Abstract: Employees were grinding concrete inside a 20 ft by 30 ft block-type building using an Edco 8 HP gasoline grinder, serial #16860. The double doors at the entrance were left open; this was the only ventilation. The employees were not wearing respiratory protection. They had been grinding in the building from 8:00 a.m. to 2:00 p.m. with a 30 minute lunch break. The employees ground at 15 to 20 minute intervals. After about 1 1/2 hours, they became dizzy. Employee #2 passed out and Employee #1 passed in and out of consciousness. Both were found to have carboxyhemoglobin levels of 22 percent, the equivalent of 200 parts per million (ppm) exposure to carbon monoxide--which has a ceiling of 200 ppm and a time weighted average of 35 ppm. Both employees were nonsmokers.