Introduction

The question of how much carbon dioxide (CO2) the average human breathes out each day is often a subject of curiosity in the fields of physiology and environmental science. Understanding this process involves delving into the intricacies of human metabolism, respiration, and the dynamics of gas exchange within the body.

Human Metabolism and CO2 Production

An average sized adult human breathes out about 1 kg of CO2 per day. At sea level, this translates to approximately 23 moles of CO2, equivalent to about 500 liters or half a cubic meter, which equates to roughly 18 cubic feet. This amount can vary depending on the individual's size, activity level, and diet.

Breathing and Metabolic Rates

The production of CO2 is directly related to the metabolic rate, which measures the rate at which the body uses energy. One liter of oxygen (O2) consumed releases approximately 5 kcal of energy. Therefore, an individual consuming a 2000 kcal/day diet and burning 2000 kcal of food daily would require around 400 liters or 16.4 moles of O2 per day.

Basis of the Calculations

Given that the burning of carbohydrate releases one CO2 molecule per O2, the burning of fats releases 0.67 CO2 molecules per O2, and the burning of proteins releases an intermediate value, an average of 0.85 CO2 molecules per O2 can be used. This leads to an estimated 9.7 millimoles of CO2 produced per minute, or 404 mg of CO2.

Respiratory Output and CO2 Concentration

Assuming 20 breaths per minute with each breath containing 500 ml of air (or 10 L in total), the concentration of CO2 in the exhaled air is about 40 mg/L. Since fresh air contains approximately 0.04% CO2 by volume, the exhaled air contains about 50 times the amount of CO2 found in ambient air. This calculation confirms the reality that an order-of-magnitude higher CO2 production would lead to the depletion of oxygen from each breath, which is physiologically impossible.

Comparison with Estimates

Michael Cromer estimates that exhaled CO2 is 4% by volume. This aligns with the calculations based on metabolic rates and respiratory mechanics, reinforcing the accuracy of our estimates.

Conclusion

Understanding the amount of CO2 breathed out daily provides insights into human physiology and the importance of respiratory processes. The calculations indicate that an average adult releases a significant amount of CO2 daily, highlighting the balance between oxygen consumption and carbon dioxide production.

Further Reading and Research

For further exploration, interested readers may consult academic journals and research papers on human physiology and respiration. Additionally, studies on the effects of diet and activity on respiratory output can provide a deeper understanding of this topic.