In the upper reaches of Earth's stratosphere, a thin layer of ozone molecules absorbs ultraviolet sunlight, making conditions at the surface conducive for living beings. The ozone layer is thin -- only about the thickness of two stacked pennies -- and certain gases interact with ozone to cause a seasonal thinning of the layer. Most of the gases responsible for these ozone holes are released as a result of human industrial or agricultural activity.

Oxygen forms about 21 percent of the earth's atmosphere, and the bulk of that exists as a stable molecule consisting of two oxygen atoms. In the upper stratosphere, however, sunlight has enough energy to split some of these molecules into free oxygen atoms that can combine with the stable oxygen molecules to form ozone -- a molecule that consists of three oxygen atoms. The three atoms create a configuration that enables the molecule to absorb ultraviolet light. Scientists believe that the ozone layer formed about 600 million years ago, allowing organisms to emerge from the sea and live on land.

Chlorine and bromine have similar atomic structures, and they both have the ability to deplete the ozone layer. When a single atom of either element comes in contact with an ozone molecule, it strips the extra oxygen atom to form a slightly more stable molecule -- either a hypochlorite or a hypobromite ion -- and leave molecular oxygen. Being far from inert, each hypochlorite and hypobromite ion reacts with another ozone molecule, this time forming two oxygen molecules and leaving the chlorine or bromine radical free to begin the process again. In this way, a single chlorine or bromine atom can convert thousands of ozone molecules into oxygen.

If chlorine or bromine gas were released at the surface, neither would make it to the stratosphere -- they would form compounds long before they got there. However, chlorine is a primary component of two classes of inert gases, called chlorofluorocarbons, or CFCs. These gases migrate into the upper atmosphere, where the sun's radiation is strong enough to break apart the molecules and release free chlorine. In the same way, expelling methyl bromide at ground level releases bromine into the stratosphere. CFCs have many uses in industry, and methyl bromide is a pesticide. Other classes of ozone-depleting gases that contain bromine, called halons, are used in fire extinguishers and agriculture.

As of February 2013, 197 countries had agreed to the terms of the Montreal Protocol, an international treaty controlling the use of certain CFCs and halons. The treaty does not specifically address carbon tetrachloride, another ozone-depleting substance, but since it is used in the manufacture of CFCs, which have been phased out, its use has declined. The treaty also doesn't address the release of methyl bromide or nitrous oxide. The latter is another ozone-depleting gas released in farming and agriculture. Like CFCs, nitrous oxide forms a reactive radical in the stratosphere that strips the extra oxygen atom from ozone.