RR496 - Isocyanate exposure, emission and control in small motor vehicle repair premises using spray rooms: Phase 1
A mock up spray room was constructed within the HSL spray booth with dimensions, ventilation conditions, extraction
rates etc set to represent typical MVR spray room conditions. A robotic sprayer was used to simulate paint spraying of
car parts under a range of conditions; including spray orientation relative to the extraction duct, spray gun type,
ventilation (air in) set up and extraction rates. Real and near time monitors were used in conjunction with the HSL
standard method for airborne isocyanate monitoring (MDHS 25/3).
The main findings were:
- High isocyanate (NCO) levels (~ thousands of µg NCO/m3 during the spraying period) arise in spray rooms during
- Airborne NCO levels were homogeneous throughout the spray room. Tracer gas studies confirmed this finding.
- Factors affecting the amount of airborne NCO are; gun type (eg HVLP give ~ 2 to 5x lower levels than
conventional types), gun condition and set-up, spray pattern and isocyanate formulation. Spraying in the
direction of the extract fan did not decrease airborne NCO.
- Airborne NCO took a significant time (~20+ minutes) to clear the spray room.
- The majority of air in the spray room was close to perfect mixing (tracer gas experiments) but short-circuiting
(ie inlet air that is extracted without mixing with the main body of air in the room) occurred. This agrees with
HSL/HSE field observations.
- Tracer gas studies found that the clearance time was proportional to the air-flow rate. If this is the case for spray
rooms in general, then the clearance rate may be estimated by calculation from the perfect mixing equation.
- The addition of false walls and filters to the spray room did not decrease airborne NCO levels in the room but
the filters did remove most (94–98%) of the NCO from the vented air and so reduced the risk of re-circulation of
the NCO mist back into the spray room, adjacent workplaces and the environment.
- The near time monitor (paper tape reader) underestimated significantly (~10x) the amount of airborne NCO but
both the real time (photo-ionization detector) and near time monitors (paper tape reader) gave clearance times
that were comparable with the HSL standard method.
- Spraying solvent through the guns to clean them produces high levels of airborne isocyanate (~ thousands of
µg NCO/m3) and should not be undertaken unless full control procedures are in place.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any
opinions and/or conclusions expressed, are those of the author alone and do not necessarily reflect HSE policy.
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