Scuba¹ diving is one of the fastest growing sports in the world. Thousands of people each year leave the frozen wastes of the north to play in the inviting tropical waters of many exotic locations, from Fiji, to Hawaii, to the Caribbean, to the Red Sea.

There is a slight downside to diving, and that is that it's a good way to get yourself killed. By international law, no one is permitted to rent equipment, fill an air tank, or charter a boat trip until all divers involved can produce evidence that they have received sufficient instruction in the dangers of diving and the use of their equipment. The hazards are many.

The Air You Breathe

Most recreational scuba diving is done using ordinary compressed air, which is pumped into a steel or aluminum tank until it reaches 3000 psi. It is vital that you, as the diver:

• Inspect your tank: The first thing you should notice is a rubber gasket at the point where the air leaves the tank. Ensure it is in good condition and properly greased, or you will lose air underwater.
• Use only repectable dive shops: A shop that performs poor maintenance on their tanks and compressors is putting you at tremendous risk. If corrosion appears in the tank, you will inhale that iron oxide/aluminum oxide dust, which is not only harmful to your lungs, but it can also cause you to go into a terrible coughing fit at depth, leading to drowning. Poor maintenance of the air compressor can lead to this as well; a poorly maintained compressor can introduce oil or carbon monoxide into your air. The best situation is to own your own tanks, and take them in to get inspected by your local dive shop once a year.

The cleanest, purest air in the world can still kill at depth. This is because of water pressure. Every ten meters (approximately) the diver descends, he experiences another atmosphere of pressure. The air in the scuba tank also experiences this. Gasses respond in an odd way to increased pressure; they shrink down to the appropriate ratio. That is, if the diver is at ten meters, he experiences two atmospheres of pressure, the gas has contracted to half its size, and the diver will receive, in effect, twice the amount of oxygen and nitrogen he would receive at the surface for each lungful of air.

Too Much Nitrogen

Nitrogen makes up approximately 78% of normal air. Your blood absorbs a bit of this at normal pressures, then disposes of it as necessary. At depth, your blood will absorb astonishing amounts of it, depending on how deep and how long you dive. This phenomenon is based on Henry's Law, which states that the solubility of gasses increases under increasing pressure. Think of how much carbon dioxide a can of soda holds, and then releases when you release the pressure via the pull tab, and you've got the general idea. Anyway, too much nitrogen in the bloodstream will prevent the further absorbtion of oxygen, which will manifest itself slowly. You will start to feel a bit light-headed and giddy, and will begin to behave as if you were drunk. This condition is known as 'nitrogen narcosis,' and as its name suggests, can lead to unconsciousness. If you realize what is going on, or your partner spots it, this is an easy condition to alleviate. Simply ascend to a new depth for few minutes, and the condition should correct itself. Once corrected, it is best to end the dive. If left unabated, it leads to drowning. This underscores the need to always dive in pairs, and to keep an eye on each other.

Narcosis is far from the only danger from excess nitrogen. The blood has still absorbed massive amounts of nitrogen throughout the dive, regardless of whether narcosis has had an effect. As the diver ascends, this gas begins to expand back to its original size, just as the carbon dioxide expands and escapes when you open a soda can. If the diver ascends too fast, it begins to form bubbles within the blood stream, which collect at the joints. The result is 'decompression sickness' (DCS), more commonly known as 'the bends.' In an effort to alleviate the intense pain at the joints, the victim will curl into a fetal position, bending every joint, which is where the nickname developed. The only treatment for this condition is a long and expensive stay at the local hyperbaric chamber.

The US Navy Divers performed research of the most primitive kind into this condition in the early days of scuba. They simply sent divers down to increasing depths, for increasing amounts of time, and noted the effects. They kept a hyperbaric chamber handy, but the science is still a bit crude, to say the least. However, this research produced a set of dive tables that are universally accepted guidelines to depth and time tolerances to nitrogen. Before diving, you simply look to see how deep you deepest point in the dive will be, then look up the maximum time allotted, and ascend when that time is reached. At depths above 20 meters, dive tables aren't really necessary, since your air will run out long before your maximum bottom time is reached. Divers who wish to go beyond 20 meters are required to undergo another level of training to acquire their 'Advanced Open Water' certification before doing so. With this, the maximum allowed depth is 45 meters.

