Lake Powell, UT-Sick children aboard a Lake Powell houseboat – including a vomiting baby who appeared to be having a seizure – were a family’s first clue something was wrong, emergency personnel say.

Before the Tuesday night ordeal ended, 21 people aboard the boat were treated for carbon monoxide poisoning in what one longtime Lake Powell physician’s assistant called the largest case of carbon monoxide poisoning he had seen.

None of the victims died or required transportation to a hospital. It was the third such poisoning in less than a week at Lake Powell, and the cases have spurred authorities to again warn boaters about dangers posed by the colorless, odorless gas.

On Saturday, a 7-year-old girl from Arizona drowned after inhaling carbon monoxide behind a cabin cruiser. Earlier Tuesday, three people who had been running a generator to power an air conditioner aboard a houseboat went to Dangling Rope Marina complaining of carbon monoxide poisoning symptoms, said Marianne Karraker, spokesperson for National Park Service.

The trio did not require treatment, she said.

The victims in Tuesday’s mass poisoning were among 40 people from Utah, Idaho and Texas who were aboard the boat for a family reunion, said Steve Luckesen, a National Park Service ranger at Bullfrog Marina.

The boat was in a canyon near Bullfrog Marina. Children were sleeping in the rear of the boat next to an open window, while a generator there was running to power the air conditioning, Luckesen said.

Rangers received a call at 10:45 p.m. saying there were five children on the boat who were having trouble breathing and one child was receiving CPR.

“The one baby was being really fussy, and the baby vomited and its eyes rolled back in its head like it was having a seizure,” Luckesen said.

The family brought the children to the marina in a speedboat. Then rangers received word there were others on the boat who also were showing symptoms of carbon monoxide poisoning, Luckesen said.

Eventually, 36 people went to the marina’s medical clinic for an examination, said Craig Humes, a physicians assistant at Bullfrog medical clinic. Of those, 21 received treatment, Karraker said.

Humes said no one was severely poisoned. Their symptoms, he said, were headache, nausea and vomiting. The last sick person left the clinic after receiving oxygen for four hours, he said.

The group returned to their boat, Luckesen said.

The number of victims taxed the marina’s medical clinic, which has three beds and typically does not have a doctor present. The family members were lined up outside the clinic waiting for oxygen treatment, Humes said.

Humes credited park rangers and staff of an area medical helicopter for helping clinic personnel.

”I’ve been in the clinic here for 25 years and this is the largest group I’ve ever” treated for carbon monoxide poisoning, Humes said.

The worst-poisoned person had 16 percent of hemoglobin saturated with carbon monoxide, Humes said. Symptoms can start to show at 7 percent and death can occur at 25 percent, depending a person’s age and health.

“If the baby hadn’t alerted everybody by having the first symptoms,” Luckesen said, “and [without] the mother waking everybody up we probably could have lost some of the children.”

Megan Evans, the child who died Saturday, showed carbon monoxide saturation of 65 percent, said her father, Matthew Evans. Her friend, Kayleen Tubbs, also 7, was found unconscious near Megan with a saturation of about 26 percent, he said. Tubbs recovered with oxygen treatments.

In recent years, rangers at Lake Powell have tried to warn the public of dangers posed by carbon monoxide emitted from boats. Houseboats, which have engines for propulsion and generators for electricity, have been the biggest source of carbon monoxide accidents.

There are signs at the lake and literature is given to visitors as they arrive, warning about carbon monoxide. Humes said news media have helped educate the public, too, and he does not know how else to spread the message.

“It’s just sometimes you can take folks to the water trough but they won’t drink,” Humes said.

Humes said people who suffer poisonings from houseboats are usually swimming near or sitting on the rear of the boat, where the engine and generator spew exhaust.

In many Lake Powell fatalities, the poisoning often caused the victim to lose consciousness and drown. But in some cases the poisoning was severe enough to kill the victim.

Poisonings inside boats are less common. In the family poisonings Tuesday, Humes said, the boat was sitting still and inside a canyon where there were no winds, which helped the exhaust accumulate.