The snorkel
Why is the optimum snorkel length not longer than 40 centimeters?
Snorkeling
A swimmer's snorkel is a tube typically about long and with an inside diameter of between , usually L- or J-shaped and fitted with a mouthpiece at the lower end, and constructed of rubber or plastic. It is used for breathing air from above the water surface when the wearer's mouth and nose are submerged. The snorkel usually has a piece of rubber that attaches the snorkel to the outside of the strap of the diving mask. An older technique is pushing the snorkel between the mask-strap and the head, but this practice increases the chances the mask will leak. The optimum design length of the snorkel tube is at most 40 centimeters (about 16 inches). A longer tube would not allow breathing when snorkelling deeper, since it would place the lungs in deeper water where the surrounding water pressure is higher. The lungs would then be unable to inflate when the snorkeler inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure. Snorkels also constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is recycled into the lungs, reducing breathing efficiency and causing a build up of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, and the smaller the tidal volume of breathing, the more this problem is exacerbated. A smaller diameter tube reduces the dead volume, but also increases resistance to airflow and so increases the work of breathing. Occasional exhalation through the nose while snorkeling will reduce the build up of carbon dioxide, and help in keeping the mask clear of water.