Free radicals are unstable molecules (e.g., oxygen) with an unpaired electron. They are very reactive, (e.g., reactive oxygen species) trying to capture the needed electron to gain stability. Generally, free radicals attack the nearest stable molecule, "stealing" its electron. When the attacked molecule loses its electron, it becomes a free radical itself, beginning a chain reaction. Once the process is started it can cascade, finally resulting in the disruption of a living cell. Some free radicals arise normally during metabolism. Sometimes the body’s immune system’s cells create them to neutralize viruses and bacteria. However, environmental factors such as pollution, radiation, cigarette smoke and herbicides can also generate free radicals.