Why breathing air matters when you spray isocyanate paints
An air-fed hood exists because the paint itself is the hazard. Two-pack isocyanate paints cure by reacting with moisture in the air, and the same reaction happens in a spray mist inside a booth, where isocyanate particles small enough to reach deep into the lungs stay suspended for some time after spraying stops. A carbon filter mask stops solvent vapour but does not stop these particles, which is why HSE requires supplied air, taken from a clean source and delivered to a hood that covers the face, for anyone spraying isocyanate paint.
Sensitisation does not give a second warning
The particular danger with isocyanates is occupational asthma brought on by sensitisation: repeated exposure, sometimes at levels too low to notice at the time, can prime the immune system to react violently to any further contact. Once someone is sensitised, even a brief exposure at very low concentration can trigger a serious asthma attack, and the condition does not reverse when they stop spraying. That makes the air fed to the hood a control on a hazard with no safe threshold established by feel.
The compressor is now part of the breathing system
A standard workshop compressor draws its intake air from wherever it happens to sit, and that air can carry engine exhaust, its own compressor oil, or moisture that turns to a fine mist under pressure. None of that is a problem when the same air is only running a die grinder. It becomes a direct hazard the moment it is fed to somebody's face, which is why breathing air for a hood has its own quality limits, its own filtration and monitoring, and its own testing regime, set out on the pages that follow.
What BS EN 12021:2014 sets as the limit
BS EN 12021:2014 sets a ceiling for each of these four contaminants in the air reaching the hood, tested at least every three months.
| Contaminant | Limit | Why it's controlled | How it's checked |
|---|---|---|---|
| Carbon monoxide (CO) | 5 ppm or less | Drawn in when the compressor intake sits near an engine, generator or vehicle exhaust; breathed directly it displaces oxygen in the blood | Checked at the same interval as the rest of the panel |
| Carbon dioxide (CO2) | 500 ppm or less | Builds up where the intake sits in a confined or poorly ventilated space | Checked at the same interval as the rest of the panel |
| Oil (aerosol and vapour) | 0.5 mg/m³ or less | Carried over from a lubricated compressor element, or picked up from contaminated intake air, and breathed as a fine mist | Sampled from the test point nearest the hood |
| Water | Set as a pressure dew point relative to the system's working pressure | Free water in the line can carry the other contaminants forward and corrode the filters and valves feeding the hood | Assessed against the dew point stated for the pressure the system runs at |
Figures are the limits set out in BS EN 12021:2014. Whether a given test report meets them is for the competent person who signs it off, against the reading their equipment actually takes.
The equipment chain from intake to hood
Air quality can fail at any point between the compressor's intake and the hood itself, so each stage needs checking on its own terms.
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Intake sited away from exhaust and contamination sources
The compressor intake must draw from a point clear of vehicle exhaust, engine fumes, solvent vapour and its own compressor exhaust, since carbon monoxide and oil vapour drawn in at this stage cannot be filtered out reliably downstream.
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Compressor type suited to breathing air
An oil-lubricated compressor puts oil into the air stream that the filtration stage then has to remove; an oil-free compressor or one fitted for breathing air duty avoids that load.
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Filtration matched to what the air needs to remove
Particulate, oil and water each need their own stage of filtration, and the filter set has to be specified for breathing air.
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Carbon monoxide monitor fitted and working
A monitor on the line gives a continuous check on carbon monoxide, which is the hazard most directly linked to intake siting and the one a filter alone will not reliably remove.
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Test point provided for sampling the air actually reaching the hood
A sampling point downstream of the filters, at or near the hood connection, lets the air be tested as delivered.
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Hood and hose rated for breathing air and free of damage
The hood and its supply hose need to hold their seal and be checked for splits, kinks or perished fittings, since a leak here lets contaminated ambient air in even when everything upstream is correct.
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Records kept of testing and any filter changes
A dated record of when the air was last tested and when filters were changed gives the next check a starting point.
A written scheme or air quality standard does not replace a competent person's assessment of your own system. Where a check depends on a fitting or setting specific to your compressor, confirm it against the equipment manual.
Test intervals and what a breathing air test report should show
HSE does not set a single calendar interval that applies to every spray booth. It expects the frequency of breathing air testing to come out of a risk assessment of the actual system: what compressor feeds the hood, how the air is filtered and monitored, how many hours a week the booth runs, and what has changed since the previous test. A booth running continuously through the week normally needs testing more often than one used for occasional touch-up work, and any change to the intake position, the filter train or the compressor itself resets that assessment, because a test result only describes the system as it stood on the day the sample was taken.
What decides the interval in practice
The person setting the interval, usually the competent person carrying out the test or advising on it, weighs how the air is used against how the equipment is maintained. Filters that are changed on schedule and a compressor intake sited well away from exhaust and solvent fumes both reduce the risk between tests; a system that has had a filter failure or a change of location deserves a shorter gap before the next check, whatever the previous interval had been.
What a test report should show
A test report is only useful if it lets you check the result against the standard yourself. Look for each of the following on the report before filing it:
- the sample point and the date the sample was taken
- the standard tested against, stated as BS EN 12021:2014
- a measured figure for each contaminant it covers, carbon monoxide, carbon dioxide, oil and water, set against that standard's limit
- a clear pass or fail against each of those limits
- the equipment or method used to take the measurement
- the name and qualification of the person or body carrying out the test
None of this decides on its own whether a particular hood and compressor combination complies with the Pressure Systems Safety Regulations 2000 or with HSE's expectations for breathing air. That judgement rests with a competent person assessing the system as installed, using the test report as one part of the evidence.
Getting a test report on your breathing air
A test tells you whether the air reaching the hood meets the limits for carbon monoxide, carbon dioxide, oil and water. Comparing quotes from more than one tester means comparing what each one actually checks and how often.