Until recently carbon monoxide poisonings were thought to occur in enclosed, poorly ventilated areas. However, open-air cases of poisoning have recently been reported including exposures from exhaust from various kinds of watercraft including houseboats, cabin cruisers and ski boats. Unlike automobiles, boat engines do not have mechanisms to reduce carbon monoxide emissions. The density of watercraft and the large numbers of people using the channel creates the opportunity for individuals to be exposed to excessive carbon monoxide emissions. This exposure investigation examines the extent of carbon monoxide exposure in recreational boaters in the Rotary Beach area near the London Bridge in Lake Havasu City, Arizona during the Memorial Day Holiday in 2003. The objective of the investigation is to determine whether a public health hazard from carbon monoxide exposure exists in an area heavily used by recreational boaters.