This document defines several key terms related to humidity: - Humidity refers to the amount of water vapor in a gas. It can be expressed in terms of absolute humidity, relative humidity, and dewpoint. - Absolute humidity is the actual amount of water vapor per liter of gas, measured in mg/L. Relative humidity expresses the amount of water vapor as a percentage of the maximum the gas can hold at a given temperature. Dewpoint is the temperature at which the gas reaches 100% relative humidity. - The maximum amount of water vapor a gas can hold depends on temperature - the warmer the gas, the more water vapor it can hold before reaching saturation.

1. What is Humidity?
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HumidificationSeminarSeries
What is Humidity?
What Is Humidity?
Objective: To define the various terms used when talking about humidity and relate
them to humidification in the hospital environment. This presentation is important as
the terms used will become relevant in subsequent presentations.

2. What is Humidity?
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HumidificationSeminarSeries
Forms of Water
Ice
+ heat / energy
Water
+ heat / energy
Water vapour
Forms of Water
Objective: To introduce concepts of water and energy.
There are three states of water:
Ice (solid state) - consists of water molecules that are tightly bound each other,
preventing any movement. When energy is added to ice in the form of heat, it turns
into liquid water.
Water (liquid state) - consists of clusters of water molecules loosely bound to each
other. If energy is added to liquid water in the form of heat, it turns into water vapour.
Water vapour (gaseous state) - consists of individual water molecules which are not
bound to one another and move independently within a gas.

3. What is Humidity?
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HumidificationSeminarSeries
• Upper airways add heat
and moisture to
inspired air
• Moisture is added as
water vapour
• In hospitals moisture is
added as
– Water vapour
– Aerosols
– Saline
Forms of Water in the Airways
LIQUID WATER
LIQUID WATER DROPLET
WATER VAPOUR
Forms of Water in the Airways
Objective: To introduce the different ways water can be added to the airway, both
physiologically and in the hospital.
In the upper airway, heat and moisture are added to inspired air. Moisture is added
as water vapour.
In the hospital environment, moisture or water can be added to the airways in
different forms:
Water vapour such as that seen with heated passover humidifiers
Liquid water droplets, or aerosols such as those produced by nebulisers or bubble
through humidifiers
And liquid water such as that added when instilling saline
Does the form of water added to the airway make a difference?

4. What is Humidity?
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HumidificationSeminarSeries
Liquid Water and Water Vapour
•Passover humidifiers
create water vapour
which cannot transport
bacteria or viruses
•Nebulisers create liquid
water droplets which
can transport bacteria
or viruses
NEBULISED LIQUID WATER 1 - 40 MICRONS
BACTERIA 0.2 - 10 MICRONS
VIRUS 0.017 - 0.3 MICRONS
WATER VAPOUR 0.0001 MICRONS
1Hamill et al., 1995; 2Orec et al., 1997
Liquid Water and Water Vapour
Objective: To introduce bacteria and viruses, how they are created and their relative
sizes. To tell the audience that different devices have different contamination
aspects.
The size of the different forms of water does make a difference in the ability to carry
pathogens.
Aerosols (liquid water droplets) produced by nebulisers are 1 - 40 microns in size.
Bacteria and viruses vary in size from 0.017 - 10 microns, which is less than the size
of some aerosols. Therefore, it is possible for the aerosols to carry pathogens to the
patient.
Water vapour molecules are thousands of times smaller than bacteria and viruses,
so they are incapable of transporting pathogens.
The best way of adding moisture to the airways is through water vapour, as this
cannot transport pathogens.
1Hamill et al., An outbreak of Burkholderia (formerly Pseudomonas) cepacia respiratory tract colonization and
infection associated with nebulized albuterol therapy. Ann Intern Med 1995;122:762–6.
2Orec et al., Mechanisms of bacterial movement in ventilator circuits. Anaesth and Intensive care 1997;25(5):568.

5. What is Humidity?
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HumidificationSeminarSeries
Humidity
•Humidity is water
vapour in a gas
•It is expressed in
terms of:
– Absolute humidity
– Relative humidity
– Dewpoint
Humidity
Objective: To introduce the three terms used to define humidity. Tell the
audience that we will use these terms through the day so they are very useful to
remember.
Humidity is water vapour in a gas.
We can express humidity in three ways:
̇ Absolute humidity - the actual amount of water vapour per litre of gas,
measured in mg/L.
̇ Relative humidity - the measure of how much water vapour is actually in the
gas, compared to its capacity to hold water vapour. Relative humidity is
measured as a percentage (%).
̇ Dewpoint - the temperature where the gas is at 100% relative humidity (100%
full) or saturated, measured in degrees Celsius. If a gas cools below this
temperature, excess water vapour is lost as condensation.

6. What is Humidity?
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HumidificationSeminarSeries
Absolute Humidity
(mg/L)
Is the actual amount of
water vapour in a litre
of gas
1L
Absolute Humidity
Objective: To define the term absolute humidity
Absolute humidity is the actual amount of water vapour per litre of gas.
It is measured in milligrams of water vapour per litre of gas. If we look at this bottle,
which represents a litre of gas , it contains 22 mg of water vapour. Therefore, its
absolute humidity is 22 mg/L.
If more water vapour is added to the litre of gas, its absolute humidity will increase.
There is however, a limit to the amount of water vapour a gas can hold and this is
called its maximum capacity (to be covered later).

