Humidity and Bland Aerosol Therapy: An Overview (2025)

The human respiratory system is uniquely equipped to condition the air we breathe, warming and humidifying it to protect delicate lung tissues and promote effective gas exchange.

However, when natural conditioning mechanisms are bypassed—such as during intubation or mechanical ventilation—external methods must be used to maintain proper humidity and temperature levels.

This is where humidity and bland aerosol therapy come into play. These therapies are essential for preventing airway injury, maintaining mucociliary function, and ensuring patient comfort and safety in clinical settings.

In this article, we’ll explore how these therapies work, why they’re important, and the various methods used to deliver them.

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Gas Conditioning and the Role of the Upper Airway

Under normal conditions, the nose and upper airway act as the body’s natural humidifier. As air passes through the nasal passages and pharynx, it is warmed and moistened before reaching the lower respiratory tract.

This conditioning process helps maintain the integrity of airway tissues and supports the immune system’s ability to trap and clear pathogens.

When a patient requires mechanical ventilation or has a tracheostomy tube in place, this natural pathway is bypassed. As a result, the air delivered directly to the trachea lacks adequate heat and humidity, which can dry out mucous membranes, impair mucociliary clearance, and even lead to structural damage in the lungs. Therefore, artificial humidification becomes not just beneficial, but critical.

Recommended Humidity and Temperature Levels

The temperature and humidity of inhaled gas should closely match what the body naturally provides. Here are the standard guidelines for respiratory gas conditioning:

• Gas delivered to the nose and mouth should be warmed to 20°C to 22°C with a water vapor content of approximately 10 mg/L, which is equal to 50% relative humidity.
• Gas delivered to the trachea should be conditioned to a higher standard—35°C to 40°C with 40 to 45 mg/L of water vapor, achieving greater than 90% relative humidity.

Note: Meeting these targets is essential for maintaining normal airway function and avoiding complications such as thickened secretions, airway occlusion, or infection.

Why Inadequate Humidification Is Harmful

Failing to properly humidify inspired gases can lead to several complications, including:

• Mucosal drying and irritation
• Impaired mucociliary clearance, increasing the risk of infection
• Atelectasis due to dried secretions obstructing the airways
• Airway inflammation and trauma from cold, dry air

Note: For these reasons, respiratory therapists and clinicians must understand how to use humidification systems effectively based on the patient’s condition and mode of ventilation.

Humidifier vs. Nebulizer: Understanding the Difference

Although both humidifiers and nebulizers are used to add moisture to the respiratory tract, they serve distinct purposes and operate differently.

Humidifier

A humidifier is a device that adds invisible molecular water vapor to inspired gas. Its primary goal is to provide adequate humidity to maintain mucosal integrity and prevent the drying of secretions.

Humidifiers are especially important when delivering dry medical gases like oxygen over extended periods or when bypassing the upper airway with devices like endotracheal or tracheostomy tubes.

Nebulizer

A nebulizer, on the other hand, produces visible liquid particles, or aerosols, by dispersing sterile water or saline into a gas stream. This is known as bland aerosol therapy, and it serves therapeutic purposes beyond just humidification. Nebulizers are used to hydrate secretions, reduce upper airway edema, and deliver medications or induce sputum samples.

Note: Understanding the difference between vapor and aerosol is essential: water vapor does not carry pathogens, while aerosols and condensate can harbor and transmit infectious organisms.

Infection Risks and Pathogen Transmission

While humidification supports airway health, it also introduces the potential for infection, especially when not managed properly.

• Condensate within breathing circuits can serve as a breeding ground for bacteria. This liquid must be treated as infectious waste and disposed of with strict precautions.
• Microaerosol particles generated at high flow rates by some bubble humidifiers can carry infectious bacteria, increasing the risk of nosocomial infections.
• Equipment like nebulizers and humidifier chambers must be cleaned, disinfected, or replaced regularly to reduce the risk of cross-contamination.

Note: Maintaining strict hygiene protocols, monitoring for condensation, and using sterile water are all essential practices for infection control in humidity and aerosol therapy.

