Human Breathing and CO2 Emissions
Every time you breathe, you inhale air containing about 0.04% CO2 and exhale air containing approximately 4% CO2 -- a hundred-fold increase. The average adult exhales about 200-250 mL of CO2 per minute at rest, which translates to roughly 900 grams (about 2 pounds) of CO2 per day. Over a year, this amounts to approximately 330 kg (730 lbs) of CO2 per person.
However, it is important to understand that human breathing is carbon-neutral in the global cycle. The CO2 we exhale comes from the food we eat, which captured that carbon from the atmosphere through photosynthesis. We are part of a closed carbon loop -- unlike fossil fuel combustion, which releases ancient carbon that was sequestered underground for millions of years.
The Respiration Formula
Cellular respiration converts glucose and oxygen into carbon dioxide, water, and energy. The CO2 production rate varies dramatically with activity level, from about 200 mL/min during sleep to over 3,000 mL/min during intense exercise.
CO2 Exhalation by Activity
| Activity | CO2 (mL/min) | CO2 (g/hour) | Breaths/min |
|---|---|---|---|
| Sleeping | 200 | 23.8 | 8-12 |
| Resting / Sedentary | 250 | 29.7 | 12-16 |
| Light Walking | 500 | 59.4 | 16-20 |
| Moderate Exercise | 1,500 | 178.2 | 20-30 |
| Vigorous Exercise | 3,000 | 356.4 | 40-60 |
Putting It in Context
- All 8 billion humans exhale about 2.6 billion tonnes of CO2 per year -- about 7% of total anthropogenic emissions.
- However, this is carbon-neutral because it was recently fixed from the atmosphere by plants we eat.
- A single car produces about 14 times more CO2 per year than a single human exhales.
- One transatlantic flight emits roughly the same CO2 as a person breathes in an entire year.
Frequently Asked Questions
Is human breathing contributing to climate change?
No. Human breathing is part of the short-term carbon cycle. The CO2 we exhale was recently captured from the atmosphere by the plants and animals we eat. It is carbon-neutral, unlike fossil fuel emissions which add carbon that was stored underground for millions of years.
How much CO2 does a person exhale per year?
An average adult exhales approximately 330 kg (730 lbs) of CO2 per year during normal daily activities. This varies based on body size, metabolic rate, and physical activity level.
Do athletes exhale more CO2?
Yes, during exercise. An athlete during vigorous exercise can produce 10-15 times more CO2 per minute than at rest. However, the total daily increase is modest because exercise periods are relatively short compared to the full day.