Our bodies are adapted to breathe on land, but not underwater. When diving, our respiration is affected by the surrounding pressure, limited air supply, and physical changes in the air we breathe.
On this page, you'll learn why certain gases become more toxic at depth. You'll also learn about the risks of oxygen and carbon monoxide poisoning, along with the steps you can take to prevent problems resulting from breathing these gasses.
Partial Pressure of Gasses
In the Physics section you learned that absolute pressure increases by 1 atmosphere (or 14.7 psi) every 33 feet you descend in salt water. So if we were to inflate a balloon at the surface and pull it down to 33 feet, the pressure would double to 2 atmospheres.
"Partial pressure" refers to the pressure of one specific atmopsheric component at a given pressure. So if we add up the partial pressures of all components, we would get the total pressure. As covered earlier in this section, the air we breath is primarily made up of oxygen and nitrogen, with oxygen making up 21% of our air. So at 1 atmosphere, the partial pressure is 21% of that 1 atmosphere, or .21 atmospheres. If we double the pressure to 2 atmopsheres, the partial pressure of oxygen would also double to .42 atmospheres.
If you've ever exercised at a high elevation you're aware of the effects of partial pressure when pressure is decreased. Atmospheric pressure decreases as elevation increases, making the air less dense, or "thinner". So with every breath of air you inhale, you're inhaling the same volume of air, but getting fewer oxygen molecules. This has several effects on our bodies because our respiratory and cardivascular systems are regulated or affected by partial pressure of gasses, not percentage. This is why Mount Everest climbers use oxygen as they near the summit. The average atmospheric pressure at the summit is 5 psi, or approximately 1/3 atmosphere. Since the pressure decreased to 1/3, the partial pressure of oxygen decreases the same amount, to about .07 atmospheres, which is too low to maintain normal body function. But when breathing pure oxygen, the partial pressure is 1/3 of 100% oxygen, or approximately .33 atmospheres, which is even higher than the partial pressure of oxygen at sea level.
Divers are most concerned with the effects of increased partial pressure. Oxygen, carbon dioxide, nitrogen, and especially any unordinary toxic gasses that end up in your tanks all have different effects on your body as their partial pressure increases.
We all need oxygen to survive, but too much oxygen can lead to serious conditions such as convulsions and death. This limit is well in excess of 100% oxygen, so we don't need to be concerned about oxygen toxicity on land, even when administering oxygen for first aid.
However, as you descend, the density of the air you breathe increases. This allows your blood cells to carry more air molecules than at the surface. When your cells carry a certain amount of oxygen, your body can have adverse reactions such as convulsions. This can lead to more serious consequences such as drowning.
Oxygen toxicity can be avoided by never filling your cylinder with pure oxygen, and diving within safe depth limits. Pure oxygen becomes toxic at about 20 feet, and the oxygen in normal air becomes toxic at about 218 feet. Notice that this is well beyond the maximum recreational diving depth of 130 feet.
Carbon Monoxide Poisoning
Carbon monoxide is a tasteless, odorless, and colorless gas that interferes with oxygen delivery to your body's tissues. It's dangerous in any quantity, but even more so under pressure. Symptoms of carbon monoxide poisoning include headaches, nausea, blue lips and fingernails, or unconsciousness. Immediate medical attention is necessary no matter how severe the symptoms appear to be.
Two common causes of carbon monoxide contamination of your breathing air are exhaust fumes and poorly maintained air compressors. Reputable dive shops take extensive measures to prevent contamination of your air supply, and this is why you should only have your cylinders filled by qualified dive shops. You should also smell your air before diving. You cannot smell carbon monoxide itself, but oily or other chemical odors are often a sign of possible carbon monoxide contamination.