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.
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.