Oxygen

Oxygen is a very reactive element that readily combines with other elements to form many different compounds. Oxygen is also one of the most plentiful elements on earth, accounting for 20.946% of the atmosphere. The vast abundance of this gas is due primarily to the tiny yet prolific phytoplankton that thrive in the shallow lighted zone of all the world’s oceans.

These phytoplankton – primarily dinoflagellates and diatoms – produce more than 85 percent of the world’s oxygen. Without these tiny creatures, life as we know it could not exist.

Like nitrogen, in its gaseous form oxygen combines with itself so that two atoms create the oxygen gas molecule O2. However, unlike nitrogen gas, oxygen gas remains very reactive and the body uses it during metabolism (the process by which food is converted to energy to support life). Although the body needs oxygen to live, under pressure oxygen can become toxic.

When breathing air within recreational depth limits, this isn’t an issue, but when using enriched air (air that has more than 21% oxygen – commonly 32% or 36% oxygen) a diver can reach depths at which oxygen toxicity can occur. (See Section Two, The Physiology of Diving, for more information about oxygen toxicity.)