A colorless, odorless, tasteless gas that is produced in only very small amounts by natural processes. By far the most important source of the gas is the incomplete combustion of coal, oil, and natural gas. In terms of volume, carbon monoxide is the most important single component of air pollution. Environmental scientists rank it behind sulfur oxides, particulate matter, nitrogen oxides, and volatile organic compounds, however, in terms of its relative hazard to human health. In low doses, carbon monoxide causes headaches, nausea, fatigue, and impairment of judgment. In larger amounts, it causes unconsciousness and death.