Types of Humidifiers

There are several types of humidifiers used in clinical practice, each with unique mechanisms and applications:

• Bubble Humidifiers: These devices direct gas through a column of water, creating bubbles that pick up moisture. They’re commonly used with low-flow oxygen delivery systems. However, their performance diminishes at high flow rates and they may generate infectious aerosols.
• Pass-Over Humidifiers: These allow gas to flow over a heated water surface, picking up moisture. They are more efficient than bubble humidifiers and often used in ventilator circuits.
• Wick Humidifiers: A porous wick is placed in a water reservoir and draws up water through capillary action. As gas flows past the wick, it becomes humidified. These are highly efficient and commonly used in mechanical ventilation systems.
• Heat and Moisture Exchangers (HMEs): Known as “artificial noses,” HMEs are passive humidifiers that capture heat and moisture from a patient’s exhaled breath and return it during inhalation. Most HMEs provide around 70% efficiency in heat and moisture conservation.

Note: Respiratory therapists must know when and when not to use HMEs, why active humidifiers are sometimes necessary, and the clinical complications that can arise with humidification systems.

Heat and Moisture Exchangers (HMEs)

HMEs are commonly used in mechanical ventilation because they’re compact, cost-effective, and require no power source. By capturing heat and moisture from the patient’s exhaled gas, they provide a simple way to condition inspired air—acting much like the natural humidifying function of the upper airway.

However, HMEs are not suitable for every clinical situation. While many standard adult patients benefit from HME use, certain populations and ventilation strategies make them ineffective—or even harmful.

When Not to Use HMEs

• Infants and Neonates: HMEs are not recommended for use with small children due to the risk of increased mechanical dead space, which can impair ventilation. Additionally, infants often use uncuffed endotracheal tubes (ETTs), allowing exhaled gases to bypass the HME, making them less efficient.
• Patients Requiring Lung-Protective Ventilation: In cases such as ARDS, where small tidal volumes are used to minimize lung injury, HMEs can contribute to CO₂ retention and hypercapnia. Their added resistance and dead space can reduce the effectiveness of ventilation.
• Patients with Copious Secretions: Thick or excessive mucus can clog the HME, increasing airway resistance and potentially compromising ventilation. In these cases, heated active humidification is a safer and more effective alternative.

Active Heated Humidifiers

Unlike HMEs, active humidifiers use electric heating elements to warm a water reservoir, delivering fully saturated gas at body temperature. These systems typically include temperature and humidity sensors, heated wires to reduce condensation, and feed systems that ensure continuous water delivery.

Active humidification is especially useful for:

• Patients with artificial airways
• Those on high-flow ventilation
• Situations where precise humidification is required (e.g., thick secretions, lung injury, or long-term ventilation)

Note: Although more complex and costly than HMEs, active systems offer superior control and are better suited for critically ill patients.

Common Complications of Humidification Systems

Even when used properly, humidification systems can introduce clinical challenges. The most common issues include:

• Condensation: Water buildup in breathing circuits can alter gas flow and harbor bacteria. It must be drained carefully and treated as infectious waste.
• Cross Contamination: Improper handling of humidifier components or poor cleaning practices can spread bacteria between patients.
• Inadequate Gas Conditioning: Failure to maintain target temperature and humidity can lead to mucosal drying, impaired secretion clearance, and increased infection risk.

Note: To prevent these issues, clinicians must routinely inspect equipment, follow strict infection control procedures, and monitor the performance of humidification systems during therapy.

What is Bland Aerosol Therapy?

Bland aerosol therapy involves the delivery of sterile water or saline particles into the airway in the form of a cool or heated mist. Unlike humidifiers, which produce invisible vapor, aerosol therapy creates visible liquid droplets that help hydrate the airways and serve specific therapeutic purposes.

Primary Uses of Bland Aerosol Therapy

Bland aerosol therapy is commonly used for the following clinical reasons:

• Treat Upper Airway Edema: Cool, bland aerosol can soothe inflamed tissues, particularly in conditions like croup, post-extubation stridor, or smoke inhalation injuries.
• Overcome Heat and Humidity Deficits: Patients with bypassed upper airways (such as tracheostomy or endotracheal tubes) often lack adequate humidification, leading to thick secretions. Aerosol therapy helps restore airway moisture.
• Aid in Sputum Induction: In patients with a weak or ineffective cough, bland aerosol delivery can loosen mucus, making it easier to obtain sputum samples for diagnostic testing.

