How Do Snorkels Work? A View From Inside the Tube

A snorkel works by keeping the open end of a short breathing tube in the air while the mouthpiece and the snorkeler’s face are underwater; inhaling draws atmospheric air down the tube, exhaling sends used air back up, and breathing stops whenever the inlet itself is submerged, even if a dry-top valve has kept the tube empty.

That last distinction gets lost in crowded feature lists. “Dry” describes water control. It does not describe an underwater air supply. From inside the tube, everything depends on which side of the waterline its opening occupies.

How does air move through a snorkel?

In a simple J-shaped snorkel, the swimmer’s breathing muscles create the pressure change that moves air. An inhale lowers pressure at the mouthpiece and draws surface air through the barrel. An exhale reverses the flow through the same passage. There is no pump, stored oxygen or regulator in that circuit.

The mouthpiece lets the swimmer breathe through the mouth while a separate mask encloses the eyes and nose. The barrel curves upward beside the head, and a keeper attaches it to the mask strap. With the face angled down, the inlet should sit above and slightly behind the crown rather than trail sideways in the water.

Extra parts alter water handling; the air still comes from the atmosphere. A splash guard deflects some spray. A reservoir below the mouthpiece gives stray water somewhere to collect. A one-way purge valve lets an exhalation push that water out through the bottom. On a dry snorkel, a float-operated valve closes the top opening when it is covered; Cressi’s Supernova Dry product description says its valve seals the breathing tube when submerged.

Once that float closes, the swimmer has only the air already in the lungs and tube. On resurfacing, the float should drop, the inlet should reopen and surface breathing can resume. Dedepu’s equipment guide warns that sand or salt can jam the float, giving a dry top a specific reason to be tested in shallow water.

Why can’t a snorkel be much longer?

A longer tube adds stale air and breathing resistance without solving the pressure problem below the surface. BS EN 1972:2015 therefore limits separate snorkels by lung-capacity class. An SGS conformity report reproduces the standard’s dimensional table and defines length as the linear distance from the center of the mouthpiece opening to the lowest part of the air inlet.

| BS EN 1972 class | Intended user | Maximum internal volume | Maximum barrel length | |---|---|---:|---:| | Class A | People with larger lung capacities | 230 mL | 380 mm | | Class B | People with smaller lung capacities, including children | 150 mL | 350 mm |

Internal volume matters because a snorkel becomes added respiratory dead space. At the end of an exhalation, the tube contains used gas; the first part of the next inhalation pulls some of it back before fresh air arrives. StatPearls’ NCBI Bookshelf chapter on lung dead space gives 500 mL as a normal tidal volume, the amount moved in an ordinary resting breath, and about 150 mL as normal anatomical dead space. Equipment can add a similar amount again.

A 2003 study by A.S. Toklu and colleagues in the International Journal of Sports Medicine measured an additional 160–170 mL of dead space from a snorkel. In 12 subjects, ventilation and tidal volume rose at rest and during light exercise while respiratory rate did not. The body answered the rebreathed gas with deeper breaths rather than simply faster ones.

Bore diameter pulls in the other direction. A narrow tube has less volume, yet it resists flow more. In a 2021 Undersea & Hyperbaric Medicine study, N.A.M. Schellart tested a classic J-tube with a 20.5 mm inner diameter in 19 volunteers. The paper calculated that classic J-snorkels with bores of at least 19.5 mm add 3%–16% breathing resistance.

N.A.M. Schellart, a biomedical engineering and physics researcher at Amsterdam UMC, wrote, “The best multipurpose snorkel (19–21 mm) has no top appendages and no water release valve.” That preference favors low resistance and simplicity. It conflicts with the beginner-friendly convenience sold by dry tops and purge valves, so the useful choice depends on which inconvenience matters more: occasional water clearing or extra moving parts and drag.

How much exhaled carbon dioxide can come back?

