For years, snorkeling in Hawaii has been by far the leading cause of tourist deaths, and snorkel-related drownings with no signs of distress have often been a mystery. A new study offers some reasons why.

A subcommittee established in 2019 by the state Department of Health recently completed the Snorkel Safety Study, funded by the Hawaii Tourism Authority.

The study rebuts the common belief that snorkeling-related deaths are just from water inhalation, according to the project director, Carol Wilcox. She said it turns out the cause of most snorkel-related fatal and near-fatal drownings is due to low levels of oxygen in body tissues prompted by excess fluid buildup in the lungs, otherwise known as hypoxia induced by rapid onset pulmonary edema, or ROPE.

The study also makes the connection between snorkel-related drownings and a cardiac condition that restricts the heart’s ability to fill with blood in between each beat. Wilcox said this cardiac condition helps explain why middle-aged men are more likely to be affected.

Another predisposing risk factor the study discovered is breathing through a snorkel that has too much resistance. This causes an increase in negative pressure in the lungs and can also result in ROPE.

The study says the ROPE diagnosis accounts for the lack of distress signals in many snorkel drownings as it causes muscle fatigue and loss of consciousness, and can also be triggered by a heart condition.

By contrast, when drowning due to the inhalation of water, there are often obvious signs of struggle. The main difference is the source of the excess fluid.

In ROPE, the excess fluid is not inhaled into the mouth and windpipe — rather, it is a buildup of bodily fluids that seeps into the lungs, reducing the ability to deliver oxygen to the body, leading to a lack of sufficient oxygen.

Unless educated and familiar with these two diagnoses, they can be difficult to differentiate.

Though it is unclear how many deaths were due to ROPE, according to the Department of Health, there were 204 snorkeling-related deaths from 2012 to 2021. Of those, 184 were tourists.

John Titchen, the chief of the Honolulu Ocean Safety and Lifeguard Services Division, says one of the obvious reasons that more tourists die from snorkeling is their unfamiliarity with Hawaiian waters and with snorkeling in general.

“A lot of people who visit here tend to treat our beaches and the ocean like you would an amusement park … the waters are tranquil and crystal clear, and they think it looks easy,” Titchen said. “It seems as though people find a way to get into trouble snorkeling all around the island.”

The study authors surveyed people who had survived near-fatal drownings.

Wilcox said 71% of the 131 survey participants in the study reported calm conditions, 87% reported good visibility, 63% reported at least sufficient snorkeling experience, and 88% reported at least sufficient swimming ability.

According to Wilcox, only one participant reported experiencing the inhalation of water. All others experienced characteristics of ROPE, including Wilcox herself.

Almost two decades ago, she went for a swim with her fins and a new snorkel mask in Waikiki and thought she wasn’t going to come back. A frequent swimmer and snorkeler, Wilcox said she swam out and suddenly became short of breath and lost strength in her arms.

“I started to hear this huge heartbeat in my ears, like deep drums … I knew I was going to die,” Wilcox said, adding that she survived because she was pulled out of the water and given oxygen.

Not only was Wilcox using an unfamiliar snorkel which may have contributed to her near-fatal drowning, but the incident occurred a day after Wilcox returned to Hawaii from Canada, which has an average direct flight time of around seven hours.

The Snorkel Safety Study is the first to hypothesize that long-haul air travel under certain circumstances may be a significant predisposing factor to snorkel-induced ROPE, which could also help explain why so many snorkeling-related fatalities are predominately tourists.

Wilcox said that exploring this possibility is especially important in Hawaii, as most incoming visitors have spent a minimum of five hours on an airplane at a cabin pressure equivalent to up to 8,500 feet elevation.

Dr. Philip Foti, a Kailua pulmonologist and the study’s principal investigator, said there have been very few studies that have been done “to determine both the effects of high altitude on those with certain types of lung disorders, as well as in those without any.”

Other than identifying traveling by plane as a possible predisposing risk factor, the study is also the first to identify the added risk of snorkeling from a boat.

“It takes a lot of effort to jump into water where you cannot touch bottom,” Wilcox said. “Adjusting your mask and snorkel and fins, and then swimming away … chances of survival are better if the incident happens where the snorkeler can touch bottom.”

What the study was unable to conclude as a risk factor was the use of the full-face snorkel mask, as only four out of the 50 masks the study group tested were full-face masks.

Foti said the study determined that whether a mask was high or low resistance was very unlikely to be confirmed without testing on land.

“The safest option is to use a mask and snorkel that doesn’t have various modifications of the tip in order to keep water from entering. Use a straight snorkel and mouth-piece that are both adequately sized, and make sure there’s no faulty or narrow valves,” Foti said.

“While these findings are compelling, they are also preliminary,” Wilcox said. “More rigorous studies are needed to verify the findings set forward in this report. Until that happens, agencies will be hesitant to adopt these findings into their programs.”

Read the full study below.