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Principles of Humidification

Since humidification and humidity therapy are so important to respiratory well-being, you need to take a moment and review the basic physical principles of humidity. Humidity is essentially the water vapor in a gas. This water vapor can be described in several ways, as:

Absolute humidity - The actual content of water vapor in a gas measured in milligrams per liter. Potential humidity - The maximum amount of water vapor that a gas can hold at a given temperature. Relative humidity - The amount of water vapor in a gas as compared to the maximum amount possible, expressed as a percentage.

When these three are presented in equation form, their relationship becomes more clear:

When a gas or air becomes heated, it expands and more spaces is created between the molecules. The resulting warmer gases have greater capacity for "holding" more water vapor than do cooler gases. Therefore, potential humidity increases as the temperature of a gas increases. As a result, warm, humidified gas traveling through tubing tends to "rain-out" water vapor as the gas cools and has a lower water-carrying capacity. The Table below illustrates the relationship of temperature and potential humidity:

Temperature&Water Content Water Vapor Pressure

Water Vapor Content of Air

One of the more important gases found in air is: water vapor. The amount of water vapor in the air can vary widely day-to-day, while gases like oxygen and nitrogen are present in relatively constant amounts. In general, water in vapor form is governed by gas laws and can be treated as a gas.

All bodies of water or moist organic bodies are capable of giving off water vapor. When water enters the air as a gas, the air's humidity increases. The measure of how much water vapor is contained in the air is identified as the humidity level, and the factors determining the humidity include:

1. The availability of water. Clearly the air over a desert has less chance of picking up water vapor than the air over a lake.
   
   
2. Temperature is also a factor since the spacing of warmer air's molecules allows water vapor's molecules to fit more easily. Recalling the principles of Charles' law, the volume of a gas increases as its temperature increases. If the number of molecules of the gases increases as the temperature rises, the humidity will not increase as much.

Summarizing, if there is water available there will be a specific amount of water vapor in the air at each ambient temperature. That amount equals the air's total capacity for water vapor at that temperature. Air that contains its total capacity for water vapor at a specific temperature is said to be 100% saturated.

Amounts of water found in air are generally measured as grams per cubic meter of air (gm/ml) or as milligrams per liter of air (mg/1). These measurements can then be converted to moles of water by dividing by the gram molecular weight of water (which is 18). The total capacity of the air for water vapor is measured in milligrams per liter, with total capacity of the air for water vapor in milligrams per liter is referred to as the air's potential water vapor. Table 1 identifies the various values of the potential water vapor content at several different temperatures.

THE POTENTIAL WATER VAPOR IS THE MAXIMUM THE AIR CAN HOLD AT A CERTAIN TEMPERATURE.

Table 1: Value of Potential Water Vapor Content

Air's capacity for water cannot be met if there is not enough water available, and to discover the actual amount of water vapor present in the air it is necessary to measure the absolute humidity.

Relative humidity is another important measurement, and it represents the actual amount of water vapor in the air (absolute humidity) compared to the total possible water vapor content of the air at the given ambient temperature (potential water vapor). The measurement of relative humidity is expressed as a percentage (of saturation). For example, if the air contained only 17 mg/l of water vapor at 25°C, then it would not be totally saturated (see Table 1).

Calculating the relative humidity involves dividing the absolute humidity (actual water vapor content of the air) by the potential water vapor (maximum possible water vapor content of the air), and multiplying by 100 to convert the decimal percentage:

For the example above where the air had 17 mg/l of water vapor at 25° C: