Phoenix Journal · LEV Testing
A downdraught booth with a valid certificate was quietly failing its sprayer. We trace the failure back through filters, pressure and clearance time - and show what to fix.
Case postmortem
A sprayer finishes a two-pack job, waits the usual few minutes, lifts his mask - and by mid-afternoon his chest is tight and he is coughing between panels.
Nothing looked wrong. The booth still roared when the fan kicked in. The lights still came on. Overspray, though, had started drifting out through the personnel door and settling as a fine haze on the tools racked against the workshop wall. That haze was the tell. A downdraught booth that is working properly holds the mist inside and pulls it down and away from the operator; a booth that lets it wander is no longer controlling the one thing it exists to control.
The bodyshop had a valid test certificate on the wall, roughly ten months old. On paper the local exhaust ventilation - the LEV - was compliant. In practice a worker was developing exactly the symptoms the system is meant to prevent. When we were called in to re-examine it, the failure traced back through a short, unglamorous chain of neglect that any operator could have caught. This is how we found it, and what it should teach you about your own booth.
The single number that gave it away was clearance time - how long the booth takes to clear its own airborne mist once spraying stops. A well-set downdraught booth clears in a handful of minutes; a room can take twenty or more. This one had been marked up at four minutes when it was commissioned. Our smoke test measured it at just over nineteen. The sprayer had been lifting his mask on the old four-minute habit while the air around him was still loaded with isocyanate.
Isocyanates matter here more than almost any other reason. They are the leading cause of occupational asthma in Great Britain, and two-pack paints release them as an invisible mist you cannot see under normal lighting. Once someone is sensitised, even a tiny future exposure can trigger an attack, and the damage does not reverse. A booth that clears slowly is not a minor housekeeping issue - it is the difference between a controlled process and a worker breathing sensitiser.
The clearance figure was a symptom, not the cause. Working backwards, three things had stacked up. The extract filters were heavily loaded and well past their change point, so airflow through the booth had collapsed while the fan sounded exactly the same from across the shop. The pressure gauge that should have flagged the loaded filters had been ignored for months because nobody had been told what reading meant "change now". And the intake filters had been swapped for a cheaper grade that did not match the booth's design, throwing the balance between supply and extract out and letting positive pressure push mist out through the door seals. Each fault was small. Together they turned a compliant booth into a hazard, and the paper certificate hid all of it.
It is worth being clear about what a booth test actually is. Under the Control of Substances Hazardous to Health Regulations, engineering controls like LEV must be maintained and put through a thorough examination and test - a TExT - at least once every fourteen months. HSE guidance for spray booths, set out in HSG276 for isocyanate paint spraying and HSG258 for controlling airborne contaminants generally, goes further: it expects a clearance time measurement as part of that examination, taken under worst-case conditions, meaning just before the filters are due to be changed. A test that skips clearance time can pass a booth that is quietly failing, which is precisely what had happened here.
If overspray escaping the booth sounds familiar, the underlying causes are often the same handful every time. We walk through them in more detail in why is my spray booth overspraying, and the fixes below follow the same logic.
Restoring this booth was not expensive - it was a matter of undoing the neglect in the right order and then making sure it could not creep back. Here is what we did, and what you can put in place on your own system.
The theme running through all five is that the equipment was never the real problem - the monitoring was. A booth degrades gradually and silently, and the fan noise stays reassuring right up to the point of failure. The only reliable defence is measurement you can trust and a person who reads it.
One last point worth making. Some of what went wrong here started at specification, not maintenance - a booth sized and balanced correctly at the outset is far more forgiving of a busy shop. If you are planning a new install or replacing an ageing one, it is worth reading how to specify a spray booth correctly before you commit, because the cheapest fault to fix is the one you design out.
Questions
Under the Control of Substances Hazardous to Health Regulations, LEV must have a thorough examination and test at least once every fourteen months. For a busy two-pack booth that interval is a maximum, not a target - the risk from isocyanates and the speed at which filters load mean many booths are better checked far more often, with clearance time measured every visit. Records must be kept for at least five years.
Clearance time is how long a booth takes to clear its own airborne paint mist once spraying stops. It is the figure that tells a sprayer when it is safe to lift their mask. A well-set downdraught booth clears in a few minutes, but a booth with loaded filters or poor balance can take twenty or more - and because the mist is invisible, nobody notices until the number is measured with a smoke test and marked on the door.
The earlier certificate did not include a proper clearance time measurement, so it passed a booth that was quietly degrading. Loaded extract filters, an ignored pressure gauge and wrong-grade intake panels had stacked up to collapse the airflow while the fan still sounded normal. A test that skips clearance time under worst-case conditions can miss exactly this kind of slow, silent failure.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.