What unit is used for humidity deficit in this context?

• A. mg/L
• B. ppm
• C. mmHg
• D. L

Answer: (A)

Humidity deficit is the amount of water that must be added to the inspired gas per liter to reach full humidification at body temperature. This is a mass-per-volume measure, so the natural unit is milligrams of water per liter of gas (mg/L). At 37°C, saturated absolute humidity is about 44 mg/L, so the deficit is 44 mg/L minus the current absolute humidity.

Other units don’t fit: ppm is a volume-based concentration and isn’t a measure of water content per liter of gas; mmHg is a pressure unit and doesn’t quantify water mass in the gas; liters describe volume, not the water content. Hence mg/L is the appropriate unit.