Human metabolic emissions of carbon dioxide and methane and their implications for carbon emissions

Carbon dioxide (CO₂) and methane (CH₄) are important greenhouse gases in the atmosphere and have large impacts on Earth's radiative forcing and climate. Their natural and anthropogenic emissions have often been in focus, while the role of human metabolic emissions has received less attention. In this study, exhaled, dermal and whole-body CO₂ and CH₄ emission rates from a total of 20 volunteers were quantified under various controlled environmental conditions in a climate chamber. The whole-body CO₂ emissions increased with temperature. Individual differences were the most important factor for the whole-body CH₄ emissions. Dermal emissions of CO₂ and CH₄ only contributed ~3.5% and ~5.5% to the whole-body emissions, respectively. Breath measurements conducted on 24 volunteers in a companion study identified one third of the volunteers as CH₄ producers (exhaled CH₄ exceeded 1 ppm above ambient level). The exhaled CH₄ emission rate of these CH₄ producers (4.03 ± 0.71 mg/h/person, mean ± one standard deviation) was ten times higher than that of the rest of the volunteers (non-CH₄ producers; 0.41 ± 0.45 mg/h/person). With increasing global population and the expected large reduction in global anthropogenic carbon emissions in the next decades, metabolic emissions of CH₄ (although not CO₂) from humans may play an increasing role in regional and global carbon budgets.