In June 2015, 15 year old Sarah Pool was hanging on to the platform at the back of a boat on Lake Travis in Austin, Texas, where she was attending a church wakeboarding camp. Unbeknownst to her, she was inhaling carbon monoxide fumes from the boat’s exhaust as the boat idled. She slipped under the water, unconscious, never to surface again, according to the local newspaper The Statesman. (Two other girls, who were wearing life jackets, survived.)

Sadly, Pool was not the first person to succumb to carbon monoxide (CO) poisoning on a boat.

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In the early 1990’s, marine patrol officer Wes Dodd and other first responders rushed to a crash scene in Antioch, California. Dodd remembers finding four people unconscious on the deck of the boat, which had plowed into a levee. The two men and two women were out cold. Their lips were blue.

The cause of the crash became evident after teams rushed the victims to a nearby hospital. The four had CO poisoning, which contributed to the crash, says Dodd, who owns Arizona-based Marine Accident Investigation. Dodd has served as an expert witness after investigating hundreds of boat accidents. In that case, all four victims survived.

According to the Centers for Disease Control and Prevention, more than 800 boat-related carbon monoxide poisonings have been identified in 35 states since the agency started collecting data 15 years ago. Of the poisonings, 140 were fatalities.

The “station wagon effect”

Newer boats usually follow stricter U.S. Coast Guard standards to help reduce carbon monoxide emissions. But carbon monoxide emissions on older craft pose a particular hazard, Dodd said.

Another cause of CO poisoning, says Dodd, may be the design of boat sterns. The aerodynamics at the back of the boat, especially when the craft is idle or moving slower than 5 mph, can allow for the buildup of this deadly gas in that area. “It’s called the station wagon effect,” Dodd said. “[The CO] builds up and builds up — and you can’t see it or smell it.”

People at the stern, particularly those who sit on or ride on the platform that allows easy access to the boat from the water, are usually exposed to the highest levels of carbon monoxide, adds Phil Odom, another boat accident investigator and owner of H20 Investigations in Boerne, Texas.

Holding on to the platform from the water while the boat is moving, a practice called teak surfing, also increases exposure and risk, says Odom.

“As they teak surf or ride on the platform on the back of the boat, the wind creates an eddy effect, causing the gases to circulate in a clockwise rotation into the rear of the boat,” Odom explains. “It’s a lot more of a widespread problem than people think it is.”

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Bigger boats, bigger danger?

The CDC warns the risk of CO poisoning may be greater for people on larger boats, such as house boats, which rely on generator systems that vent at the rear of the craft. Carbon monoxide levels on these boats can be more intense as gases intensify in the air space beneath the stern or near the swim deck. “CO that builds up in the air space beneath the stern deck or on and near the swim deck can kill someone in seconds,” writes the CDC on its website. In fact, 300 of the 800 reported CO poisonings occurred on houseboats, with more than 200 of those caused by generator exhaust.

How to stay safe

Carbon monoxide poisoning symptoms include headache, dizziness and nausea, but many victims succumb to the gas before any warning signs appear, Odom says. The best way to avoid accidental exposure is to never let the gas build up in the first place.

Experts and the CDC recommend the following precautions:

  • Never ride on the back of a boat or platform, particularly when the craft is moving slowly or is stationary.
  • Shut off the engine and generator when the boat isn’t moving or people are in the water nearby.
  • Install a carbon monoxide detector in the cabin area.
  • Educate passengers on the signs and symptoms of carbon monoxide poisoning.
  • Follow Coast Guard recommendations and wear a life jacket at all times.
  • Dock, beach or anchor your craft at least 20 feet away from the nearest boat running its engine or generator.

Some states are taking carbon monoxide poisoning by boat more seriously than in the past. In California, Oklahoma and Tennessee, the states passed laws that outlaws teak surfing, Odom said.

Dodd adds that boaters generally need more education about the danger of carbon monoxide.

“When you get your first car you have experience driving it and a driver's license,” Dodd said. “You can buy a boat with no experience. (The dealer) may show you how to operate it but you don’t necessarily have any knowledge of the rules of the road.”

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Ronald Agrella is a freelance writer and former editor of The Boston Globe’s Boston.com.