Quick answer: Spray painting is the one workshop activity where most people know they need respiratory protection. The problem is that most of the protection being used is wrong for the hazard. A dust mask -- FFP2 or FFP3 -- filters particles. It does not filter chemical vapours. 2K automotive paint, 2K primer, 2K lacquer and many industrial coatings contain isocyanates -- the most common cause of occupational asthma in the UK. Isocyanates are gases and vapours at the point of spray application. They pass straight through a dust mask. The correct protection for isocyanate-containing products is an air-fed breathing hood or a full-face respirator with the correct combination filter. No exceptions. This is the one workshop activity where getting the PPE wrong causes irreversible harm very quickly.
Watch any automotive restoration or custom paint channel on YouTube and the respiratory protection during spray painting varies enormously -- from full air-fed hoods in professional bodyshops, through half-face respirators of varying specification, to dust masks, to nothing at all. The creators who know what they are doing use air-fed protection. The ones working in garages and home workshops frequently do not -- and neither do most of their viewers who are inspired to try the same technique.
Spray painting is the one area where the hazard is visible -- you can see the mist -- and people respond to the visible hazard by wearing a mask. The problem is that the visible mist is particles. The invisible component -- the vapour phase -- is the chemical hazard. And a particle filter does nothing about a chemical vapour.
What isocyanates are and why they matter
Isocyanates are a family of highly reactive chemical compounds used as the hardener component in 2K (two-component) paints, primers, lacquers and coatings. They react with the resin component to form a hard, chemical-resistant coating. The same reactivity that makes isocyanate-cured coatings so durable makes isocyanates highly reactive with biological tissue -- particularly the delicate mucosal surfaces of the airways.
Isocyanates are the most common cause of occupational asthma in the UK. Occupational asthma caused by isocyanate sensitisation is:
- Immediate -- sensitisation can occur after a single significant exposure event, not just after years of cumulative low-level exposure
- Irreversible -- once sensitised, the asthmatic response to even trace isocyanate exposure is permanent. You cannot desensitise. The condition does not resolve on its own.
- Disabling -- for a vehicle bodywork professional or spray painter, isocyanate sensitisation is a career-ending diagnosis. They can never work with 2K products again.
The HSE WEL for the most common isocyanate used in automotive coatings (HDI -- hexamethylene diisocyanate) is 0.02 mg/m3 as an 8-hour TWA and 0.07 mg/m3 as a 15-minute STEL. These are among the lowest WELs of any commonly encountered substance, reflecting how hazardous isocyanates are at very low concentrations.
Which products contain isocyanates
Isocyanates are present in:
- 2K automotive topcoats, basecoats and lacquers
- 2K automotive primers and fillers
- 2K industrial and commercial coatings
- Some 2K floor coatings and concrete sealers
- Some polyurethane spray foams
- Some adhesives and sealants
If a paint product has two components that are mixed before use and the product is described as polyurethane or isocyanate-cured, it very likely contains isocyanates. Check Section 3 of the SDS -- isocyanates will be listed as components. Section 8 will specify the required respiratory protection.
Single-component (1K) paints, water-based paints and most rattle-can aerosols do not contain isocyanates. The hazard is specifically in 2K hardener-cured products.
Why a dust mask fails for isocyanate protection
An FFP2 or FFP3 dust mask is a particulate filter. It works by physically capturing particles on a fibrous filter medium. It has no mechanism for capturing gaseous or vapour-phase molecules. Isocyanates are present in the vapour phase at the point of spray application -- the concentration of isocyanate vapour in a spray booth or garage during 2K paint application can reach or exceed the WEL within minutes.
An FFP3 mask worn during 2K spray painting provides essentially no protection against the isocyanate vapour hazard. It may filter the visible spray mist particles -- but the vapour that causes sensitisation passes straight through.
The correct protection for isocyanate-containing products
Air-fed breathing hood (the professional standard)
A compressed-air-fed breathing hood supplies clean filtered air from outside the spray environment directly to the face and breathing zone. It provides the highest protection factor of any respiratory protection and eliminates all reliance on face fit. In a professional bodyshop, air-fed hoods are the required standard for 2K spray work.
Full-face respirator with combination filter
A full-face respirator with the correct combination cartridge -- organic vapour (OV) plus P3 particulate filter -- provides protection against both isocyanate vapour and spray mist particles. The full-face seal is important -- a half-face respirator leaves the eyes exposed to isocyanate vapour, which can also sensitise through the ocular mucosa. The filter cartridge must be rated for isocyanates specifically -- some OV cartridges are not effective for the specific isocyanate compounds used in automotive coatings.
Full-face respirators must be face-fit tested for each individual wearer. The protection they provide depends entirely on the seal between the face piece and the wearer's face. A respirator that fits one person does not necessarily fit another.
What is not acceptable
- FFP2 or FFP3 dust mask -- no vapour protection
- Half-face respirator with P3 filter only -- no vapour protection
- Half-face respirator with OV/P3 filter -- provides vapour protection but exposes eyes to isocyanate vapour
- No protection -- any exposure is potentially sensitising
Ventilation -- the engineering control that reduces but does not eliminate the need for RPE
Ventilation -- either a spray booth with cross-flow or downdraft airflow, or forced ventilation of a garage or workshop during and after spraying -- reduces isocyanate concentration in the workspace by diluting and removing contaminated air. Good ventilation reduces the concentration of isocyanate vapour significantly. It does not reduce it to zero. Respiratory protection is required in addition to ventilation, not instead of it, for any 2K spray application.
Part of the series: What is Really in the Air in Your Workshop
Series hub | Metal grinding and welding fume | Woodworking and timber dust | Resin and fibreglass | Concrete and stone
Related pages
M-Class extraction | H-Class extraction | COSHH Regulations 2002 | HSE EH40
