Boats puff up outdoor carbon monoxide risk.

Following a series of deaths and hospitalizations from carbon monoxide poisoning on an Arizona waterway, medical investigators have established that large congregations of motorboats can produce enough of the invisible, odorless gas in open air to be hazardous to people.

Investigations along Bridgewater Channel at Lake Havasu City, Ariz., began after two deaths and eight hospitalizations occurred among carbon monoxide--poisoned vacationers between 1997 and 2002. An additional fatality and four serious poisonings have happened since then on boat decks or in nearby water.

During holiday weekends in September 2002 and May 2003, researchers from the Havasu Regional Medical Center, the Centers for Disease Control and Prevention, and other cooperating organizations sampled air along the boat-choked channel and conducted breath tests among vacationers and workers in the vicinity. The breath tests were used to estimate how much carbon monoxide each study participant's blood contained.

In the afternoon, when carbon monoxide was thickest in the air surrounding the channel, the gas was also most abundant in people's blood. Average estimated hemoglobin concentrations of the gas rose from a morning concentration of 1 to 11 percent for nonsmokers who were on boats or wading in the water. Nonsmokers working near the channel had peak carbon monoxide concentrations of 6 percent. Smokers typically had carbon monoxide readings 1 to 2 percentage points above those of their nonsmoking cohorts.

According to World Health Organizations recommendations, carbon monoxide concentrations in blood hemoglobin shouldn't exceed 2.5 percent.

In addition to the abundance of boats, calm air and high temperatures contribute to elevated carbon monoxide concentrations at the Bridgewater Channel, the researchers report in the April 23 Morbidity and Mortality Weekly Report.--B.H.