The amount varies with equipment volume, valve layout, breathing depth and workload. Toklu’s team reported increased carbon dioxide in inspired gas from the 160–170 mL of expired air retained in a conventional tube. Schellart’s 2021 paper reached a different safety judgment for the classic J-snorkels it modeled and tested, stating there was no hypercapnia risk with the dimensions it recommended. The studies did not use the same protocol or make the same endpoint claim.

Full-face snorkel masks add another airflow layout. Their channels are supposed to separate inhaled and exhaled gas, but the seal between the eye pocket and the mouth-and-nose compartment has to work. A 2023 University of Auckland trial in Diving and Hyperbaric Medicine tested 20 adults with two full-face models and a conventional snorkel. The paper reports that earlier full-face-mask testing measured inspired carbon-dioxide partial pressure of 1–2 kPa, roughly 1%–2% of the gas at sea-level atmospheric pressure.

During light exercise in the Auckland trial, 18 of 40 full-face-mask runs, or 45%, were stopped after end-tidal carbon dioxide exceeded 7.0 kPa. The corresponding conventional-snorkel figure was 4 of 20 runs, or 20%. Those measurements came from dry cycle-ergometer tests, so they do not establish an accident rate in the sea.

Dr Hanna van Waart, an anaesthesiology researcher at the University of Auckland and the trial’s corresponding author, and her co-authors concluded, “Use of FFSMs may result in rebreathing due to non-unidirectional flow, leading to hypercapnia and hypoxaemia.” A full-face design deserves its own evidence; results for a plain 20.5 mm J-tube cannot simply be transferred to a mask with multiple channels and valves.

What happens when a snorkel goes fully underwater?

An open-top snorkel floods as its inlet passes below the surface. A dry top closes instead. Both stop access to atmospheric air at that moment. The dry model may spare the swimmer a tube full of water, but it cannot deliver another breath until the inlet is back in air.

People still descend while wearing snorkels because they are making breath-hold dives to look more closely, then using the tube again at the surface. PADI’s comparison of snorkeling and scuba gives 3–4 meters (12–15 feet) as an average snorkeler’s descent and up to 7 meters (25 feet) for experienced snorkelers. Those figures measure breath-hold descents; the snorkel supplies no air at those depths.

Divers Alert Network likewise describes snorkelers as surface swimmers who may make breath-hold dives. Its medical guidance warns against hyperventilation before submersion because it can delay the urge to breathe while oxygen continues to fall. DAN recommends no deliberate hyperventilation; even under favorable conditions, it limits preparation to one or at most two deep breaths.

Water that enters a simple tube has to leave before the next inhale. After the inlet resurfaces, a forceful exhalation can blast water out of the top. A bottom purge valve shortens that path by opening outward under exhaled pressure, then sealing inward during inhalation.

There is no single published opening-pressure figure that covers every purge valve. Two Dedepu equipment pages even disagree with each other. Its snorkel-versus-cylinder comparison lists 0.01 bar, equal to 10 mbar or 1,000 Pa, while its buying guide gives 0.05 psi, about 3.4 mbar, for a “high-quality” diaphragm. The SGS report on BS EN 1972 specifies a different measurement, requiring total peak respiratory pressure during inhalation and exhalation to remain within ±10 mbar; it does not isolate purge-valve cracking pressure. A buyer needs a model-specific test sheet to treat any lower number as a verified product specification.

How does a dry-top snorkel compare with scuba and an open-top snorkel?

The three systems solve different problems. Cressi says a dry-top float seals the tube when submerged. PADI describes scuba as carried breathing gas used through dedicated equipment, allowing substantially deeper and longer underwater travel. A plain snorkel keeps the fewest components between the swimmer and surface air.

