Breathing is a necessary function for human beings. It supplies oxygen for a process called cellular respiration, which is how cells manufacture the energy they need for their life functions. There are lots of things that can affect the rate of breathing in humans. Exercise is one of those things, particularly aerobic exercise. Aerobic exercise is any exercise that causes the person to breathe more, such as running, jogging, playing basketball, football, running track, or playing soccer. Another factor that can influence breathing rate is allergic reactions to stimuli from the environment, such as pollen. This can inflame the passageways leading to the lungs, making them smaller, requiring more air. Smoking is a habit that can influence the development of lung cancer, which in turn influences the rate a person breathes. Nervous conditions and reactions to stimuli also influence the breathing rate, such as the famed "fight or flight" syndrome.