whether the system controls the contaminant in practice.
Why both
Relying on only one side leaves a gap. Numbers alone can give false reassurance: a system hitting its airflow figures looks compliant, but if the hood is poorly positioned the contaminant may still be escaping, and only observation would reveal it. Observation alone is also incomplete: seeing capture that looks adequate does not tell you whether it is robustly within design or marginally scraping by, nor does it give the comparable evidence to track performance over time - that needs the measured readings.
Used together, they cross-check each other and build a complete picture. The measurements confirm the system is moving air to specification and provide the objective, trackable evidence; the observation confirms that this translates into the contaminant actually being captured where the work happens. A conclusion that the system is controlling exposure is only sound when both agree - the numbers are right and the capture is real. That is why competent LEV testing is built on both, not a choice between them.
The takeaway
Quantitative and qualitative LEV assessment are the two halves of a proper test. Quantitative is the measurement of airflow, velocities and pressures against what the system needs; qualitative is the observation, through smoke or dust-lamp techniques, of whether the contaminant is actually captured at source. They answer different questions - how much air, and does it work - and a system can pass one while failing the other.
That is why a thorough LEV test uses both. Numbers give objective, comparable evidence of performance but can mask a hood that lets contaminant escape; observation shows real-world capture but needs the numbers to know it is robust and to track it over time. When you commission an LEV test, it is worth knowing it should include both - because a test that only measures, or only observes, is only answering half the question of whether your extraction is really controlling exposure.
Questions
The measurement side of an LEV test - taking readings of airflow, air and capture velocities and pressures with calibrated instruments, and comparing them against what the system needs to achieve for the contaminant it controls.
The observational side - using techniques like smoke tubes or a dust lamp to see whether the contaminant is actually being drawn into the hood at source, rather than escaping around it into the worker's breathing zone.
Because a system can pass one and fail the other. Good measured airflow can still let contaminant escape a badly positioned hood, which only observation reveals; and observed capture needs the numbers to know it is robust and to track it over time.
No. Hitting the airflow figures makes a system look compliant, but if the hood is poorly positioned the contaminant may still escape. Only qualitative observation confirms the measured performance translates into real capture where the work happens.
How capture behaves in the real geometry of the work - the hood's position relative to the source, how the operator works, room draughts, and whether the contaminant is thrown past the airflow. These defeat capture even when the meter reads well.
They cross-check each other. Measurements confirm the system moves air to specification and give trackable evidence; observation confirms the contaminant is actually captured. A sound conclusion needs both to agree - the numbers right and the capture real.
We assess LEV both ways - measuring performance against design and observing whether the contaminant is genuinely captured - so the conclusion rests on both, not half the picture.