7. What is Humidity?
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HumidificationSeminarSeries
Relative Humidity
•Expressed as a %
•Is the actual amount
of water vapour in a
gas compared to the
maximum amount
this gas can hold
44 mg/44 mg = 100% RH
22 mg/44 mg = 50% RH
100% 50%
44 mg 22 mg
Relative Humidity
Objective: To define the term relative humidity.
Relative humidity is a way of describing how saturated (full of water) a gas is.
That is, how much water vapour is actually in the gas compared to its maximum
capacity. We express this as a percentage. As we learnt in school, to work out a %
we use the formulae
% = Content/capacity x 100
For example, if we look at the bottle on the left which represents one litre of gas, it
contains 44 mg of water vapour (content). At maximum capacity of a gas to hold
water vapour at 37 °C is 44 mg per litre (capacity), therefore the gas is full or 100%
relative humidity.
The bottle on the right represents another litre of the same gas. It too could hold a
maximum of 44 mg of water vapour (capacity), but has only 22 mg (content).
% = Content/capacity x 100
% = 22mg/44mg x 100 = 50%
The gas is then only 50% full or is at 50% relative humidity.
The maximum capacity, however, is not constant, it changes with temperature.

8. What is Humidity?
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HumidificationSeminarSeries
•Warming the gas
increases its
maximum
capacity to hold
vapour
•Cooling the gas
reduces its
capacity to hold
vapour
Maximum Capacity
Temperature (oC)
44
30
30 37
AbsoluteHumidity(mg/L)
Maximum Capacity
Objective: To introduce the relationship between temperature and capacity to hold
water vapour - warming a gas increases its ability to hold water vapour
The maximum capacity of a gas to hold water vapour changes with temperature.
Warming a gas will increase its maximum capacity to hold water vapour while
cooling a gas will reduce its maximum capacity.
This graph shows how the maximum capacity (red line) of a gas varies with
temperature. For example;
At 30 °C, the gas can hold a maximum of 30 mg/L. At 37 °C the gas can hold a
maximum of 44 mg/L. If the gas is warmed from 30 °C to 37 °C, its capacity to hold
water vapour increases from 30 mg/L to 44 mg/L.
If the gas is then cooled from 37 °C back to 30 °C its capacity to hold water vapour
reduces from 44 mg/L to 30mg/L.

9. What is Humidity?
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HumidificationSeminarSeries
Temperature and Capacity
30 mg/L
32 mg/L
34 mg/L
36 mg/L
38 mg/L
40 mg/L
42 mg/L
44 mg/L
30 °C 31 °C 32 °C 33 °C 34 °C 35 °C 36 °C 37 °C
Temperature & Capacity
Objective: To reinforce the concept that the warmer a gas is, the more humidity it
can hold.
The capacity of a gas to hold water vapour changes with temperature.
This graph shows that as a gas is warmed it can hold more water vapour.
At 30 °C the gas can hold a maximum of 30 mg/L. At 37 °C, the gas can hold a
maximum of 44 mg/L.

10. What is Humidity?
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HumidificationSeminarSeries
Dewpoint
•Expressed in °C
•Is a temperature
where the gas is at
100% relative
humidity (RH)
•Below this
temperature water
vapour is lost as
condensation
Dewpoint
Objective: To introduce the concept of dewpoint. To help the audience
understand how and why condensate forms.
The third way of expressing humidity is dewpoint.
Dewpoint is a temperature where the gas is at 100% relative humidity. Below this
temperature water vapour is lost as condensation.
To illustrate dewpoint, think of a bottle that you take out of the refrigerator. This
bottle will be colder than the ambient temperature of the room. When the air in the
room touches the cold glass, it cools down. The room air can no longer hold the
same amount of water vapour at this colder temperature and the lost water
vapour appears as condensate on the glass.

11. What is Humidity?
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HumidificationSeminarSeries
15 °C
2% RH
0.3 mg/L
37 °C
100% RH
44 mg/L
40 °C
86% RH
44 mg/L
22 °C
100% RH
20 mg/L
AH, RH & Dewpoint
AH, RH & Dewpoint
Objective: To use a practical example to bring the terms together. To use this
example to explain how a heated humidifier and heated circuit works.
Let’s work through how heating & cooling a gas changes the absolute humidity,
relative humidity and dewpoint of a gas.
A container of dry medical gas is cold and virtually devoid of any moisture.
If we heat the gas to 37 °C and add humidity to achieve 100% relative humidity, the
gas can hold 44 mg of absolute humidity per litre of gas. At this temperature, the
gas is at its dewpoint (37 °C). This is what happens in a chamber.
If the gas is heated to 40 °C and no water vapour is added as would occur in a
heated circuit the amount of water vapour stays the same. Its capacity to hold
water vapour increases to 51 mg/L, therefore the relative humidity will reduce to
44/51 x 100 = 86%.
Cooling the gas back to 37 °C will increase the relative humidity back to 100%.
If the gas is cooled further the maximum capacity reduces and condensation will
form.