Phoenix Journal · LEV Testing
Your extraction runs all day, yet the air still feels gritty and surfaces film over by knocking-off time. Here is why that happens - and how to fix the fine dust your system is quietly missing.
Workshop air quality
If your extraction hums away all day yet the air still feels gritty and the surfaces still film over by knocking-off time, the machine is not necessarily broken - it is very likely capturing far less than you think.
Here is the uncomfortable truth about dust: the particles you can see settling on a windowsill are not the ones that matter most. Those visible flecks are the heavy end, and they drop out of the air within seconds. The dust that hangs, drifts and gets breathed in is the fine, near-invisible fraction. The HSE splits airborne dust into inhalable particles, which run from under 0.01 up to 100 micrometres, and respirable particles, which sit below roughly 10 micrometres and travel deep into the lungs. That respirable fraction can stay suspended for minutes or hours, which is exactly why a workshop can look clean, run its extraction all shift, and still leave a haze that catches the light and coats your throat.
So when the air feels dusty despite extraction, the question is not usually "is my extractor on?" - it is "how much of the fine dust is my system actually catching at source, and where is the rest going?" That is a very answerable question, and the rest of this piece walks through the common causes and what to do about each.
Extraction only controls dust it captures at the moment the dust is made. Local exhaust ventilation - LEV - is built to pull contaminant away right at the source, before it reaches your breathing zone. The weak point is almost always the hood, the capture point nearest the tool or bench. HSE guidance HSG258 puts the idea of capture velocity front and centre: the air has to be moving fast enough, at the point the dust is released, to overcome the speed the dust is thrown off at and drag it into the hood. Get the hood a hand-span too far away, or let the airflow drift, and capture collapses. Fine dust does not need much of a draught to escape a poorly placed hood and end up back in the room.
The usual suspects behind "extraction is on but the air is still dusty" are worth naming plainly:
Any one of these will leave you with a machine that sounds busy and achieves surprisingly little. A sudden change in the noise your system makes is itself a clue worth reading - it often signals a fan working against a blockage or a failing filter, and we cover that in detail in why has my extraction got louder.
The honest answer is that you cannot tell by eye or ear alone, which is precisely why the law does not ask you to. Under the Control of Substances Hazardous to Health Regulations 2002 - COSHH - regulation 9 requires that any LEV system controlling a hazardous substance is given a thorough examination and test at set intervals. For most workshop systems that interval is at least every 14 months, and for some higher-risk processes it is more frequent. You are also required to keep the records for at least five years. This is not box-ticking; it is the mechanism that catches the slow decline that makes your air feel dusty in the first place.
A thorough examination and test is more than a visual once-over. A competent tester - one holding the recognised BOHS P601 qualification - measures the things you cannot see: face and capture velocities at each hood, static pressures through the ducting, filter condition and airflow, and whether the system as installed still achieves the control it was designed for under regulation 7 of COSHH. The report tells you not just pass or fail, but where performance has drifted and by how much. That turns a vague "the air feels off" into a specific "hood three is running at half its target capture velocity because the duct behind it is choked."
It helps to know what "good enough" looks like for the dust you generate, because control is judged against legal exposure limits set out in the HSE's EH40 document. Common workshop figures include:
Those numbers are strikingly small. A haze you can barely see can still sit at or above them, which is the whole reason measurement beats guesswork. If your last test is more than 14 months old, or you have never had one, that alone explains a good deal of the uncertainty about your air.
Clearing a persistently dusty workshop is rarely one big fix - it is a short chain of sensible steps, done in order, so you are not throwing money at the wrong link.
Start at the source. Get the hoods back where they belong, as close to each dust-making point as the work allows, because capture velocity falls away sharply with distance. Then work back through the system: change or clean blinded filters, check ducting for blockages and slipped joints, and make sure the fan has not been quietly asked to serve more machines than it was ever sized for. If you run recirculating units, treat their final filter as a consumable, not a fixture - a tired one hands fine dust straight back to the room.
Next, get the whole thing measured rather than guessed. A thorough examination gives you the airflow and capture figures that tell you whether your improvements have actually landed, and it keeps you the right side of COSHH at the same time. Book it on a schedule - inside the 14-month window - so decline is caught early rather than discovered as a haze.
Finally, remember why this is worth the effort. Cleaner air is not only a compliance matter; dust drags on concentration, comfort and output across a working day, a link we explore in the link between air quality and workshop productivity. Getting your extraction genuinely capturing at source pays back in health, in fewer settled surfaces to wipe down, and in people who are simply sharper by mid-afternoon.
Questions
Under COSHH regulation 9, an LEV system controlling hazardous dust must have a thorough examination and test at least every 14 months, and some higher-risk processes need it more frequently. You must also keep the records for at least five years. If your last test is older than that window, or you have never had one, that gap alone often explains why your air still feels dusty.
Yes, easily. The dust that matters most is the respirable fraction below about 10 micrometres, which is near-invisible and stays suspended for a long time. Limits are small - 0.1 mg/m3 for respirable crystalline silica and 3 mg/m3 for hardwood dust over an eight-hour shift - so a faint haze you can barely see can still sit at or above the legal figure. That is exactly why measurement beats a visual check.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.