What percentage of water is lost through the respiratory system?
The kidneys maintain the balance between water input and output.
The normal daily input is about 2.5 L from beverages food, and metabolism.
The normal daily output of 2.5 L consists of 1.8 L in urine, sweat, and feces, plus 0.7 L in insensible water loss.
Insensible water loss (so-called because we are not aware of it) is water that evaporates through the skin and from the respiratory tract.
Insensible loss differs from sweat, because sweat contains solutes.
Insensible losses account for 0.7 L of water per day.
The average human exhales 0.35 L of water each day.
The amount varies with air temperature, relative humidity, and level of activity, so the range is probably 0.3 L to 0.45 L per day.
This corresponds to about 10 % to 20 % of water lost through respiration.