PRINCIPLES OF TOXICOLOGY

546 CONTROLLING OCCUPATIONAL AND ENVIRONMENTAL HEALTH HAZARDS
deliver/scavenging nosepiece. The nosepiece should seal against the face of the wearer to minimize
loss from around the face to facepiece seal. A prototype local exhaust ventilation system was developed
for use in the dental operatory. With proper positioning it was found to reduce exposure levels to nitrous
oxide to below 25 ppm (procedure TWA). Further studies indicated that positioning of the ventilation
system is critical. The total volumetric flow rate must be kept to less than about 600 cfm (cubic feet
per meter) to prevent backflow down chimneys in home-type offices, while the capture velocity must
be kept above 80 fpm at the mouth to efficiently capture nitrous oxide emitted from the mouth.
Fiberglass Layup Operation
Many of the boats, golf carts, and even some of U.S. cars are made from fiberglass, which reinforces
a polystyrene outer coating. The process involves spraying a polyester resin in a styrene monomer
solvent, called the gel coat, onto a mold. The spray gun mixes the resin/solvent at the tip with MEK
peroxide, a catalyst used to hasten the polymerizing reaction. The gel coat is allowed to dry and then
fiberglass is applied along with a mixture of polyester resin and styrene monomer, MEK peroxide, and
acetone (to liquify the surface and allow the fiberglass mixture to adhere to the gel coat), through a
“chopper gun,” which is much like a spray-paint gun, except that it has an additional blade, which
rotates around the opening to cut the long fiberglass strands. After the fiberglass mixture dries, the
product is separated from the mold, and surfaces are smoothed by sanding. Some pieces require actual
cutting and grinding to provide the proper fit.
In one such operation, the gel coat sprayer was located in a separate room with an exhaust fan
located in the wall. The sprayer wore a full-body covering, synthetic rubber gloves, and an air-supplied
hood while spraying gel coat. His 8-h TWA exposure to styrene vapor outside the hood was 82 ppm,
which was below the current OSHA PEL and the then-current ACGIH TLV ® of 100 ppm. However,
at the time the ACGIH was in the process of lowering the TLV ® from 100 to 50 ppm, with a STEL of
100 ppm. An evaluation of the wall fan indicated that it had an average exhaust rate of 1000 cubic feet
per minute (cfm) and produced 17 air changes per hour, which should have been more than adequate
to reduce levels in the room to well below 50 ppm. However, when the air flow patterns in the room
were observed with smoke tubes, the majority of flow into the room was observed to come from a
doorway at the end of the room, which did not dilute the air in the sprayer’s breathing zone; the flow
of fresh air was short-circuited.
Recommendations for this area included moving the wall fan to an area farther from the side door
and closer to the spraying area, which would place the sprayer between the front door and the fan;
closing off the lower part of the side door to increase resistance of air movement in that direction; and
instructing the sprayer to stay between the fan and the front door, spraying toward the fan so that the
overspray would not be pulled back into his breathing zone. The company decided not to accept the
recommendations, since the operator was wearing a supplied air respirator.
Airborne exposure of the three chopper gun operators ranged from 65 to 103 ppm. Exposure
variation was caused by the individual work practices and the location relative to the vane axial fan
located on one side of the spray area, which workers used to cool themselves. An exhaust ventilation
system with a single slot about 3 feet from the floor was in place at the back of the layup area.
Observations with smoke tubes indicated that at more than one foot from the slot there was virtually
no air movement attributable to the ventilation system. Spraying was done eight feet from the
ventilation hood, so there was little chance that air contaminants were being moved out of the
employee’s breathing zone. Visualization of the air movements with the smoke tubes indicated the
airflow was across the work area from the floor fan toward an open window. The workers nearest the
window were exposed to the higher levels. In addition to the air exposures, one chopper gun operator
was observed to have reddened hands, apparently caused by the defatting action of acetone and styrene
on the hands. Further investigation revealed that acetone was used to clean her hands of overspray
material.
To reduce exposures in the layup area, the ventilation system was remodeled. A larger fan was
installed to increase the airflow, the hood was angled to have greater width at the bottom than at the