A minor but important constituent (chemical symbol O3) of the earth's atmosphere. While it is essential for life as we know it today, it is also a toxic gas that can result in significant physiological and ecological damage if exposures exceed critical limits. In both the stratosphere and troposphere, ozone concentration levels depend on many linked chemical and meteorological mechanisms, which vary significantly with space and time. Human pollutant emissions often perturb these linkages, resulting in significant increases or decreases in ozone concentration. While many facets of ozone's atmospheric behavior are well understood, a large number of important uncertainties remain, the resolution of which will require substantial combined efforts by the meteorological and chemical communities. See air pollution.