PRINCIPLES OF TOXICOLOGY - Biology East Borneo

546 CONTROLLING OCCUPATIONAL AND ENVIRONMENTAL HEALTH HAZARDSdeliver/scavenging nosepiece. The nosepiece should seal against the face of the wearer to minimizeloss from around the face to facepiece seal. A prototype local exhaust ventilation system was developedfor use in the dental operatory. With proper positioning it was found to reduce exposure levels to nitrousoxide to below 25 ppm (procedure TWA). Further studies indicated that positioning of the ventilationsystem is critical. The total volumetric flow rate must be kept to less than about 600 cfm (cubic feetper meter) to prevent backflow down chimneys in home-type offices, while the capture velocity mustbe kept above 80 fpm at the mouth to efficiently capture nitrous oxide emitted from the mouth.Fiberglass Layup OperationMany of the boats, golf carts, and even some of U.S. cars are made from fiberglass, which reinforcesa polystyrene outer coating. The process involves spraying a polyester resin in a styrene monomersolvent, called the gel coat, onto a mold. The spray gun mixes the resin/solvent at the tip with MEKperoxide, a catalyst used to hasten the polymerizing reaction. The gel coat is allowed to dry and thenfiberglass is applied along with a mixture of polyester resin and styrene monomer, MEK peroxide, andacetone (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, whichrotates around the opening to cut the long fiberglass strands. After the fiberglass mixture dries, theproduct is separated from the mold, and surfaces are smoothed by sanding. Some pieces require actualcutting and grinding to provide the proper fit.In one such operation, the gel coat sprayer was located in a separate room with an exhaust fanlocated in the wall. The sprayer wore a full-body covering, synthetic rubber gloves, and an air-suppliedhood 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 of100 ppm. An evaluation of the wall fan indicated that it had an average exhaust rate of 1000 cubic feetper minute (cfm) and produced 17 air changes per hour, which should have been more than adequateto reduce levels in the room to well below 50 ppm. However, when the air flow patterns in the roomwere observed with smoke tubes, the majority of flow into the room was observed to come from adoorway at the end of the room, which did not dilute the air in the sprayer’s breathing zone; the flowof fresh air was short-circuited.Recommendations for this area included moving the wall fan to an area farther from the side doorand 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; andinstructing the sprayer to stay between the fan and the front door, spraying toward the fan so that theoverspray would not be pulled back into his breathing zone. The company decided not to accept therecommendations, since the operator was wearing a supplied air respirator.Airborne exposure of the three chopper gun operators ranged from 65 to 103 ppm. Exposurevariation was caused by the individual work practices and the location relative to the vane axial fanlocated on one side of the spray area, which workers used to cool themselves. An exhaust ventilationsystem 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 virtuallyno air movement attributable to the ventilation system. Spraying was done eight feet from theventilation hood, so there was little chance that air contaminants were being moved out of theemployee’s breathing zone. Visualization of the air movements with the smoke tubes indicated theairflow was across the work area from the floor fan toward an open window. The workers nearest thewindow were exposed to the higher levels. In addition to the air exposures, one chopper gun operatorwas observed to have reddened hands, apparently caused by the defatting action of acetone and styreneon the hands. Further investigation revealed that acetone was used to clean her hands of overspraymaterial.To reduce exposures in the layup area, the ventilation system was remodeled. A larger fan wasinstalled to increase the airflow, the hood was angled to have greater width at the bottom than at the