Under an interagency agreement with the United States Coast Guard, working in collaboration with an industry consultant, National Institute for Occupational Safety and Health (NIOSH) researchers evaluated carbon monoxide (CO) exposures on ten express cruiser boats from several manufacturers. The evaluated boats were new and included several different models. These boats had gasoline-powered propulsion engines and used gasoline-powered generators to provide electricity for onboard appliances. This study was performed for the U.S. Coast Guard to better understand how CO poisonings may occur on express cruisers, identify the most hazardous conditions, and begin the process of identify controls to prevent/reduce CO exposures. Boats were evaluated while stationary and at multiple speeds, ranging from 5 to 25 miles per hour. CO concentrations were measured by multiple real-time instruments, which were placed at different locations on the boats with overhead, enclosing canopies set at various configurations. Many of the evaluated boats generated hazardous CO concentrations: peak CO concentrations often exceeded 1,100 parts per million (ppm), while average CO concentrations were well over 100 ppm at the stern (rear). Two boats with a combined exhaust system (exhausting at the sides and underwater) had dramatically lower CO concentrations than any of the other evaluated boats (about 40% lower). Based on the results and observations made in this report, the following major findings are summarized below: 1. When the canvas is deployed and boat is underway, CO concentrations exceeded the immediately dangerous to life and health (IDLH) level near the swim platform for many of the evaluated boats. 2. The combination of travel at low speeds into the wind with the canvas fully deployed and no forward hatches, windows or front panels opened maximized the station wagon effect, pulling significant amounts of CO into the cockpit. 3. Different exhaust configurations have a major impact on how CO concentrations are entrained into the cockpit and other occupied areas. Accordingly, boats equipped with underwater exhaust exhibited significantly lower CO concentrations than vessels equipped with other exhaust designs. 4. CO concentrations are typically higher at the stern of the boat and become gradually lower toward the front of the boat. 5. Stationary smoke tests in the engine compartment showed satisfactory sealing of the bulkhead between the engine and adjacent compartments on all boats. Based on the preceding findings, the following recommendations are made to reduce CO concentrations on express cruisers: 1. Boat manufacturers should consider underwater exhaust that will significantly reduce CO concentrations inside the cockpit and other occupied areas compared to surface exhaust. 2. Because of the station wagon effect, some canvas configurations should not be used while boat is moving or propulsion and/or generator engines are running. 3. The possibility of adding force draft blowers into the cabin, creating a positive pressure to minimize potential CO intrusions, should be studied. Auxiliary blowers can be fitted and routed to ventilate the cockpit and swim platform areas in order to minimize negative pressure areas throughout the vessel. 4. Since properly sealed cabin doors directly influenced the CO concentration in the cabin area, door suppliers should be encouraged to develop better sealing methods and designs. 5. Windshield manufacturers should be encouraged to study the possibility of maximizing ventilation of occupied areas by improving the design of the center and side wings of the windshield. 7. Due care should be exercised when designing the powered ventilation system on the engine compartment, locating the air intake on the opposite side of the generator exhaust. Also, potentially moving the intake much farther forward on the vessel would help minimize the intake of CO exhaust into the engine compartment. 8. The development of cleaner burning engines (propulsion and generators) with catalytic converters should continue since they have the potential to greatly reduce CO concentrations to safer levels. 9. The American Boat and Yacht Council (ABYC) should examine their standards and emphasize ventilation problems that can lead to CO intrusions, taking a strong position against surface exhaust designs for propulsion engines.