They had been there only briefly. Their mother looked through a peephole in the swim deck just as one boy sank. Despite quick rescue, the boys could not be revived.

Autopsies found lethal levels of CO in their blood. Two good swimmers dying together by an exhaust vent convinced Baron that CO was causing similar deaths. That and press attention brought McCammon in to help investigate. She is an industrial hygienist with the National Institute for Occupational Safety and Health.

When she began testing CO levels in cavities under houseboat swim platforms, she found concentrations that could be lethal after brief exposure. In 2001, the Coast Guard issued an advisory on houseboats of similar design. Recalls were issued and boats were redesigned.

But McCammon said problems were not just with houseboats. She and Baron found CO poisoning was occurring with many other types of motorboats.

Another tragedy would bring more action. After graduating high school in 2001, Chad Ethington of Centerville was "teak surfing" behind a boat. That is holding onto the rear of a slow-moving boat and riding the wake. He was overcome by CO and sank — even though he was a water polo player who could tread water for an hour without a break.

McCammon said, "After the investigation of his death ... and related intense press coverage, we began to hear about several similar deaths" nationally. It would lead to warnings against teak surfing and bans against it in many states and by the Park Service.

But McCammon and Baron also found probable CO poisonings not just from teak surfing but among people merely riding or swimming near motor exhaust. Motorboat engines can emit the same amount of CO (caused by incomplete combustion of fuel) as up to 166 cars.

Baron and McCammon went back to re-evaluate all "drownings" at Lake Powell in recent years. They found that 48 percent of reportable boat-related "drownings" there between 1994 and 2004 "were CO poisonings first."

Extrapolating that to drownings nationwide, McCammon says CO could be causing hundreds of deaths a year.

But she said until testing for CO in blood of drowning victims becomes routine, "We will never know the true number of CO-related drownings that occur near boats. We are left to guess from data we do have."

But their work helped reduce CO deaths at Lake Powell. Chief ranger Mike Mayer said, "We haven't had a carbon-monoxide death since 2002. We've had poisonings, but people have recognized the symptoms" because of park signs, newspaper warnings and education — and were able to take life-saving action.

Early symptoms include irritated eyes, headache, nausea and dizziness, symptoms often confused with seasickness or having too much to drink.

David Harris, boating coordinator for the Utah Division of Parks and Recreation, said his agency has been trying to spread the word with brochures and even decals for boat owners to put at stern and helm to remind passengers of CO danger. Parents and relatives of victims have also appeared at boat shows to warn others.

But he says too many still don't realize the danger.

"We had a carbon-monoxide death last year on Utah Lake. A young college student was swimming behind a boat (where the engine was running). She hadn't been swimming there long, but went under."

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E-mail: lee@desnews.com