Devices for Bland Aerosol Delivery

Several devices are available to generate and deliver bland aerosol, each with specific applications:

• Large-Volume Jet Nebulizers: These are the most commonly used devices for bland aerosol therapy. They use compressed gas to generate mist and can be connected to trach collars, aerosol masks, face tents, or T-tubes.
• Ultrasonic Nebulizers (USNs): These devices use high-frequency sound waves to produce a dense aerosol mist. They’re efficient and quiet but more expensive and prone to overheating with continuous use.
• Mist Tents: Though less common today, mist tents or croup tents are used primarily in pediatric settings to provide cool aerosol around the patient’s head and upper body.

Note: Each of these systems can vary in particle size and aerosol density, affecting how deeply the particles penetrate into the respiratory tract.

Potential Complications and Safety Considerations

While bland aerosol therapy can be highly beneficial, it’s not without risks. Common complications include:

• Cross Contamination and Infection: As with humidifiers, aerosols can spread pathogens if equipment is not kept clean or if circuits are not handled properly.
• Overhydration: Excessive aerosol delivery can lead to fluid overload, especially in small children or patients with compromised renal function.
• Bronchospasm: Some patients may react to cold or hypotonic aerosol with airway irritation or spasm. Monitoring for coughing, wheezing, or increased respiratory effort is essential.
• Inadequate Mist Production: Device malfunction, water depletion, or low flow rates can compromise therapy effectiveness. Regular checks are necessary.
• Environmental Safety and Noise: Continuous aerosol generation can increase ambient humidity in the room, potentially affecting electrical equipment or staff comfort. Ultrasonic nebulizers may also generate distracting noise in quiet clinical settings.

Humidity and Bland Aerosol Therapy Practice Questions

1. What are the types of humidifiers?
Bubble humidifier, passover humidifier, heat and moisture exchanger (HME), and room humidifier

2. What does humidity therapy involve?
It involves adding water vapor and sometimes heat to an inspired gas.

3. Heat and moisture exchange is a primary function of what?
It is a primary function of the upper respiratory tract, mainly the nose.

4. Which is more effective at providing heat and moisture, the mouth or nose?
The nose is more effective.

5. What is ISB?
It stands for isothermic saturation boundary and is the point that is normally 5 cm below the carina.

6. What happens above the ISB?
The temperature and humidity decrease during inhalation and increase during exhalation

7. What happens below the ISB?
The temperature and relative humidity remain constant (BTPS).

8. What is BTPS?
It stands for: body, temperature at the pressure to which the patient is exposed and 100% saturated with water vapor.

9. What is the primary goal of humidification?
To maintain normal physiologic conditions in the lower airways

10. What are the indications for humidification therapy?
Humidifying dry medical gases, flows greater than 4 L/min, overcoming a humidity deficit created when the upper airway is bypassed, managing hypothermia, and treating bronchospasm caused by cold air

11. What are the clinical signs and symptoms of inadequate airway humidification?
Atelectasis, dry or nonproductive cough, increased airway resistance, increased incidence of infection, increased work of breathing, patient complaint of substernal pain and airway dryness, and thick dehydrated secretions

12. What is absolute humidity?
The actual amount of moisture contained in a gas

13. What is relative humidity?
The amount of water vapor a sample could maximally hold at a given temperature

14. What is body humidity?
The amount of water vapor in a gas sample compared to the capacity for water vapor at body temperature.

15. What is the expected body humidity value at normal body temperature?
44 mg/L

16. What best describes a humidity deficit?
The amount of water vapor needed to achieve full saturation at body temperature

17. What is bland aerosol therapy used for?
It is used to treat upper airway inflammation from croup, epiglottitis, and post-extubation edema.

18. How does a heater improve the effectiveness of a humidifier?
Heat improves the water output of humidifiers.

19. When should heat be delivered with a large-volume nebulizer?
It should be used if secretions are thick and hard to remove or if the patient’s upper airway is bypassed.

20. What are the four variables that affect the performance of a humidifier?
Temperature, surface area, time of contact, and thermal mass

21. What are the low-flow humidifiers?
Bubble and jet humidifiers

22. What is a hazard of using a bubble humidifier?
At a high flow rate, it can produce aerosols which can transmit pathogenic bacteria from the humidifier reservoir to the patient.

23. When should a bubble humidifier be used?
When a patient is on a nasal cannula or simple mask with a flow greater than 4 L/min or if the patient complains of a dry nose.

24. What problems could be wrong with a bubble humidifier if no sound is heard when pinching the tubing?
The inlet could be clogged, the reservoir could be cracked or loose, the gasket could be worn or missing, or the connection could be broken.

