Phoenix Journal · LEV & Air Quality
Two-pack paints are among the most dangerous things sprayed in any workshop. The isocyanates in them are the leading cause of occupational asthma in Britain, and the mist that carries them is usually invisible. Getting spraying right is about the booth, the timing and the air you breathe while the mist clears.
Two-pack and polyurethane paints do a job nothing else does, but they carry a hazard out of all proportion to the size of the task. The isocyanates in them are the single biggest cause of occupational asthma in Britain, and the sting in the tail is that sensitisation is permanent. Once a sprayer's airways have reacted, even a trace of isocyanate can trigger a severe attack for the rest of their working life, so there is no going back to the job.
What makes this harder is that the danger is invisible. The HSE's guidance on isocyanate paint spraying makes the point plainly: the paint mist that does the damage usually cannot be seen under normal workshop lighting. A sprayer can feel perfectly safe, breathe in a fine aerosol of sensitiser, and never know until symptoms arrive weeks or months later. You cannot manage a hazard by eye when the hazard is designed to be invisible.
There is also a common and costly misunderstanding about water-based paints. The HSE is blunt that water-based does not mean isocyanate-free, and almost every modern top coat still contains them. Switching to an emulsion-style product cuts the solvent load but does not remove the sensitiser, so the same booth, timing and breathing protection still apply. The safe working assumption is that any two-pack coating you spray contains isocyanates until proven otherwise.
A limit measured in hundredths
The workplace exposure limit for isocyanates is 0.02 mg/m3 averaged over an eight-hour day, with a short-term limit of 0.07 mg/m3 over 15 minutes, both measured as the reactive NCO group. To put that in perspective, the general limit for inhalable dust is 10 mg/m3, five hundred times higher. Isocyanates sit near the very bottom of the exposure table precisely because a tiny quantity can sensitise, and the limit exists as a ceiling to stay well below rather than a target to sit under.
Because asthma is in play, the law goes further than the number. COSHH requires that exposure to a substance capable of causing occupational asthma be reduced as low as is reasonably practicable, so a shop that measures just under the limit and calls it done has still missed the duty. For isocyanate spraying that means engineering the exposure down towards nothing, not managing it to a hairsbreadth beneath a figure most people will never actually measure.
The practical version of all this is a chain of controls that back each other up: spray only in a properly designed booth or room, run it under slight negative pressure so contamination cannot drift into the workshop, and never rely on any single measure on its own. The HSE is explicit that no one control should be trusted in isolation, which is why booth, timing and breathing protection are always described together rather than as alternatives.
Clearance is a number, not a feeling
A booth only protects people if the timing around it is understood and followed. Every booth has a clearance time, the interval after spraying stops during which mist is still being cleared from the air, and that time should be established, displayed and obeyed rather than guessed. Air-fed breathing apparatus has to be worn throughout spraying and right through the clearance period, because taking the visor off early is exactly when an invisible aerosol reaches the lungs. Spraying isocyanates is one of the clearest cases where the mist is a genuine respiratory hazard rather than a nuisance, and it belongs in the same family of airborne contaminants covered in dust, fume and mist, knowing what you are dealing with, where the physical form of the contaminant decides how you catch it. A spray mist that hangs invisibly in the air behaves nothing like coarse dust, and the controls have to reflect that.
Proving it works
The two engineering pillars are capture and protection, and both have to be checked rather than assumed. The booth's extraction is local exhaust ventilation and, like all LEV, it is subject to thorough examination and testing at least every 14 months under COSHH, with the real question being whether it captures the mist at source, not merely whether a fan is turning. The air-fed system that protects the sprayer is only as good as its fit and maintenance, which is why respiratory protective equipment, getting it right is a discipline of its own rather than a box on a form. Alongside the hardware, sprayers should have health surveillance for asthma and dermatitis, and vehicle sprayers are advised to give an annual urine sample so biological monitoring can confirm the controls are holding.
Questions
The HSE sets a limit of 0.02 mg/m3 over an eight-hour day and 0.07 mg/m3 over 15 minutes, measured as the NCO group. Because isocyanates cause asthma, exposure must also be kept as low as reasonably practicable, not simply held under the number.
No. The HSE warns that water-based does not mean isocyanate-free, and almost all modern top coats still contain isocyanates. Treat any two-pack or polyurethane coating as needing booth extraction and air-fed breathing apparatus.
Yes. Anyone exposed to isocyanates should have health surveillance for asthma and dermatitis, and vehicle sprayers are advised to give an annual urine sample for biological monitoring. Records should be kept for 40 years because an asthmagen is involved.
Phoenix Duct Clean · by the numbers
Isocyanate spraying needs LEV that actually captures the mist and a booth that clears when it should. Phoenix examines and tests LEV to COSHH Regulation 9 and reports on real capture, not just airflow. UK-wide.