Prevention methods for DCS:

• Always stay within the allotted time on the dive tables. In fact, always stay on the conservative side of your math on this. Remember, these guidelines were produced by young males in the peak of physical condition, and conditioning has been shown to play a role in dive tolerances.
• Except for extreme emergencies, never ascend at a rate faster than one foot per second. The best way to gauge this is to look at your exhaust bubbles. Don't go any faster than your slowest ones.
• When in doubt, do a safety stop. This is when you ascend to a depth of five meters and hover for five minutes. This gives you extra time to breathe off more of that excess nitrogen before exiting the water entirely.

There are also air mixtures with greater concentrations of oxygen, and therefore lower concentrations of nitrogen, which alleviate these dangers. They are produced by mixing in a precise amount of pure oxygen into a tank of normal air. However, these require special equipment and special training, and are not recommended for recreational divers. They are somewhat of a tradeoff, providing less danger of DCS and narcosis, but greatly increasing the risks posed by...

Too Much Oxygen

You would think that too much oxygen in the blood would be something of an oxymoron, but it does exist. It's not a danger to people who take in 100% oxygen, but at the right depths, your body will take in more. We know that air is 21% oxygen by volume. If we look at the mathematical model of pressures, we see that, in effect, at 10 meters we will be inhaling 42% oxygen. At 20 meters, it increases to 63%, and so on. When you start taking in over 100% oxygen, it becomes toxic, and you run the risk of suffering, oddly enough, 'oxygen toxicity.' The very first symptom of oxygen toxicity is convulsions, which can be deadly underwater. You can injure yourself against rocks, diminishing your ability to escape, or you can knock youself out completely. You can also easily lose your mouthpiece, which would drown you in short order. Even the most attentive dive partner will likely find it impossible to restore your air to you. There is only one preventive measure for oxygen toxicity: never dive below 45 meters! If you find yourself straying below, ascend immediately to a lesser depth.

No Air At All

Divers are not allowed the comfort of believing that things work properly all the time. An air valve can get stuck. A hose can get ruptured. A leak can develop. Or, you may simply lose yourself in the environment, and lose track of your air consumption. Whatever the cause, lack of air is an immediate emergency, and must be dealt with swiftly.

Many divers spare the expense for a 'pony bottle,' or 'spare air.' This is a tiny cylinder² of compressed air, with a little mouthpiece and a demand regulator, that can be worn hooked to the belts or straps of the your gear. It provides air for just a few minutes, but will allow you to proceed to make a controlled ascent to the surface.

It is highly adviseable that you attach a second air regulator, known as an 'octopus,' to your tank. It provides you with a spare in case your primary malfunctions, but, more importantly, it provides your partner with a spare in case his air fails him entirely. Insist that your dive partner has one as well.

Most dive shops no longer teach a procedure called 'buddy breathing,' which allows two divers to share a single regulator, in case of emergencies and the lack of spares. The divers grab on to each other's straps with one hand to keep them from drifting apart. The working regulator's owner takes his regulator in hand, and the other diver places his hand over that. They work in turns; the owner will take two quick breaths, releasing control of the regulator as he begins a slow exhale of the second breath. The other diver will bring the mouthpiece to his mouth and repeat the process. All the while they perform this exercise, the divers are working together to make a slow, controlled ascent. It is a procedure that requires a large amount of concentration, coordination, and trust in each other's personal hygiene.

When all else fails, do an emergency ascension. If the problem occurs early in the dive, or if you haven't descended beyond 15 meters, you probably won't have any difficulties. But if you do, it is still better to deal with the pain of DCS than to drown. If your tank was empty, you'll find that a couple of new breaths appear as you ascend.

