allows easy and accurate control of ascent rate and stop depth for safer decompression. Various surface detection aids may be carried to help surface personnel spot the diver after ascent. In addition to the surface marker buoy, divers may carry mirrors, lights, strobes, whistles, flares or emergency locator beacons. Accessories and tools Divers may carry underwater photographic or video equipment, or tools for a specific application in addition to diving equipment. Professional divers will routinely carry and use tools to facilitate their underwater work, while most recreational divers will not engage in underwater work. Medicine Antihistamines Scopolamine: Motion sickness, including sea sickness, leading the use of scopolamine use by scuba divers (where it is often applied as a transdermal patch behind the ear) Promethazine Breathing from scuba Breathing from scuba is mostly a straightforward matter. Under most circumstances, it differs very little from normal surface breathing. In the case of a full-face mask, the diver may usually breathe through the nose or mouth as preferred, and in the case of a mouth held demand valve, the diver will have to hold the mouthpiece between the teeth and maintain a seal around it with the lips. Over a long dive this can induce jaw fatigue, and for some people, a gag reflex. Various styles of mouthpiece are available off the shelf or as customised items, and one of them may work better if either of these problems occur. The frequently quoted warning against holding one's breath on scuba is a gross oversimplification of the actual hazard. The purpose of the admonition is to ensure that inexperienced divers do not accidentally hold their breath while surfacing, as the expansion of gas in the lungs could over-expand the lung air spaces and rupture the alveoli and their capillaries, allowing lung gases to get into the pulmonary return circulation, the pleura, or the interstitial areas near the injury, where it could cause dangerous medical conditions. Holding the breath at constant depth for short periods with a normal lung volume is generally harmless, providing there is sufficient ventilation on average to prevent carbon dioxide buildup, and is done as a standard practice by underwater photographers to avoid startling their subjects. Holding the breath during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a gas supply malfunction until it is too late to remedy. Skilled open circuit divers