Ozone (O3) is a gas composed of three atoms of oxygen, and it can be good or bad, depending on where it is found.
Stratospheric ozone (in the upper atmosphere) is good, while ozone found at ground level is harmful due to its effects on health and the environment.
Depending on the level of exposure, ozone causes coughing, throat soreness, airway damage and pain when taking a deep breath. These effects can be found even in healthy people but are usually more serious in people with lung diseases such as asthma, which aggravation is linked with long-term exposure to ozone.