Oxygen is a colorless, odorless, tasteless gas that is slightly soluble (1.2 cubic in [3.08 cubic cm] per 39.4 cu in [100 cubic cm] of water) at room temperature. It is considerably more soluble in some organic solvents, such as ethyl alcohol, carbon tetrachloride, and benzene. Oxygen is less soluble in sea water than in pure water, although still soluble enough to support the survival of marine organisms.
Oxygen exists in three allotropic forms, monatomic oxygen (O), diatomic oxygen (O2), and triatomic oxygen (O3). The first of these is sometimes called nascent oxygen, and the last is more commonly known as ozone. Under most circumstances in nature, the diatomic form of oxygen predominates. In the upper part of the stratosphere, however, solar energy causes the breakdown of the diatomic form into the monatomic form, which may then recombine with diatomic molecules to form ozone. The presence of ozone in the earth's atmosphere is critical for the survival of life on Earth since that allotrope has a tendency to absorb ultraviolet radiation that would otherwise be harmful or even fatal to both plant and animal life on the planet's surface.
Some scientists are now concerned about the possible depletion of the ozone layer in the upper stratosphere. There is strong evidence that certain synthetic chemicals, such as the Freons and compounds known as the chlorofluorocarbons (CFCs) may be causing destruction of ozone molecules in the atmosphere. The most widely accepted theory says that solar radiation causes such chemicals to break apart, releasing a free chlorine atom to the stratosphere. That chlorine atom then reacts with ozone molecules, converting them into diatomic oxygen molecules. One of the disturbing aspects of this theory is that it suggests that a single chlorine atom can cause the decomposition of many thousands of ozone molecules.
The environmental hazard posed by this series of reactions is that the level of ultraviolet radiation reaching the earth would be expected to increase as more and more ozone molecules are destroyed. Ultraviolet radiation has been implicated in a number of biological problems for plants, animals, and humans, including an increase in skin cancer and in eye problems. In response to this threat, most of the world's nations have agreed to reduce the amount of Freons, CFCs, and other ozone-depleting chemicals produced and sold each year.