The Water Around You

Your mother once prodded you into the bathtub, or sent you to the store in the rain, with the words, 'A little water won't kill you.' Then you took up diving, and you found out different.

Pressure Lecture Part II

Just as the water affects the gasses in your tanks, it also affects the gasses in you. A benefit to this is that your partner won't see bubbles emanating from both ends of you at once. Offsetting that is the danger to your hearing.

The eardrum is simply a thin membrane stretched across the ear hole. Changes in air pressure affect the eardum, and would break it, except for the natural methods of regulating that pressure that are built into the human ear. The eustachian tube runs from the inside of the eardrum to the mouth, right about where the jaws meet. As external air pressure decreases, the air inside the eustachian tube expands, and flows right down the tube and out the mouth. When external pressure increases, air must be taken up into the tubes, which doesn't occur as naturally and easily as it flows out. It occurs in fits and spurts, and creates a sensation referred to as 'popping your ears.' The location of the tubes explains why chewing gum during rapid descents above ground is a good way to help.

Increasing pressure due to diving is an even greater challenge, because the pressure changes are occurring much more rapidly. Luckily, your ears will remind you when it is time to take steps; they'll hurt. Eardrum pressure is regulated the following ways:

• Make a slow, controlled descent.
• Every ten feet or so, your ears will remind you to 'equalize.' There are several ways to accomplish this, but the easiest, most effective, and silliest looking way is to pinch your nose and blow air into your head, without releasing any out of your mouth. Don't blow too hard, or you'll break your eardrums the other way.
• Never dive with sinus problems. Clogged sinuses will block the eustachian tubes. If in doubt, test your equalization capacity before jumping into the water. Many divers take an analgesic before every dive, just as a bit of insurance.

As you ascend, the eustachian tube will expel the excess pressure without intervention. However, this means that anytime to ascend a bit and the descend back to your previous level, you'll have to equalize again.

Because of the placement of the eustachain tube, it is quite easy to get air bubbles trapped in your jaw. These will also expand as you ascend. The end result is that, as the post-dive euphoria begins to wear off, you'll notice that your jaws are a little sore, and that your teeth don't fit together like they used to. This condition will pass, and is perfectly normal.

One result of external pressure on your internal gasses will not pass so easily. This involves the air in your lungs. As you ascend rapidly, the air in your lungs also expands rapidly. If you hold your breath, your lungs will explode. Remember to blow out anytime you are forced to ascend rapidly, and it's best to never hold your breath at any point while diving.

Another possibility is that a bit of air may be squeezed out of your alveoli³ and into your blood. As you continue to ascend, that air bubble will expand, and it will float toward your brain. When it becomes as large as the blood vessel it travels in, it will cut off circulation, causing tremendous pain. This condition is known as "air embolism." The only treatment for an embolism is the hyperbaric chamber. Cautious dive practices are the best prevention.

The Motion of the Ocean

The ocean is constantly in motion, as anyone who has ever watched the dance of the sandpipers⁴ can attest. Currents, waves, riptides, they all have the ability to carry away the incautious diver.

Waves and riptides are not a problem to the diver who manages to acquire a boat. For those who must brave a beach entry, however, they can be quite a problem. Scuba gear tends to make one quite awkward and slow in the shallow waters, and thus a better victim for these sorts of things. Use good judgement when surveying a beach entry. If the surf looks too rough, scrap the dive, or find a better spot. If you can handle it, though, it's a good idea to empty your buoyancy compensator of all air. That way, you'll be able to swim under the waves. Stick your regulator in your mouth, and maintain a good grip.

Currents can be a problem, or a boon, depending on how well you plan. If you, or someone in your dive group⁵ knows the currents at that site, then the dive profile can be planned with a certain entry point, and a certain exit point downstream from that. You will then be required to make minimal effort, as you can allow yourself to be carried along. Sure, they call it a sport, but you really didn't get involved for exercise, did you? This sort of dive is so popular, it has even been given a name: drift diving.

