Carbon-monoxide (630080) (CO) levels produced by operating a gas powered pressure washer in enclosed spaces were measured. An experiment to simulate CO exposures generated by such equipment in confined structures was conducted inside a 25.5x28.5 foot garage using a gasoline powered pressure washer. The garage was ventilated passively by a system consisting of a 12 inch (in) diameter turbine vent located near the peak of the roof, four individual 4in diameter vents, and a 2in wide soffit vent. CO exposures during operation of the washer were monitored with the passive ventilation system sealed off with plastic and all doors and windows closed (worst case scenario) and with the passive ventilation system open and all doors and windows fully open (best case scenario) using a precalibrated gas detector/data logger and Draeger gas data loggers. Under the worst case scenario, the CO concentrations in the center of the garage increased to more than 200 parts per million (ppm) within 5 minutes and reached 1,500ppm, the upper range limit of the monitoring instrument, within 19 minutes. At the garage perimeter, CO concentrations also increased to more than 200ppm within 5 minutes and to 1,500ppm within 19 minutes. Under the best case scenario, CO concentrations at the center of the garage increased to +200ppm within 3 minutes and peaked at 658ppm within 12 minutes. At the perimeter, CO concentrations increased to more than 100ppm within 3 minutes and peaked at 203ppm within 23 minutes. The authors conclude that acutely toxic CO concentrations, above the 200ppm NIOSH limit, can be quickly generated by a gasoline powered pressure washer operating indoors.