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How Does Mold Grow in HVAC Systems?

Updated: Nov 18, 2022

Justin H. Joe, PhD, CIH, CSP, CPE, Nicole Lee, and Lucas Kim

Air conditioning or HVAC systems are an ideal solution for humid, hot summer weather. However, these units are a breeding ground for mold growth, as it provides the ideal conditions for it to flourish. Oftentimes, mold is undetected by occupants within a household. HVAC systems are connected throughout the home, allowing disturbed mold spores to disperse in rooms where air ducts are present. The dark space in air vents, debris buildup, moisture, and oxygen provide ideal mold growth conditions. It is essential that HVAC systems are routinely inspected, especially with regular use to ensure the safety of your family.

Recognizing mold in air conditioners

The presence of mold may not always be easy to detect, as the individual spores are microscopic. However, several factors may be considered when determining and locating mold growth in the HVAC system:

Smell: oftentimes, a musty smell will linger where mold is present, especially where the AC unit or vents are located within a room.

  • Appearance of mold reservoirs: surface mold growth will appear on grills of the vents or air conditioner unit. Sometimes, black dust will be visible on and around air vents which may indicate black mold. However, mold may not always be apparent on the surface of the grills but rather, inside the heart of the unit or HVAC system.

  • Health symptoms: Another way to check the presence of mold in your air conditioners is to check for health symptoms such as:

  • Allergic symptoms: Itchy, irritated eyes, sneezing, or irritation to mucous membranes of the throat and nose, or rashes.

  • Respiratory symptoms: Chronic coughing or difficulty breathing,

  • Gastric symptoms: Nausea, vomiting and/or diarrhea.

  • Nonspecific symptoms: Fatigue, headaches, or impaired concentration.

Causes of mold growth in air conditioners

Mold growth is more likely to occur in HVAC systems as the ideal thriving conditions are satisfied within ducts. These necessities include:

  • Oxygen

  • Moisture

  • Organic matter (dust)

  • Dark spaces

Most commonly, mold growth is present on the evaporator coil and/or blower fan of the HVAC system. Due to condensation, mold will attract on colder, moist surfaces.

Common mold types in air condition systems

The most common types of mold found in air conditioning units are:

  • Aspergillus: common allergen

  • Cladosporium: common allergen

  • Stachybotrys Chartarum: also known as black mold; a harmful mold that releases mycotoxins

How does mold in the ventilation systems affect me?

While the presence of mold is greater in more damp rooms, such as basements, mold in air conditioners can be more detrimental to your household’s health.

In any ventilation system where mold is present, air blowing through the grills of an air duct or air conditioning unit will disturb any surface spores. Because air ducts are connected throughout the home, an HVAC system infested with mold growth may spread the spores when air is blown out through the vents. As a result, occupants may experience allergic reactions or more severe respiratory consequences.

Remediation and Maintenance

While there are home remedies to remove mold in HVAC systems, the EPA highly recommends that mold remediation in HVAC systems should only be performed by qualified and licensed professionals. To address mold in air conditioning systems, a mold inspector evaluates the potential source of mold and takes samples. These samples include surface swab and/or spore count air samples. Surface swab samples are taken from areas where mold is visible or suspected to be growing. Spore count air samples measure mold spores lingering in a room as surface spores are disturbed and dispersed through air ducts. Once samples are taken, they are analyzed by a lab and received by the inspector. The inspector then evaluates the concentration of spores and recommends remediation plans to be completed by qualified contractors.

Qualified contractors loosen and remove debris using HEPA vacuuming equipment. In severe cases, parts or the whole HVAC system may be replaced if the components are outdated or are functioning improperly.

To prevent further growth of mold in the HVAC system, it is recommended that the following measures be taken:

  • Routine inspections and regular duct cleaning

  • Observing moisture patches

  • Changing AC filters on a regular basis

  • Monitoring humidity levels

  • Monitoring allergies and symptoms for mold


  • Mold in air conditioning systems is very common, especially with regular use or when inactive. It can be detected primarily through smell, appearance, and health symptoms.

  • The most common mold types in HVAC systems are aspergillus, Cladosporium, and black mold. While Aspergillus and Cladosporium are common allergens, black mold is highly toxic and should be remediated professionally immediately.

  • Regular maintenance and inspection is required to prevent further mold growth.

How can BNF Consulting help with mold in HVAC systems?

BNF Consulting, Inc. is a licensed mold inspection company that abides by New York State & US EPA regulations for mold inspection and testing. Does your home or business have a mold problem? Our company can provide a reliable mold inspection service to determine potential risks.

If you are concerned for you and your family’s safety or curious about air pollution in your home, schedule a free call with us today at 914-297-8335 to learn more about our services. We also offer a free phone/video consultation.

About the Authors:

Justin H. Joe, PhD, CIH, CSP, CPE, is a Certified Industrial Hygienist and a principal consultant of BNF Consulting, Inc. Dr. Joe graduated from the University of Illinois at Urbana-Champaign with PhD and MS degrees in nuclear engineering. Dr. Joe has provided industrial hygiene consulting as a core function with his diverse background of experience and education.

Nicole Lee is a sophomore pharmacy major at Rutgers University and BNF Consulting internship program participant.

Lucas Kim graduated from Pohang University of Science and Technology with PhD and MS degrees in chemistry and BNF Consulting Environmental Consultant.


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Mold growth in HVAC systems can sometimes be a hidden problem, but HVAC Contractor Riviera Beach technicians utilize specialized equipment and techniques to detect and address any signs of mold infestation. Our goal is to provide peace of mind to our customers by ensuring their HVAC systems are mold-free.

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