Ozone is a molecule composed of 3 atoms of oxygen. Two atoms form the basic oxygen molecule- the oxygen that we breathe and is essential for life. Ozone is considered a major outdoor pollutant. Ozone is a very unstable gas and breaks down spontaneously into one oxygen molecule, and one reactive atom of oxygen. This active atom of oxygen can reattach or react with a molecule of another substance causing oxidation and alteration in the chemical properties of the substance. It is this property what makes ozone a useful and harmful gas. Ozone in the upper atmosphere helps shield us from ultra violet rays. However, in the lower atmosphere it can be useful and harmful to our health. Ozone has been used extensively for water purification and in food processing plants. High concentration of ozone is sometimes used to decontaminate unoccupied spaces after fire, to get rid of strong odors due to tobacco smoke or chemicals, sewage or mold. At high concentration ozone can react with finishes on household material and cause damage. Ozone generators are marketed as air cleaners. The efficiency of these devices is a controversial subject. When inhaled, ozone has the potential of damaging lung tissue and can cause throat irritation. People with asthma and other respiratory diseases are cautioned to stay away from being exposed to ozone even at low concentrations. When associated with ozone air purifiers, the use of terms such as ‘energized oxygen” or “pure air” can be misleading. When it comes to our health, there is nothing energizing about ozone. The FDA requires ozone output in indoor medical devices not to exceed 0.05 ppm. NIOSH recommends an upper limit of 0.10 ppm for any time, while the EPA standard for ozone is a maximum 8 hr average of no more than 0.08 ppm.