One day in 2004, after flying home to Honolulu from Canada, Carol Wilcox went snorkeling at Sans Souci Beach, began to lose consciousness and nearly drowned, although she had not aspirated sea water through her snorkel or mask.

“After about eight minutes in the water, I was having trouble breathing, couldn’t hold onto the flagpole (on the reef), and was losing all of my muscular capability,” said the experienced snorkeler, who nonetheless managed to keep struggling shoreward and was rescued and then treated in a hospital emergency room.

Dr. Philip R. Foti, a Kailua private physician and pulmonologist, diagnosed Wilcox as having suffered hypoxia (asphyxiation) when her bodily fluids filled her lungs in a form of pulmonary edema known as rapid onset pulmonary edema.

The condition was not previously associated with snorkeling, which has been implicated in 206 Hawaii drownings — 189 involving visitors — from 2009 to 2018, according to the state Department of Health. During that period, snorkeling-related drownings nearly doubled in what officials called an “alarming” rise.

“People think snorkeling is benign, easy to do, but unfortunately it isn’t,” said Ralph Goto, a retired director of the Ocean Safety Division of the Honolulu Emergency Services Department who now directs operations for the Hawaiian Lifeguard Association. “Lot of times at Hanauma Bay you found people floating face-down and what caused it, we don’t know.”

In 2017, the DOH established a Snorkel Safety Sub-Committee that ordered a two-year “Snorkel Safety Study” be conducted. On Thursday, the study team, including Wilcox as project manager, Foti as lead investigator and Goto as administrator, released an interim report finding that “hypoxia induced by (rapid onset pulmonary edema) is the cause of some, probably most, snorkel related fatal and near fatal drownings” and “unravels the mystery” of such incidents.

In the report, Foti examined data collected by Honolulu Ocean Safety regarding types of equipment used in drowning incidents. Forty-­nine snorkels were tested, including four full-face masks, versions of which were worn in several recent drownings and have been banned by Snorkel Bob’s and some other Hawaii snorkel tour operators.

Foti determined that equipment with high air-flow resistance, which makes it harder to inhale air, may lead to ROPE.

“Resistance to inhalation sets off this negative pressure in your lungs,” Foti said, “and then your bodily fluids start to intrude into this really delicate mechanism. Your lungs start to fill up with fluid and very quickly, there’s no oxygen going into your blood stream.”

Full-face masks “may be a risk factor” in developing ROPE but posed “no inherent advantage or disadvantage” in air-flow resistance, the report concluded.

Goto noted the type of mask had been identified in only 16% of the incidents recorded, among which a full-face mask was worn in five drownings and a conventional two-piece mask and snorkel was worn in 11 drownings.

However, the report pointed out, full-face masks can’t be removed easily, even with quick-release features, and the mouthpiece can’t be “spit out” in urgent situations. Additionally, water can’t be cleared from the snorkel tube by exhaling forcefully, as with conventional gear; the snorkeler can’t dive beneath the surface safely; and “valve malfunction may lead to serious aspiration (of sea water).”

In general, simple snorkels produced least air-flow resistance, the tests showed.

Wilcox was not wearing a full-face mask when she nearly drowned, but her snorkel was tested and found to have high air-flow resistance.

For the next phase of the study, members of the public who have experienced trouble while snorkeling, or know someone who has, are asked to take an online survey at snorkelsafetystudy.com. Foti said he analyzed 36 survey responses for the interim report but more are needed.

Sixty-six percent of the survey respondents said they were using full-face masks when they ran into trouble. Eight-three percent of those respondents believed the masks contributed to their problems.

Foti also examined 98 Hawaii Medical Examiner’s Office autopsies and reports for drownings in 2017, 2018 and 2019 and interviewed individuals who experienced nonfatal drowning.

Higher risk factors included having an underlying cardiac condition known as left ventricle end diastolic pressure, and being a male age 50 or older.

Ocean inexperience, Foti said, could also be a factor in the higher rate of snorkeling drownings as well as ocean drownings generally among visitors, compared with residents.

There were 87 fatal drownings in Hawaii in 2019, comprising 53 visitors and 34 residents, according to statistics collected by Dan Galanis, epidemiologist with the EMS and Injury Prevention System Branch of the state Department of Health.

From 2015 to 2019, there were an average 48 drownings per year among nonresidents and 35 among residents — except in 2020, when deaths by ocean drowning in Hawaii plummeted by more than 50%, reflecting the plunge in tourism after the state’s pandemic lockdown in March, Galanis said.

From January through September, his preliminary data show a total of 23 Hawaii drownings, 11 among visitors and 12 among residents, with a projected total of 40 drownings comprising 19 visitors and 21 residents.

Four visitors and no residents drowned while snorkeling in the ocean during that period.

However, Goto said, the survey, most of whose 36 respondents to date have been Hawaii residents under age 55, brought home to him that many local oceangoers who are strong swimmers, like Wilcox, also drown or come close to it, and ROPE may be a reason.

With ROPE, often snorkelers lose strength and consciousness so rapidly they give no signs of distress, Foti said.

He added that ROPE was not the same kind of pulmonary edema one gets with blood clots on a plane or with a heart attack, but physiological effects of recent airplane travel, as was the case with Wilcox, might play a part in developing the rapid pulmonary edema.

“You need a lot of science on that to really see if there’s really a connection between long-distance travel and this,” Wilcox said, “but if there is, people would be well-advised not to go snorkeling within several days of a long-distance trip.”

Meanwhile, she stressed that people can provide needed information that can help save lives by taking the survey, which allows for anonymous participation if desired.

“I’m terrified,” Foti said. “You should be. Everybody should realize snorkeling is not a low-risk recreational activity to be taken lightly.”

Next steps, he said, include submitting the study for publication in peer­-reviewed journals and developing clear public education messages. The interim report’s advice includes:

>> Always snorkel with a buddy.

>> Choose snorkel devices thoughtfully; avoid constrictive mouthpieces, which may increase airway resistance.

>> Learn to use the simplest snorkel device safely before trying more complex models and before venturing to depths beyond one that allows standing with water at chest level.

>> If in doubt about your cardiovascular health, don’t go out.

>> Shortness of breath can be a sign of danger. Stay calm, stand up, remove snorkel, get out of water immediately.

The “Snorkel Safety Study” was funded by the Hawaii Tourism Authority and prepared in cooperation with the state Department of Health, Hawaiian Lifeguard Association, Honolulu Medical Examiner’s Office, the four county first responder and emergency medical service agencies and Friends of Hanauma Bay.

For more information, visit snorkelsafetystudy.com.

Researchers are asking the public to share snorkeling mishaps in an online survey at snorkelsafetystudy.com.