Two nine-year-old Jackson girls suffered carbon monoxide poisoning while hanging behind an inboard ski boat on Jackson Lake on Tuesday afternoon.

Park emergency medical providers responded to the 4:30 p.m. emergency call from Colter Bay Marina.

Park officials say that a family, and two young guests from another family, was out on the lake for an afternoon outing. The two girls took turns holding onto a swim platform attached to the back of a ski boat near the exhaust ports while being pulled across the water.

Both girls lost consciousness and slumped down behind the platform.

The National Park Service said in a news release that the activity is called "teak surfing" and is potentially deadly. The service says that levels of carbon monoxide in teak surfing are more dangerous due to the boating environment because they can lead to drowning.

Carbon monoxide-an odorless, colorless, and tasteless gas-is a leading cause of accidental poisoning deaths each year in the United States. According to the Centers for Disease Control, approximately 500 people perish annually due to carbon-monoxide poisoning. Symptoms of carbon-monoxide poisoning may include severe headache, dizziness, confusion, nausea, fainting, and death. Low levels can cause shortness of breath, slight nausea, and a mild headache.

"This incident serves as a harsh lesson that a seemingly innocent activity can actually be quite dangerous," said Superintendent Mary Gibson Scott. "We are so relieved that these two young ladies were revived, and that this incident had a positive ending for the families involved."

The young girls were transported to St. John's Medical Center where they received a highly concentrated oxygen treatment and were released from the hospital.