Ground-level ozone is a highly unstable, poisonous gas.  It is the main ingredient in smog and has been the most difficult pollutant for Hamilton County to control because it is not released directly into the air but is caused by the interaction of other airborne pollutants and is transported hundreds of miles by winds.  When inhaled, ozone can cause respiratory problems and aggravate asthma in children, the elderly, those with respiratory disease, and those who work or exercise outdoors on smoggy days.  High levels of ozone damage vegetation and reduce crop growth and yield.