How a Snorkel Works, and the Failure Mode That Drowns People

A snorkel is a tube. One end goes in your mouth, the other stays above the water, and you breathe through it while your face points down. That is the whole mechanism, and it is why the equipment looks so simple.

The risk is not simple. Hawaii's Snorkel Safety Study, whose final report was published in June 2021, investigated why people drown while snorkelling and found something most guides do not mention: inhaled water is rarely the trigger. The mechanism is usually fluid accumulating in the lungs, and the resistance of the tube you are breathing through is one of the risk factors.

The physics, properly

Dead space

The air inside the tube does not leave when you exhale. You push it up the barrel, and on your next breath you pull it back down and rebreathe it. That volume is called dead space, and it is exhaled air: lower in oxygen, higher in carbon dioxide.

A longer or wider tube holds more dead space. This is the reason snorkels are not sold in three-foot lengths, and it is one of two constraints on how long a snorkel can be.

Why you cannot use a long snorkel

The other constraint is pressure. At depth, water presses on your chest with a force your diaphragm has to overcome to inflate your lungs, while the air at the top of the tube stays at atmospheric pressure. By about a metre down, the pressure difference is more than the respiratory muscles can work against for any length of time.

That is why snorkels are around 30 to 40cm and why there is no snorkel that lets you breathe at depth. Dive below the surface and you are breath-holding, with a tube full of water to clear on the way back up.

Inhalation resistance

Every constriction in the airway makes breathing harder: a narrow bore, a small mouthpiece, a tight bend, a valve. That extra work is trivial when you are sitting still. It is not trivial when you are also swimming, cold, and possibly anxious.

The Snorkel Safety Study lists degree of the snorkel's resistance to inhalation as the first of its three risk factors for SI-ROPE. Its equipment guidance follows directly: "Avoid constrictions in bore size or mouthpiece caliber, which may increase resistance to inhalation."

What the study actually found

This is the part worth reading carefully, because it changes what you should worry about.

SI-ROPE, Snorkel Induced Rapid Onset Pulmonary Edema, is fluid accumulating in the lungs during snorkelling. The study identified it as a common factor in snorkel-related drownings and near-drownings.

The three risk factors it names:

1. Degree of the snorkel's resistance to inhalation
2. Certain pre-existing medical conditions
3. Increased exertion

Among the near-drowning survivors it surveyed:

• Aspiration was rarely the trigger or even a factor
• Lack of swimming or snorkelling experience was rarely a factor
• Almost all events happened where the person could not touch the bottom
• 38 percent were using a full-face mask, and 90 percent of those considered it a contributing factor

The typical sequence the study describes:

1. Sudden shortness of breath, fatigue, loss of strength
2. Feeling of panic, doom, need for assistance
3. Diminishing consciousness

Full-face masks

A full-face mask covers the eyes, nose and mouth in one sealed unit and you breathe normally rather than through a mouthpiece. That is a genuine comfort advantage, particularly for people who dislike a mouthpiece or cannot maintain a seal on one.

The structural concerns are specific and worth stating plainly:

• You cannot spit out a mouthpiece, because there is not one. In a conventional setup, dropping the snorkel gets your face to open air in one movement.
• You cannot clear the tube with a sharp exhalation. The standard clearing technique does not transfer.
• You cannot remove it quickly. Even with a quick-release strap, getting a sealed mask off a panicking face in water takes longer than dropping a mouthpiece.
• A valve malfunction has more serious consequences, because the whole airway runs through the mask.

Combined with the survey finding, that is enough to make a considered choice rather than a default one. If you use a full-face mask: buy one from a manufacturer that publishes its standards compliance, fit it properly, practise removing it in shallow water, and stay where you can stand.

How to use a snorkel properly

Fit the mask first. Press it to your face without the strap and inhale through your nose. It should hold itself in place. If it does not, it will leak all day, and no amount of strap tension fixes a bad seal. Hair under the skirt is the most common cause of a leak.

Set the snorkel angle. The barrel should sit roughly vertical when your face is in the water and your eyes are looking down and slightly forward. If you have to lift your head to keep the top clear, the attachment point is wrong.

Breathe slowly and deeply. Short shallow breaths waste effort on the dead space and do less to exchange the air in your lungs. This is the single technique change that makes snorkelling less tiring.

Clear with one sharp blast. Do not blow gently; the tube empties on a forceful exhalation. Practise it standing in waist-deep water before you swim out.

Defog before you get in. Anti-fog solution, or the well-known method of a smear of saliva rubbed round the inside of the lens and rinsed. Do it once on dry glass and it lasts most of a session.

The rules that matter most

• Swim with a buddy, and one who is actually watching you.
• If you cannot swim, do not snorkel. A flotation vest changes the odds, it does not change that rule.
• Stay where you can touch the bottom. Almost every incident the study surveyed happened where the person could not.
• If in doubt about your cardiovascular health, do not go out.
• Consider waiting several days after arriving by air before snorkelling, which the study proposes as prudent.
• Check your position often. Drifting away from your entry point is how a pleasant afternoon becomes a long swim back.

Recreational snorkelling is not a benign low-risk activity, and that phrasing is the study's, not ours. It applies to strong swimmers as much as to beginners.

What to do first

Buy a snorkel with a wide, smooth bore and a mouthpiece that does not constrict, and fit the mask before you buy anything else. Then practise clearing the tube standing in the shallows.

On the day, pick a spot where you can stand, go with someone, and treat unexpected breathlessness as the emergency it is rather than as a reason to push on to the next patch of reef.