If you use a portable oxygen breathing machine, you have probably been told not to bring the oxygen within 5 feet of an open flame. This proximity is dangerous not because oxygen is flammable but because oxygen is an accelerator. This means that in order for a substance to burn, it needs oxygen -- or some other strong oxidizing agent -- but the oxygen itself is not what goes up in flames. Rather, oxygen combines with fuel at the right temperature and initiates a chain reaction known as fire.

Oxygen is a fundamental element -- the third most common element in the universe. Symbolized by the letter "O" on the periodic table, this gas has an atomic number of 8, meaning it has 8 protons and, normally, 8 electrons. Because of its atomic structure, it is a highly reactive gas, so it readily forms compounds, such as water and carbon dioxide. The Earth's atmosphere is approximately 21 percent oxygen, but its crust is about one-half oxygen.

Fire is the result of a process called combustion. In this process, an oxidizer, such as oxygen, combines with a fuel, such as wood or paper, when it is heated to a certain ignition temperature. As the fuel reacts with the oxidizer, the molecules get excited and break apart. The molecules then recombine to form new combustion products, such as carbon dioxide, and release energy, which people perceive primarily as light and heat. The combination of an oxidizer, fuel and heat is sometimes called the fire triangle, and as long as the fire has these three things, it will continue to burn.

An oxidizing agent, also called an oxidizer or oxidant, can either be a chemical compound that easily gives away its oxygen atoms or a substance that takes on electrons. Oxygen is the former variety, as are ozone and hydrogen peroxide, but any oxidizing agent can support combustion, even if no oxygen is present. While these materials are not combustible themselves, they are still very dangerous because they make other substances burn faster and more readily.

When using a portable oxygen breathing apparatus, you should always stay far away from any open flames, and you should never light a cigarette or be near smokers. Many people believe that because oxygen is not flammable, it does not pose much risk. Even though the oxygen by itself will not burn and even though a flame may be relatively small, the oxygen will help the flame grow much larger, and you could easily ignite a lethal conflagration. Many insurance companies will not pay for oxygen for smokers because so many accidents have resulted from the small blaze of a cigarette.