Carbon dioxide (CO2) Carbon dioxide occurs normally in Earth's atmosphere in concentrations of about 0.036% (360 parts per million). Because humans breathe out carbon dioxide as a basic byproduct of metabolism, levels can increase if there is poor or no ventilation to the outside air. When the CO2 level in the air increases:
Carbon dioxide (CO2) should not be confused with carbon monoxide (CO). Carbon monoxide, a common product of combustion, is lethal at comparatively low levels: CO concentrations above 400 ppm are life-threatening and above 1600 ppm, will cause human death within an hour.
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