CO2 Breathing Emission Calculator

Exhaled carbon dioxide is a direct output of cellular respiration and scales with metabolic rate, which is why this calculation runs through oxygen consumption rather than lung volume. One metabolic equivalent of task, or MET, is defined as an oxygen uptake of 3.5 mL per kg of body mass per minute, approximately the resting requirement of an average adult. Multiplying by the MET value of an activity gives VO2, and multiplying VO2 by the respiratory quotient converts oxygen consumed into carbon dioxide produced. RQ is 1.0 for pure carbohydrate oxidation, about 0.70 for fat and roughly 0.80 to 0.85 for a mixed diet. The volumetric result is converted to mass using a CO2 density of 1.977 g per litre at 0 degrees C and 101.3 kPa. For a 70 kg adult at rest this yields roughly 200 mL CO2 per minute, near 0.55 to 0.9 kg per day depending on how much of the day is sedentary, and on the order of 250 to 330 kg per year. Vigorous exercise at 7 METs raises the instantaneous rate about sevenfold. Three limitations apply. The 3.5 mL/kg/min constant is a population average that overestimates VO2 in obese individuals, because adipose tissue is metabolically quiet yet counts fully in body mass. Standard MET values assume a 70 kg reference adult and are not adjusted for age or fitness. Most importantly, exhaled CO2 is biogenic carbon that entered food through photosynthesis, so it circulates within the short carbon cycle and does not add net carbon to the atmosphere the way fossil fuel combustion does.

The link between breathing and combustion was established by Antoine Lavoisier, working with Pierre-Simon Laplace in Paris in the early 1780s. Using an ice calorimeter they measured the heat released and the fixed air, as carbon dioxide was then called following Joseph Black's identification of it in 1754, produced by a guinea pig, and showed that respiration is a slow oxidation obeying the same chemistry as burning charcoal. Nineteenth-century physiologists turned this insight into a quantitative technique. Henri Victor Regnault and Jules Reiset built closed-circuit apparatus in 1849 to measure the ratio of carbon dioxide produced to oxygen consumed in animals, and the resulting respiratory quotient became a standard indicator of which fuel the body was burning. Practical measurement in humans followed with Nathan Zuntz's portable respiration apparatus around 1900 and the Douglas bag, introduced by Claude Gordon Douglas in 1911, which allowed expired gas to be collected during real exercise and analysed afterwards; indirect calorimetry still rests on this principle. The MET scale used here emerged from exercise epidemiology's need for a common unit of activity intensity. Barbara Ainsworth and colleagues published the first Compendium of Physical Activities in 1993, assigning MET values to hundreds of coded activities, with major updates in 2000 and 2011 that remain the reference for activity classification in both research and public health guidance.