25. What is wrong if a bubble humidifier whistles by itself?
The oxygen flow may be too high, or the tubing may be kinked or obstructed.

26. What is wrong when no bubbling occurs in a bubble humidifier?
The capillary tube could be obstructed, there could be a loose connection, the oxygen flow may not be turned on, or there is inadequate pressure in the line.

27. What are the factors that influence the output of a bubble humidifier?
Temperature, relative humidity, and absolute humidity

28. What is the purpose of the pop-off valve on a bubble humidifier?
To warn of an obstruction and to prevent the bursting of the humidifier bottle

29. What is the expected water vapor output of a bubble humidifier?
80%

30. What liter flows should be used with a bubble humidifier?
Flows of at least 2 L/min but not more 6 L/min

31. What type of probe does a servo-controlled heating system use at or near the patient’s airway?
Thermistor probe

32. What are the three most common problems with humidification systems?
(1) Dealing with condensation, (2) Avoiding cross-contamination, and (3) Ensuring proper conditioning of the inspired gas.

33. What are some factors that would influence the amount of condensation in a humidified system?
(1) Temperature difference across the system, (2) Ambient temperature, (3) Gas flow, (4) Set airway temperature, and (5) The length, diameter, and thermal mass of the breathing circuit

34. What does particle size depend on?
It depends on the substance that is being nebulized, the methods used to generate the aerosol and the environmental conditions.

35. How does an HME work?
It acts as an artificial nose and captures exhaled heat and moisture and uses it to heat and humidify the next inspiration.

36. What type of humidifier uses a filter?
Heat-moisture exchangers (HME)

37. When should an HME be used?
When the patient’s upper airway is bypassed during mechanical ventilation.

38. What is the correct placement of an HME?
It should be placed 10 cm away from the endotracheal tube and proximal to the ventilator circuit.

39. What are the three types of HMEs?
Simple (high thermal conductivity) condenser humidifier, hygroscopic (low thermal conductivity) condenser humidifier, and hydrophobic (water repellent element) condenser humidifier.

40. What type of humidifier is known as an “artificial nose?”
Heat-moisture exchangers (HME)

41. What are the contraindications for using an HME?
Thick, copious, or bloody secretions; expired tidal volume less than 70% of the delivered tidal volume; body temperature less than 32 degrees C; high spontaneous minute volume greater than 10 L/min; and in patients who are receiving an in-line aerosol drug treatment

42. How can you tell if the HME is working properly?
If the patient is receiving proper heat and moisture

43. When should HME be changed to a large-volume nebulizer with a heater?
When secretions can’t be broken up

44. Why is it a problem that large reservoir systems must be manually refilled?
This is problematic because when the system is opened for refilling, cross-contamination can occur.

45. What is Bernoulli’s principle?
When gas in a tube exerts lateral wall pressure due to the gas velocity

46. What type of humidifier deals with Bernoulli’s principle?
Jet humidifiers

47. What is the purpose of a room humidifier?
To help with sinusitis and drainage.

48. How does a room humidifier work?
It creates an aerosol that exits the device and evaporates in the ambient air, increasing the humidity of the room.

49. What is a hazard of a room humidifier?
It has the potential to spread infections.

50. How does condensation occur?
Saturated gas cools as it leaves the point of humidification and passes through the delivery tubing to the patient. As the gas cools, its water vapor capacity decreases, causing condensation.

51. How is condensation disposed of?
It is treated as infectious waste and drained into an infectious waste container.

52. What is the formula for relative humidity?
%RH = content/capacity x 10

53. What is the capacity of water at body temperature?
44 mg/L

54. What is the formula for body humidity?
Absolute humidity / 44 mg/L x 100

55. What is a humidity deficit?
Inspired air that is not fully saturated at body temperature

56. What is the formula for humidity deficit?
44 mg/L – absolute humidity

57. What are the indications for humidification and warming of an inspired gas?
Dry gases that are at a flow greater than 4 L/min; following intubation; managing hypothermia; and treating bronchospasm caused by cold air.