| Property | Simple open-top snorkel | Dry-top snorkel | Scuba | |---|---|---|---| | Air source | Atmosphere through an open tube | Atmosphere through a float-controlled tube | Compressed breathing gas carried in a cylinder and delivered by a regulator | | What happens below the waterline | Tube floods; no new air arrives | Top valve closes; no new air arrives | Regulator continues supplying gas at surrounding pressure | | Underwater time | One breath-hold | One breath-hold | Limited by gas supply, depth and the dive plan | | Water management | Clear through the top after surfacing | Float limits flooding; many models also have a bottom purge | Regulator exhaust valve releases exhaled gas; mask is separate | | Typical role | Surface breathing with simple clearing | Surface breathing with less splash and flooding | Sustained underwater breathing after training | | Published depth comparison | PADI: average snorkeler descends 3–4 m on one breath | Same breath-hold limit; “dry” adds no gas | PADI Open Water training permits dives to 18 m/60 ft after certification |

The verbs expose the difference. A dry top closes. A scuba regulator supplies. Anyone planning to remain down for repeated breaths is describing scuba or another surface-supplied diving system.

How should a beginner use a snorkel at the beach?

Practice where standing up ends the exercise immediately. PADI says basic snorkeling skills can be learned quickly, while DAN notes that many participants receive little formal instruction. Four actions cover the actual breathing sequence.

1. Fit and float in standing depth. Seal the mask over the eyes and nose, place the mouthpiece between the lips without clamping it, and adjust the keeper until the barrel stands upright beside the head. Put the face in the water only after confirming the inlet is clear of the surface.

1. Establish surface breathing. Inhale and exhale through the mouth with slow, full breaths. Shallow panting makes the fixed dead-space volume a larger fraction of every breath. Stay horizontal enough to keep the opening up, and lift the head if airflow feels restricted.

1. Handle a wave or flooded tube. When a dry valve closes under a wave, wait for the inlet to reopen before inhaling. For water in an open tube, resurface and blow firmly through the mouthpiece to clear it, or remove the mouthpiece and drain the tube when lung air is limited.

1. Duck-dive on one breath. Breathe normally at the surface, take the final breath without repeated hyperventilation, then descend knowing the snorkel supplies no air below the waterline. Return to the surface, make sure the inlet is open, clear any water and resume breathing. DAN says the urge to breathe should end the breath-hold.

If a valve remains shut, the response is immediate and mechanical. Remove the mouthpiece, put the airway in open air and stand, float or roll onto the back.

What else do beginners ask about how snorkels work?

Why do people go fully underwater with snorkels?

People descend to inspect reefs or marine life more closely, carrying one breath in their lungs. PADI reports 3–4 meters as an average snorkeler’s duck-dive depth and up to 7 meters for experienced snorkelers. The snorkel becomes useful again after its inlet returns to the air.

Does a snorkel allow you to breathe underwater?

A snorkel allows breathing while the face is underwater and the tube’s inlet remains above the surface. It cannot supply air once that inlet is submerged. A person seen descending with one is holding a breath; sustained underwater breathing requires scuba or another system that delivers pressurized breathing gas.

How does a snorkel work when it is submerged?

A simple open snorkel fills with water when submerged. A dry snorkel uses a float valve to close the air inlet and reduce flooding. Neither version provides a new breath underwater. After resurfacing, the dry valve reopens; an open tube must be cleared by exhaling or emptied outside the mouth.

How do snorkels keep water out?

Plain J-snorkels do not keep all water out; their raised opening avoids water during normal surface swimming. Semi-dry guards deflect splashes. Dry snorkels add a float-operated top valve that closes when covered. Many models also collect stray water below the mouthpiece and release it through a one-way purge valve.

What is the point of a snorkel underwater?

Its purpose is continuous surface breathing with the face immersed, which lets the wearer watch below without lifting the head for every breath. During a duck dive, the tube supplies nothing; the swimmer is breath-holding. On resurfacing, the same snorkel permits observation to continue once its inlet is clear.

What is a dry snorkel?

A dry snorkel is a surface-breathing tube with a float-operated valve at the top. Cressi describes the valve as sealing the breathing tube when submerged, then reopening after ascent. “Dry” refers to limiting water entry. It does not mean the snorkel stores air or supports breathing below the surface.