A Breath Of Fresh Air

The world’s human population—people breathing—is a significant source of carbon dioxide added to our atmosphere daily. Although I’d expect it to be mentioned in our discussions of the “CO₂ problem,” I’ve missed references to this fact (C&EN, Aug. 2, page 35). Of course, every person directly or indirectly contributes additional CO₂ by the combustion of fuels in the production of energy for various reasons, but breathing as a source of CO₂ is excluded from the current discussion. Very simple calculations show that simply by being alive, the global population of human beings produces 5–10% of the estimated total CO₂ generated from all sources.

These calculations are based on data available in the open literature, including Wikipedia. Certainly, more sophisticated models can be developed, but I’ve attempted only to provide a simple estimate. For the record, I’ll summarize the data used, with references, and state my approach. Actually, this might be an interesting problem for high school students to work on.

A resting adult breathes 15 times a minute (1), and a breath is 0.5 L (2). Breathing by an adult increases the amount of CO₂ from a mole fraction of 0.000395 inhaled to 0.042 exhaled (3), which amounts to an increase of 0.88 kg CO₂/person/day. Of course, a mole of CO₂ occupies 22.4 L at standard temperature/pressure (STP) and weighs 44 g. This simple model ignores the fact that we don’t all live and breathe in an STP environment, we aren’t all resting, and we aren’t all adults. World CO₂ production is 29.3 billion metric tons per year (4), and world population is 6.9 billion people (5). I’ll leave it to the reader to consider the implications of these numbers.

Perhaps of greater interest is the amount of CO₂ generated from all living systems. Estimation of that CO₂ source is formidable indeed.

For those who are interested, the footnotes refer to (1) en.wikipedia. org/wiki/Respiratory_rate; (2) hypertextbook.com/facts/2001/LaurenCalabrese.shtml; (3) Altman, Philip L. and Dorothy S. Dittmer. “Respiration and Circulation.” Bethesda, MD: FASEB, 1971; (4) en.wikipedia.org/wiki/List_of_countries_by_carbon_diox ide_emissions; and (5) en.wikipedia.org/wiki/World_population.

By James P. Hardy
Franktown, Colo.