The amount of water vapour per unit volume of gas at given temp and pressure g/m3 or mg/l.

Absolute humidity divided by the amount of water vapour present if fully saturated (at same temp and pressure) ie the percentage.

Dry gases
Heat loss.
Mucosal drying with cilial damage and keratinisation.
Tenacious secretions and mucous plugging
atelectasis and infection.

High humidity uncomfortable.
Low humidity increases risk of sparks.


Hair hygrometer
Hair gets longer as humidity . 30-90% RH
Wet and dry bulb hygrometer
1 thermometer reads ambient temp while another surrounded by a wet wick reads a cooler temp because of loss of latent heat of vapourisation.
Difference in temp compared to tables.
Regnaults hygrometer
Air bubbled through ether in a silvered tube cooling of ether and tube.
Condensation occurs at the dew point which is the temp at which the ambient air is fully saturated.
Ratio gives the relative humidity.

Resistance and capacitance sensors.
Mass spectrometer
Very accurate and rapid (breath by breath).

Methods of humidification

HME filter
Wetted and heated during expiration heating and humidification of inspired gas.
Up to 80% efficient.
Cold water bath
25% efficient.

Hot water bath
Heated to 60% to prevent microbial growth.
Up to 100% efficient.
Venturi or ultrasonic (US >100%).
Droplet size- 1micron get to alveoli; 5 trachea.