Abstract: Employees #1 through #8 were working for a company engaged in resin coating of porous castings on a job-shop basis. They began to experience symptoms of carbon monoxide exposure and were transported to the hospital emergency room. Blood-gas readings of between 7 and 24 percent were obtained, and Employees #1, #3 through #5, and #7 were admitted for treatment in a hyperbaric chamber. There were no permanent effects from the exposure. Back-calculation from the blood-gas readings suggest carbon monoxide exposures of between 50 and 233 ppm. The company was the only occupant in the single-story 100 ft by 100 ft building. Subsequent investigation determined that the source of the problem was a combination of a natural gas-fired boiler, two natural gas-fired heaters, and two propane-powered forklifts. There was a negative pressure in the building due to exhaust fans in the roof and side wall. The primary source of exposure was traced to combustion products from the boiler being sucked into the building by the exhaust fans.