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Principles of Humidification

Humidity Deficit

Another factor to consider is the humidity deficit. For example, if the atmosphere's relative humidity is less than 100%, the air of the atmosphere has what is referred to as a humidity deficit. If outside air at 20°C has 14 mg/l of water vapor, and needs to have 17.3 mg/l to be fully saturated, it is said to have a primary humidity deficit of 3.3 mg/l. To calculate the primary humidity deficit simply subtract the absolute humidity of the air from its potential water vapor at the appropriate temperature and the difference between the two is the primary humidity deficit:

Primary Humidity Deficit = Potential Water Vapor Content - Actual Water Vapor Content

The secondary humidity deficit also needs to be considered. This is the moisture deficit in the inspired air that the nose and upper airway need to compensate for. When air is breathed into the nasal cavity and heated to body temperature, its potential water vapor rises to 44 mg/l which is the potential water vapor content of air at 37°C.

Therefore, unless the air of the atmosphere is at least 37°C and fully saturated, there exists a moisture deficit. For example, if the atmosphere's air was 98.6°F (37°C) and the relative humidity 100%, people in such conditions would be very hot and sweaty! Luckily, inspired air is generally not that warm or humid, so most inspired air does have a secondary humidity deficit.

The secondary humidity deficit is calculated by subtracting the absolute humidity of the air from the potential water vapor content. The difference from the calculations for primary humidity deficits is that the potential water vapor content is always 44 mg/l, the potential water vapor of air at body temperature:

Secondary Humidity Deficit = 44 mg/l - Absolute Humidity.

Therefore, if inspired air's absolute humidity is 16 mg/l at 25°C before being warmed in the nasal cavity, there is a primary humidity deficit of:

Primary Humidity Deficit = 23 mg/l - 16 mg/l = 7 mg/l

For this same absolute humidity for the atmosphere, the secondary humidity deficit is:

Secondary Humidity Deficit = 44 mg/l - 16 mg/l = 28 mg/l

Here are some other calculations for you to consider:

• If the absolute humidity of the same inspired air at 25°C were 23 mg/l, there would be no primary humidity deficit because the potential water vapor of air at 25°C is 23 mg/l (see Table above). There would still be a secondary humidity deficit of 21 mg/l because 44 mg/l minus 23 mg/l is 21 mg/l. This illustrates that at 100% relative humidity, it is still possible that the nasal cavaty's lining will have to supply moisture to the inspired air. You may sometime need to calculate a primary or secondary humidity deficit when you only know the relative humidity. To do perform this calculation, you first need to convert the relative humidity into absolute humidity before calculating the humidity deficit. Remember the formula for relative humidity:
  

  Relative Humidity=

  Absolute Humidity x 100 Potential Water Vapor Content

  
  
  
• When the temperature and relative humidity are known, you can look up the potential water vapor for the air temperature by using the Table shown above. Then substitute into the above formula the values for relative humidity and potential water vapor and calculate the absolute humidity. For example, the relative humidity at 10°C is 75% for the air in the atmosphere. Using the Table, you can see that the potential water vapor content at 10°C is 9.5 mg/l, so:
  

  Relative Humidity=

  Absolute Humidity x 100 Potential Water Vapor Content

  Absolute Humidity = 7.125 mg/l

This measurement of the absolute humidity can be used to calculate the primary and secondary humidity deficits of the same air. At 10°C, we know that the potential water vapor content is 9.5 mg/l, so the primary humidity deficit is:

Primary Humidity Deficit = Potential Water Vapor Content - Absolute Humidity
9.5 - 7.125 = 2.375 mg/l

The secondary humidity deficit for the same air would be:

Secondary Humidity Deficit = 44 - Absolute Humidity
44 - 7.125 = 36.875 mg/l

To summarize:

• The primary humidity deficit occurs in the atmosphere and represents the difference between what humidity there is and what there could be.
• The secondary deficit occurs in the body and represents the difference between what humidity there is and what there needs to be at body temperature (37°C).

Water Vapor Correction

Since we've already ascertained that water vapor acts in most ways like any other gas, air creates a partial pressure when its in a mixture of gases. That partial pressure depends on the amount of water vapor present, which in turn depends on the temperature. However, water vapor differs from the behavior of other gases in the air since changes in the barometric pressure of the atmosphere under normal conditions do not have much impact on the partial pressure of water.

As a result, it is best to calculate the partial pressures of the other gases in the air after the partial pressure of water vapor has been determined--especially when measuring the air within the lungs. Inside the lungs, the partial pressure of water vapor is approximately 47 mm Hg. This value is relatively constant because the air entering the lungs is normally saturated and at 37°C. By subtracting the partial pressure of the water vapor from the total atmospheric pressure, you will find what is referred to as the dry gas pressure.

In the lungs, Dry Gas Pressure = Atmospheric Pressure - 47 mmHg

At one atmosphere (760 mmHg), the dry gas pressure would be:
Dry Gas Pressure = 760 - 47 = 713 mmHg