58. What is a humidifier?
A device that adds molecular water to a gas, occurring by the evaporation of water from a surface

59. What are the factors that affect a humidifier’s function?
Temperature; the higher the temperature of a gas, the more water it can hold; surface area, time of contact, thermal mass; the greater the amount of water in the humidifier, the greater the thermal mass

60. What are the types of humidifiers?
Bubble, Passover, wick, HME, and cascade

61. How much deadspace does an HME add?
30-90 mL

62. What are the types of heating elements that require an electrical source?
Hot plate, wraparound, yolk or collar element, immersion type, and a heated wire-ventilator circuit

63. What amount of humidity is used for intubated patients?
At least 30 mg/L

64. What piece of equipment is used to measure humidity?
Hygrometer

65. How much can an unheated large-volume nebulizer put out?
26-35 mg H2O/L

66. How much can a heated large-volume nebulizer put out?
35-55 mg H2O/L

67. What is an ultrasonic nebulizer?
An electrically powered device that uses a piezoelectric crystal to generate aerosol. The crystal transducer converts radio waves into high-frequency mechanical vibrations that produce aerosols.

68. What does the amplitude do to the ultrasonic nebulizer?
It directly affects the volume of aerosol output. You cannot change the frequency, but you can increase the amplitude.

69. What are the types of aerosol masks?
Aerosol mask, trach collar, t-piece, face tent, mist hoods, and tents for small children and infants

70. What is the most effective humidifier/heater on the body?
The nose

71. What puts stress on the lower airway in order to provide heat and moisture?
An artificial airway

72. What is indicated following the intubation of a patient?
Humidification and warming of inspired gases

73. What is a device that adds molecular water to gas?
Humidifier

74. The higher the temperature of the gas in the humidifier?
The more water it can hold

75. What type of humidifier breaks an underwater gas stream into small bubbles?
Bubble humidifier

76. Are bubble humidifiers heated?
No

77. What is the goal of a bubble humidifier?
To raise the water vapor content of a gas to ambient levels

78. What type of humidifier directs gas over a water surface?
Passover (blow-by) humidifier

79. What type of humidifier is normally used for mechanical ventilation?
Reservoir humidifier

80. What type of humidifier does not have bubbles or aerosol and has a tube coming out of the top end?
Wick humidifier

81. What type of humidifier allows vapor to pass but not water?
Membrane humidifier

82. What kind of patients use a passover humidifier?
Patients on home CPAP units or those in the neonatal nursery

83. Do passover humidifiers have high efficiency and high exposure time?
No, they have low efficiency and low exposure time.

84. How often should an HME be changed?
Every 24 hours

85. What does heat improve in bubble and passover humidifiers?
Heat improves the water output (absolute humidity).

86. Heating systems are usually used on what type of patients?
They are used on patients with bypassed upper airways and those who are receiving mechanical ventilation.

87. What risk are patients exposed to when inhaling heated gases?
Airway burns

88. How much humidity is recommended for intubated patients?
30 mg/L

89. Inhaled gas is supposed to be maintained at what temperature?
35-37 degrees C

90. What can be used to minimize the risk of inhaling condensation?
Water traps or heated circuits

91. What can cause bacterial colonization in the circuit?
The built-up of condensation

92. What are the most common devices used for bland aerosol therapy?
Large-volume jet nebulizers

93. How are large-volume jet nebulizers powered, and what are they connected to?
They are pneumatically powered and are connected directly to a flowmeter.

94. How does a large-volume jet nebulizer work?
Liquid particles are generated by passing gas at a high velocity through a small jet orifice

95. What is the optimal size for particles passing through a large-volume jet nebulizer?
2-5 micrometers

96. What device is electronically powered and uses a piezoelectric crystal to generate aerosol?
Ultrasonic nebulizer

97. Particle size is inversely proportional to what?
Signal frequency

98. Mist tents and hoods are normally used to deliver aerosol therapy to which type of patient?
Infants and children

99. What do high flows in mist tents help to do?
They help to “wash out” CO2 and reduce heat buildup.

100. What are six problems with bland aerosol therapy?
Cross-contamination and infection, environmental safety, inadequate mist production, over-hydration, bronchospasm, and noise

Final Thoughts

Humidity and bland aerosol therapy are cornerstones of effective respiratory care, especially for patients who have compromised airway function or are receiving mechanical ventilation. Properly conditioning inspired gas not only prevents complications like mucosal drying and infection but also promotes more comfortable and effective breathing.

By understanding the different types of humidifiers and aerosol devices, their clinical uses, and potential complications, respiratory therapists and healthcare providers can deliver safer, more effective care.

As with all aspects of respiratory therapy, attention to detail and adherence to best practices are key to ensuring the best outcomes for every patient.