Mold in Heating and Air Conditioning Systems (HVAC)

by | Aug 3, 2022 | Mold Prevention Tips, Mold Removal Tips

One of the favorite access points and hiding places for toxic molds (fungi) to get into your home is through your heating and air conditioning systems (HVAC). Our company comes across a lot of properties that have heating and air conditioning systems that are contaminated with various types of toxic molds (fungi).

What you need to understand is that your HVAC system is one of the main entrance points and hiding areas for mold.

The main reason is the molds that we have indoors are brought inside from the outdoor air. The other factors would be if you have mold elsewhere in the home that has become airborne, these spores can float inside your air intake system and ductwork causing it to settle there.

This can then lead to your whole home being contaminated with mold spores at an alarming rate. Every time you turn on the heat or AC, these spores will become airborne, agitated and release dangerous mycotoxins into your indoor air. This air can make you ill and cause disease.

In a 2001 study, scientists discovered that this is a massive problem that often contributes to the occupants becoming ill with hypersensitivity diseases, such as allergic rhinitis, asthma, or contracting a disease like cancer from the mycotoxins. The researchers stated:

“Fungal contamination of air-handling units (AHUs) is a widespread phenomenon in buildings with central heating, ventilation, and air-conditioning (HVAC) systems and is a potential source of contamination for occupied spaces. Fungi have been found growing on air filters, insulation, and cooling coils, as well as in ducts.

This contamination often contributes to building-related diseases, including both infectious diseases and hypersensitivity diseases, such as allergic rhinitis, asthma, and hypersensitivity pneumonitis. In addition, acute toxicosis and cancer have been attributed to respiratory exposure to mycotoxins.”(1)

As you can see, this can be serious. Now, let me explain the simple science of how this happens.

Molds and bacteria need water as a food source to grow and thrive. It is dark and can be very humid in these heating and air conditioning systems. The perfect environment for mold to grow and thrive!

For example, your HVAC coils are the perfect breeding ground for microscopic pathogens because the constant heating and cooling of the coils will cause condensation, aka water vapors and also become dusty, dirty and can be covered by what is called bio slime, which is sometimes referred to as Dirty Sock Syndrome.

Let me explain.

When you are cooling your home, the coils use a liquid coolant to create cold air that will then flow throughout the ducts to cool the building. The management of your air and keeping the costs down requires that your air goes on and off at the opportune times per your thermostat.

All air has water vapors, so when you are running the AC or the heat and then it shuts off, these water vapors settle in the ducts and on the coils. The changing of the air from warm to cool or cool to warm will cause further condensation and if you live in a humid area such as Florida or North Carolina, the humidity will only add on to the problem.

The dust and moisture now activate the mold spores and bacteria to life. They grow, reproduce, die and the new generation lives off the old. The result produces a slime and smell from the mold off-gassing. This smell of a dirty sock now flows through the home. Hence, the name, Dirty Sock Syndrome.

All the while, spores, mycotoxins, and bacteria are being released by the millions and possibly billions into the indoor air. Unbeknownst to you, you and your family are breathing them in causing your health to decline.

You may also have mold growing in your condensation pan. Here is an image of a clogged condensation pan with various molds growing. When was the last time you checked to see if it was clean?

I hope I have opened your eyes to the toxic realities of the problems that can arise from simply operating your heating and air conditioning systems. If you haven’t cleaned your HVAC in a long time or never and you suspect that you may have a mold problem, I recommend that you have your home and HVAC system tested or professionally inspected.

Get a Mold Test

The first thing you need to do if you suspect that your heating and air conditioning system is contaminated is to either conduct a do it yourself (DIY) mold test or have a professional mold inspector come out and test it for you.

The Occupational Safety and Health Administration (OSHA) recommends:

“Air sampling may provide tangible evidence supporting a hypothesis that investigators have formulated. For example, air sampling may show a higher concentration of the same species of mold when the HVAC is operating than when it has been turned off. This finding may convince the investigators that the mold is growing within, and being disseminated by, the HVAC system.

Conversely, negative results may persuade investigators to abandon this hypothesis and to consider other sources of mold growth or dissemination. If you know you have a mold problem, it is more important to spend time and resources removing the mold and solving the moisture problem that causes the moldy conditions than to undertake extensive testing for the type and quantity of mold.”(2)

If you have mold, can it be cleaned safely?

If it is confirmed that you have a problem, the Environmental Protection Agency (EPA) suggests, “Do not run the HVAC system if you know or suspect that it is contaminated with mold – it could spread mold throughout the building.”

This is what the Center’s for Disease Control (CDC) advises if you suspect that you have mold in your HVAC:

  • You may need to hire a professional to inspect your system. Any needed repairs or cleaning of vents and air ducts should be performed before restarting the system.
  • Throw away wet or water damaged filters.
  • Do not run your HVAC system if you know or think that it is contaminated with mold — it could spread mold throughout your home.
  • Turn off your HVAC system and cover vents and ducts during cleaning to prevent contaminating it.(3)

Of course, if it is the dead of winter in the cold states or high noon in the summer, it can be dangerous to not have heating or cooling running. With that said, you are going to have to get to work to handle this ASAP.

The first step is to determine if it can be cleaned properly and safely or if you have to replace the ducts in your home. If you are renting and there is no way to clean the system or replace it, then you will have to consider moving to a safe place.

In order to figure out the next step, you need to find out what materials your ducts are made of. This is crucial!

Many modern duct systems are made entirely of sheet metal. Others either have sheet metal with insulation on the exterior or with internal insulation, and some are made entirely of fibrous glass insulation.

If you have a duct system that is made entirely of bare sheet metal or sheet metal with exterior insulation, you are most likely in luck. More often than not, they can be cleaned properly and safely if you hire a professional HVAC cleaner who has extensive experience with cleaning mold.

Sheet metal duct systems with internal glass insulation or made entirely of insulation will have to be removed and replaced if they have water damage and or mold. There is no safe way around this fact and it can be very expensive.

Here is what the EPA says, “If you have insulated air ducts and the insulation gets wet or moldy it cannot be effectively cleaned and should be removed and replaced.”

Please keep in mind that you do not want to hire amateurs to do this. Your health and life may be on the line here.

If you need help with testing, a professional inspection, remediation, or a remote consultation, please contact us at this link or call us at 760-818-6830.


  1. Effectiveness of Germicidal UV Radiation for Reducing Fungal Contamination within Air-Handling Units
  2. Occupational Safety and Health Administration (OSHA)
  3. CDC


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