An average human exhales around 2.3 pounds of CO2 in a day. That rate increases by up to a factor of eight during heavy physical exertion and falls somewhat during periods of relaxation, such as during sleep.
Human bodies cannot exhale carbon without first taking it up from food. Humans are, therefore, part of a closed carbon-capture loop. In this loop, atmospheric CO2 is taken up by plants, which are then eaten directly by humans or indirectly by herbivores that are later eaten by humans. Exhalation simply returns some carbon that had been temporarily sequestered in plants to the atmosphere.
