The ratio of o2 consumed when people inhale to co2 exhaled in the total air.
Inhalation: nitrogen (about 75%), oxygen (about 24%), water vapor (less than 1%) and carbon dioxide (less than 1%).
Exhaled: nitrogen (about 75%,) oxygen, (about 24%, slightly lower than when inhaled) water vapor (less than 1%), carbon dioxide (less than 1%, slightly higher than when inhaled).