A poorly planned dive can be a nightmare, however, as you find yourself fighting futilely against the current to reach a decent exit point. A beach dive performed in a rocky area can turn it from a nuisance to an emergency. Current information can be obtained from the local dive shop, or you can rely on your powers of observation, if it is a familiar site.

Caves and Wrecks

Divers are, by definition, an adventurous sort, and tend to seek out interesting things while diving. Two of the most adventurous dive types are cave dives and wreck dives. They are also, of course, the most dangerous.

The hazards of diving into a cave or a wrecked vessel are the same. You can find yourself in very tight quarters, with jagged points reaching out to snag your dangling hoses and other accessories. You can kick up silt as you go, completely obscuring your exit and becoming hopelessly lost. It is also worth noting that, when you start getting deep into caves or wrecks, the option of emergency ascent is closed to you.

Because of the inherent dangers, caves and wrecks should only be explored by experienced divers who have received special instruction. The instruction will teach you how to bring along some line, and mark your path as you go, so you won't become lost. It'll teach you to bring along extra air rigs, so emergency equipment will be available, to help compensate for the unavailability of an emergency ascent. It'll teach you how to avoid protrusions, and how to adjust your equipment to reduce the chance of snags. It'll also teach you how to control your kicks., to reduce the amount of silt you disturb.

Sea Life

Sharks are the most commonly imagined and dreaded sea creature. Sure, they might eat you, but I wouldn't count on it. Sharks tend to avoid scuba divers, unless they tape steaks to their nipples. Divers are big, and sharks hate to work for a meal. They blow bubbles, and that really annoys them. And since sound travels so much better in liquids, the sound of your exhaust is loud. You're not going to sneak up on anything, and likewise, a shark will choose not to sneak up on you. Anyway, the hunting patterns of large sharks reveal that surfers are in much greater danger than divers.

The real dangers to divers are difficult to see and identify. Most are not well known. Many have tricky camoflage techniques. It is important that divers acquaint themselves with the local habitat, and what they can expect, before entering any new environment. Keep a watchful eye, and be sure to point out potential dangers to your partner. An exhaustive list would be incredibly tedious, and beyond my knowledge, so here's a random sampler:

• Coral: It will scratch you. It will cut you. If it gets into your skin, it will continue to live and grow inside you. It leaves disfiguring scars. And there's a particularly nasty version called Fire Coral that irritates the skin rather well. Stay off of it. Besides, every time you touch the reef, you kill it, and damage the ecosystem as a whole. Wear gloves to protect your hands while you reach for pretty shells.
• Urchins: They're basically balls of spines. Totally inert in the day, but entertaining to see in motion at night. Not entertaining to bump into. It's comparable to hugging a porcupine.
• Eels: They hide in caves and in corals, and you may never see anything of them but the head. They usually won't mess with you, as long as you don't dangle your fingers in front of them. The nasty part is that their teeth bend in at an angle. This means that once you're bitten, you stay bitten. The only way to cure this is to behead the beast, then split his jaw. Don't expect much cooperation from the eel in this endeavor. Keep your dive knife sharp.
• Sea snakes: Common in the Red Sea, Persian Gulf, and the Great Barrier Reef. Far more poisonous than their land-slithering cousins.
• Lionfish, scorpionfish, and others of that ilk: They lie still against the coral reefs, and their coloring blends in remarkably well. You likely won't see them until you're two feet away. They have very sharp fins, sort of like the needles of a sea urchin. Unlike the urchin, however, a puncture also introduces you to a toxin that makes you even more uncomfortable. Hover a few feet above the reef, and you'll be okay.
• Rays: They're passive plankton eaters, and absolutely beautiful in their graceful motion. When they're being less attractive, however, they like to bury themselves in the sandy parts. Stepping on their barbed tail is a hazard to divers and beach-goers alike.