Ozone is a reactive environmental oxidant molecule and is the key component in oxidant smog. The respiratory tract is the primary target of the ozone toxicity. Already after short-term exposure a wide variety of morphological and biochemical changes have been noted in several animals species. The extent of the changes depend upon the ozone concentration, exposure-time and type of exposure, as well as antioxidants such as Vitamin E and Selenium, which serve as protectors in ozone induced toxicity. The dose-response curve of ozone vs. human lung function is influenced by the minute ventilation. The effect of ozone on lung function varies strongly among individuals. The biological marker for human ozone sensitivity is not known yet. The increased susceptibility of animals to airborne infections following very modest ozone concentrations is to date one of the most sensitive measures of ozone toxicity.