A standard snorkel is a curved tube with a shape usually resembling the letter "L" or "J", fitted with a mouthpiece at the lower end and made from plastic, synthetic elastomers, rubber, or light metal.

This involves a sharp puff in the early stage of exhalation, which is effective for clearing the tube of remaining water, and a fairly large but comfortable exhaled volume, mostly fairly slowly for low work of breathing, followed by an immediate slow inhalation, which reduces entrainment of any residual water, to a comfortable but relatively large inhaled volume, repeated without delay.

[16] In older designs, some snorkels were made with small "ping pong" balls in a cage mounted to the upper open end of the tube to prevent water ingress.

The use of this equipment allows the snorkeler to observe the underwater environment for extended periods with relatively little effort, and to breathe while face-down at the surface.

The rigid forehead bracket and central position minimise off-centre loads and keep the snorkel stable at relatively high speeds.

On 9 December 1929, Barney B. Girden filed a patent application for a "swimming device" enabling a swimmer under instruction to be supplied with air through a tube to the mouth "whereby the wearer may devote his entire time to the mechanics of the stroke being used."

In 1938, French naval officer Yves Le Prieur introduced his "Nautilus" full-face diving mask with hoses emerging from the sides and leading upwards to an air inlet with a ball valve which opens when it is above water and closes when it is submerged.

[32] The device resembles a full-face diving mask incorporating two breathing tubes topped with valves projecting above the surface for inhalation and exhalation purposes.

[33] Throughout their heyday of the 1950s and early 1960s, masks with integrated tubes appear in the inventories of American, Australian, British, Danish, French, German, Greek, Hong Kong, Israeli, Italian, Japanese, Polish, Spanish, Taiwanese, Turkish and Yugoslav swimming and diving equipment manufacturers.

Meanwhile, in 1957, the US monthly product-testing magazine Consumer Reports concludes that "snorkel-masks have some value for swimmers lying on the surface while watching the depths in water free of vegetation and other similar hazards, but they are not recommended for a dive 'into the blue'".

[43] In the first French monograph on spearfishing La Chasse aux Poissons (1940), medical researcher and amateur spearfisherman Raymond Pulvénis illustrates his "Tuba", a breathing tube he designed to be worn on the front of the head over a single-lens diving mask enclosing the eyes and the nose.

[49] In the late 1960s, Dacor launched a "wraparound big-barrel" contoured snorkel, which closely follows the surface of the wearer's head and has a wider bore to improve airflow.

[51] In April 1950, a West-Coast American newspaper showcased the work of The Spearfisherman diving equipment company, listing "snorkles (rubber breathing tubes)" among the manufacturer's underwater hunting products.

[52] In November of the same year, the Honolulu Sporting Goods Co. introduced a "swim-pipe" resembling Kramarenko and Wilen's side-mounted ball-float- and flutter-valve breathing tube design, urging children and adults to "try the human version of the submarine snorkel and be like a fish".

[56][57] The following year saw the première of the 1958 British thriller film The Snorkel, whose title references a diving mask topped with two built-in breathing tubes.

In February 1980 and June 1991, the Deutsches Institut für Normung published the first[60] and second[61] editions of German standard DIN 7878 on snorkel safety and testing.

The orientation and shape of the tube must also be taken into account when matching a snorkel to its use while seeking to optimise ergonomic factors such as streamlining, airflow, water retention, interrupting the field of vision, and work of breathing.

To date, all national and international standards on snorkels specify two ranges of tube dimensions to meet the health and safety needs of their end-users, whether young or old, short or tall, with low or high lung capacity.

The table above shows how snorkel dimension limits have changed over time in response to better understanding of the effects of dead space and breathing resistance on carbon dioxide retention.

Early designs of snorkel-mask typically comprised a tube for breathing with an immersion-activated shut-off valve at the top, and the bottom opening into the mask, which might cover the mouth as well as the nose and eyes.

Early manufacturers and retailers generally classed integrated snorkel masks by the number of breathing tubes projecting from their tops and sides.

[74] An integrated snorkel consists essentially of a tube topped with a shut-off valve to block water ingress during immersion, and opening at the bottom into the interior of a modified diving mask via sockets moulded into the rubber of the skirt or the plastic of the frame.

Second generation snorkel-masks may enclose the nose and the mouth within an inner orinasal pocket which may be directly connected to the single snorkel with its float valve at the upper end.

When the snorkel tops submerge, their float activated valves are supposed to shut off automatically, blocking nasal respiration and preventing mask flooding.

Conversely, the 1950s Typhoon Super Star and the modern-retro Fish Brand M4D diving half-masks came with sealed but snorkel-ready sockets at the top of the skirt moulding.

They did not use an orinasal pocket, and had a two-directional airflow, with a large dead space, and mixing of fresh and exhaled air in the mask volume.

[4] The orinasal pocket and one way airflow prevent warm, moisture saturated, exhaled air from flowing over the inside surface of the viewport, which reduces the amount of fogging.

Other positions for exhaust ports have been through alternative snorkel channels, and at the temples, which reduce hydrostatic back pressure at the cost of a longer flow path with more frictional resistance.

Small amounts of water can be purged through the lower exhaust port where present, but in the event of major flooding, the whole mask must be removed to continue breathing, which requires quick release straps.

[4] For comparison, the US Occupational Safety and Health Administration (OSHA) has set limits for the acceptable concentration of carbon dioxide in workplace environments.