CO, a colorless, odorless, noncombustible gas that is uniformly distributed over the earth's surface at a concentration of about 0.033% or 330 ppm of air. In the atmosphere it results from the combustion of organic matter, including fossil fuels such as oil, natural gas, and coal. It is also produced by various microorganisms from fermentation and cellular respiration. CO is used in carbonated beverages, as a pressurized gas, in fire extinguishers, and as a solvent. Plants uptake CO in photosynthesis. CO is a greenhouse gas, and its concentration is rising as a result of the combustion of fossil fuels and land use changes; thus it has become the focus of public concern in recent years.