The Arizona Department of Health Services conducted a carbon monoxide exposure survey among recreational boaters at the Maricopa County Lake Pleasant Regional Park during the 2003 Labor Day weekend. To investigate the extent that recreational boaters are exposed to carbon monoxide from a variety of watercraft, ADHS measured exhaled carbon monoxide to determine the amount of carboxyhemoglobin (COHb) present in their blood. To determine whether a widespread public health hazard exists because of carbon monoxide exposure from watercraft, the ADHS health consultation, "Investigation of Carbon Monoxide Exposure, Rotary Beach at the London Bridge, Lake Havasu, Arizona, May 25-26, 2003", recommended conducting further biomonitoring studies at other recreational lakes in Arizona. ADHS asked the Agency for Toxic Substances and Disease Registry (ATSDR), under its Cooperative Agreement Program with ADHS, to interpret data from the Lake Pleasant survey and to determine whether carbon monoxide exposure is a public health hazard among recreational boaters at Lake Pleasant.