During the period of 1990 to 2000, 111 carbon monoxide (CO) poisoning cases occurred on Lake Powell near the border of Arizona and Utah. Seventy-four of the poisonings occurred on houseboats, and sixty-four of the poisonings were attributable to generator exhaust alone. Seven of the 74 houseboat-related CO poisonings resulted in death. In February 2001, the National Institute for Occupational Safety and Health (NIOSH) conducted an initial evaluation of an engineering control installed on a houseboat generator to reduce the hazard of CO poisonings from the generator exhaust. The control consisted of a water separator and an exhaust stack that extended 9 feet above the top deck of the houseboat outfitted onto a gasoline-powered generator. This previous investigation, conducted at Lake Powell, showed substantial reductions (greater than 99%) in the concentration of CO on and around the swim platform of a Lakeview houseboat retrofitted with the "dry stack" generator exhaust system. The Lake Powell evaluation was conducted on a single stationary boat docked under moderate ambient temperature and wind conditions. The current study investigated the effect of additional variables on the performance of the dry stack exhaust system. These variables included elevated ambient temperature, boat in motion, and the effect of tying two or more boats together. Alternative configurations including side and rear exhaust were also tested for comparison with the dry stack. While the boat was stationary, CO concentrations on the lower rear deck of the houseboat, averaged approximately 3 ppm with the dry stack configuration, 2 ppm with the wide exhaust configuration (exhaust terminating at starboard side) and 285 ppm for the rear exhaust configuration (exhaust terminating at transom underneath swim platform). Exhausting the generator through the dry stack or side exhaust resulted in a reduction of approximately 99% when compared to the rear exhaust. Peak concentrations on the upper deck of the houseboat exceeded the ACGIH excursion limit of 125 ppm for the side and rear exhaust configurations, but were dramatically lower for the stack exhaust. The relatively low CO concentrations measured on the lower rear deck with side exhaust were surprising because previous NIOSH investigations have shown dangerously high CO concentrations (several hundred ppm) under similar conditions. When the boat was in motion (boat underway), CO concentrations were much higher due to the CO generated from the propulsion engines. For the dry stack, average CO concentrations on the lower rear deck of the houseboat ranged between 18-87 ppm compared with 175 ppm and 129 ppm for the side and rear exhaust, respectively. Finally, when 3 boats were tied together, only the dry stack performed well. Average CO concentrations on the lower rear decks of the houseboats ranged from 6-14 ppm, compared with 104-777 ppm for side and rear exhaust configurations. Although average concentrations provide for a good comparison between generator exhaust configurations, the peak exposures provide information more relevant to the important health hazards. In the current study, CO concentrations exceeded the NIOSH Immediately Dangerous to Life and Health (IDLH) concentration of 1,200 ppm on the swim platforms of several boats while tied together and exhausting the generators through a combination of rear and side exhaust ports. Peak concentrations approaching the IDLH were also measured at the swim platform for the rear-exhausted boat while stationary and for the side-exhausted boat while in motion. The concentration of CO inside the exhaust pipe of a generator during normal operation averaged 81% (81,000 ppm). This concentration is over 67 times greater than the IDLH. Therefore, it is important to direct the generator exhaust away from the water level and any occupied region of the boat. The results of this and prior NIOSH studies indicate that houseboats retrofitted with an exhaust stack that extends well above the upper deck will greatly reduce the risk of generator related CO poisoning and possible death to individuals on or near the houseboat.