The U.S. Navy and other diving organizations have modeled how your body absorbs nitrogen as you follow various dive profiles, and made various dive tables that you can use to calculate how much nitrogen will be absorbed by your body (see NOAA No Decompression Tables). In all of the tables, there are corresponding times and depths at which you won't have to undergo decompression -- these are called "no decompression" limits. Generally, the deeper you dive, the shorter you can stay there -- the "no decompression" time decreases as depth increases. Recreational divers must plan their dives to stay within these limits so that they minimize the risk of decompression sickness. If you follow the tables, you have less than a 0.5-percent chance of getting "the bends". Your diving instructor will show you how to use these tables to plan a safe dive. Also, dive computers have these tables programmed into them and use the algorithms to calculate your safe bottom time.
High-pressure oxygen can cause convulsions, seizures and drowning. Oxygen toxicity comes on quickly and without warning. For most divers breathing compressed air, this won't occur until they've reach about 212 ft (65 m) below the surface -- usually deeper than "no decompression" limits. However, for divers breathing Nitrox, oxygen toxicity will occur at a shallower depth because the oxygen partial pressure in the gas mixture is higher. The best advice for avoiding oxygen toxicity is to be aware of your depth limit and stick to it.
One final note about gases under pressure: They must flow freely in and out of your lungs at all times during your dive. If you hold your breath while ascending, the gases inside will expand and could block the circulation in your lungs (embolism) or even rupture your lungs (pneumothorax). Therefore, never hold your breath while breathing from scuba gear!
Effects of scuba diving on the body
Ears and Sinuses
Within your head and skull bone are air spaces, sinuses within the bone itself, and air pockets in the ear canal. As you descend in the water, water pressure squeezes the air in these spaces, causing a feeling of pressure and pain in your head and ears. You must equalize the pressure in these spaces by various methods, such as closing your nostrils and gently blowing your nose. If properly equalized, your sinuses can withstand the increased pressure with no problems. However, sinus congestion caused by cold, flu or allergies will impair your ability to equalize the pressure and may result in damage to your eardrum.
A water temperature below body temperature draws heat from the body. It is important to have proper thermal protection (wet or dry suits) to avoid hypothermia. Shivering is your body's response to lower body temperature and one of the beginning symptoms of hypothermia; you should end your dive if you begin to shiver.
Increased physical exercise underwater can lead to fatigue, dehydration, and intestinal or skeletal muscle cramps. Divers should be aware of their physical limits and not push their boundaries.
While there are many risks involved in scuba diving, new divers can minimize the dangers through proper education and training. Open-water certification programs emphasize diving physiology, diving hazards and safe diving practices. A trained diver can enjoy the sport safely with minimal health risks.