It is the amount of water vapour present in the atmosphere and is subdivided into two types: Absolute humidity: - The total mass of water vapour present in the air per unit volume (kg.m -3 or g.m -3 ). -It is temperature dependent: at 20 C it is 17g.m -3 whereas at 37C it is 44 g.m -3 Relative humidity: The ratio of the amount of water vapour in the air compared with the amount that would be present at the same temperature if the air was fully saturated (RH, %).

Humidity graph

Dew point: The temperature at which the relative humidity of the air exceeds 100% and water condenses out of the vapour phase to form liquid (dew). Hygrometer: An instrument used for measuring the humidity of a gas. Hygroscopic material: One that attracts moisture from the atmosphere.

Methods of humidification HME (heat and moisture exchange HME) filter: -Passive. -Efficiency may reach 70%. -It contains a hygroscopic material within a sealed unit. As the warm expired gas cools so the water vapour condenses on the element, which is warmed both by the specific heat of the exhaled gas and the latent heat of the water.

Methods of humidification HME (heat and moisture exchange) filter: -Inhaled, dry and cool gas is thus warmed during inspiration, during which process the element cools down prior to the next exhalation. -Problems include moderate inefficiency with prolonged use, increased dead space and infection risk.

Methods of humidification Water bath (cold): -Passive. -Dry gases bubble through water at room temperature. -Inefficient (30%) and becomes even more so as the loss of latent heat of vaporization cools the water further.

Methods of humidification Water bath (warm): -Active. -Dry gases bubble through water which is heated, usually to 60 ° C (to inhibit microbial contamination). -Efficiency is greater than 90%. -They are more complex and there is a risk of thermal injury to the patient (which is minimized by thermostats).

Methods of humidification Cascade humidifier: -This is a variation on the warm water bath. -Gas is allowed to bubble through a perforated plate. -This process maximizes the surface area which is exposed to water.

Methods of humidification Nebulizers: -Active. -Gas driven nebulizers: A high-pressure gas stream is directed on to an anvil and entrains water which then breaks into droplets. -Ultrasonic nebulizers: Water is nebulized by a plate that vibrates at ultrasonic frequencies.

Droplet size -Droplets of 1 micron (mm) will be deposited in the alveoli. This is optimal. -Smaller droplets may simply pass in and out with the respiratory cycle. -Larger droplets (5 mm) risk being deposited in the trachea, which may help loosen secretions, but will not humidify the distal airways. -Larger droplets still, of 20 mm and above, will not travel further than the upper airway and may condense out in the equipment tubing itself.

Methods of measuring humidity Hair hygrometer: the hair, which is linked to a spring and pointer, elongates as humidity increases. It is accurate between relative humidity measurements of about 30% and 90%. Wet and dry bulb hygrometer: this is a cumbersome technique. The temperature difference between two thermometers relates to evaporation of water round the wet bulb, which in turn relates to ambient humidity. The figure is calculated from tables.

Methods of measuring humidity Regnault’s hygrometer: this is a more accurate technique in which air is blown through ether within a silver tube. The temperature at which condensation appears on the outer surface is the dew point, the temperature at which ambient air is fully saturated. The ratio of the saturated vapour pressure (SVP) at the dew point to the SVP at ambient temperature gives the relative humidity. The result is determined from tables.

Methods of measuring humidity Transducers: as a substance absorbs atmospheric water, there is a change either in capacitance or in electrical resistance. Mass spectrometer: this is very accurate and has a rapid (breath-by-breath) response time. The equipment is expensive.

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Thank YOU DR. Ahmed Mostafa