Black Mold In The Air Conditioner? Don't Ignore the Warning Signs

By: Licensed Mold Assessor Brad Fishbein

February 8, 2023

Your air conditioning system is probably the most important thing in your home.

The air you breathe is sent through the system and distributed throughout the home. But what happens if you have toxic mold in your air conditioning?

No, there likely isn't Black Mold in your Air Conditioner.


Is it even possible?

You see:

The presence of black mold spores in a home requires immediate action. That's because this type of mold is a danger to your health.

Stachybotrys chartarum—the scientific name for black mold—goes by another name: toxic mold.

Kitchens, bathrooms, basements, attics—these are some of the places where toxic, black mold can accumulate. Here's the problem with black mold: it can be airborne, and it CAN off-gas. That means you can inhale it into your lungs.

This brings us to the question: Can black mold grow in your air conditioner?

We're going to answer that question in this article. We're also going to cover:

  • Why or Why Not Black Mold Is in air conditioners
  • How to prevent black mold in your air conditioning system
  • Can the air conditioning ducts have mold growing

Let's examine the air conditioner first.


Can Black Mold Spores Grow in the AC Unit?

The short answer is no: Black mold normally doesn't grow in the air conditioner.

To understand this, you need to understand the factors which need to be present to allow mold growth including:

  • Water
  • A food source
  • Humidity

Knowing these factors will help you prevent the growth of black mold around your air conditioner.

We'll discuss the lack of black mold growth in air conditioners first.

Black Mold Growth Doesn't Happen in Air Conditioners

Air conditioners remove hot air from your home and replace it with cooler air. The process of cooling the air requires condensation, which means that the inside of an AC unit is often damp and humid.

This environment is not conducive to black mold growth because the temperature isn't warm enough for long periods of time or consistent enough for the spores to take hold and begin growing.

Additionally, most AC units are designed with good drainage systems so that excess moisture doesn’t accumulate and create an environment where mold can thrive.


When a air conditioner doesn’t drain properly, it sets up a whole new set of issues.

That’s because this can cause your air conditioning to leak.

And when your air conditioning is leaking it can cause the areas surrounding the air conditioning to get wet. Depending on what type or material surrounds the air handler, it can cause black mold in surrounding areas even if not in the air conditioning system itself.

Air Conditioners Lack Cellulose Materials

Another reason black mold doesn’t grow in air conditioners is that most of them lack cellulose material.

Cellulose is a polysaccharide material found in many plants and trees, including products derived from them like:

  • Cotton
  • Wood
  • Paper

In older homes, the materials used consistent of plaster which is lime based and resistant to mold. But as of the last quarter or so of the 20th century, home builders started to use more sheetrock type materials which is cellulose based.

Cellulose is what gives these materials their strength and durability. It's also an excellent insulator and helps keep moisture from entering or leaving the material it's bound to—making it an ideal breeding ground for mold spores. Mold needs food to survive, which is where cellulose comes into play.

Cellulose-based materials contain organic compounds like sugars and starches that mold can feed on. Furthermore, since cellulose is an excellent insulator, it helps trap moisture—creating a warm and humid environment in which molds can thrive. This combination of food sources and a controlled climate (high relative humidity) makes it easy for mold spores to grow rapidly in any material with a high cellulose content.

Stachybotrys tends to love drywall much more than wood so you can expect to find black mold growing there.

But why doesn’t Stachybotrys grow on air conditioning units?

It’s because they are mostly consistent of building material that is metal or plastic which doesn’t support black mold growth.

Does that mean mold can’t grow on your air conditioning at all?


It’s quite the opposite. Mold can absolutely grow in air conditioning systems, it’s just the type of molds that are typically found are:

  • Penicillium
  • Aspergillus
  • Cladosporium

Cladosporium is the most prominent type of mold found in the air conditioning system. While Cladosporium does not cause Mycotoxins it is certainly still a nuisance for humans and animals.

Many people tend to be allergic to Cladosporium. And even the ones that are not, Cladosporium mold in your air cause such symptoms as coughing or itchy eyes.

The Forgotten Part Of The Air Conditioner That Black Mold CAN Grow On

I don’t want to contradict earlier statements, Stachybotrys does not typically grow within the air handler.

However, there is a section of the air conditioning system which is often forgotten about:

Underneath the air handler.

Depending on how your air conditioning system is set up the area underneath is typically called a plenum.

Well, the thing about air conditioning unit coils is they condense. It’s a normal part of the air conditioner coils especially in humid environments.

And when they drip water, it goes right into the plenum.


There are two types of plenums, and insulated plenum and uninsulated plenum. The difference between the two is one will more likely produce Stachybotrys than the other.

Can you guess which one will produce Stachybotrys easier?

If you guessed the uninsulated plenum you would be correct!

The reason which is because when it’s not insulated, there will typically be sheetrock underneath which if you have learned anything from this article you would know that means its cellulose based.

Here is an example of an uninsulated plenum: Mold%20Underneath%20Plenum

When a plenum is insulated it can still produce visible mold growth but it’s typically Cladosporium. It is hard to prevent mold in a plenum This is what a plenum insulated with fiberglass that has been water damaged looks like: Mold%20on%20Insulated%20Plenum

Both have mold that is “black” in appearance but one is topic while the other is not.

How Can I Prevent Black Mold From Growing In My Air Conditioner?

Even though black mold normally won’t grow in an AC unit itself, it can still form around the edges if:

  • Debris accumulates near the coils
  • The AC Units system is leaking


To prevent these from happening, ensure you remove any dust or debris that has built up around the AC unit.

People tend to have their Air Conditioner serviced once or twice a year which is great to extend the life of the Air Conditioner system and also ensure the system has all it’s proper fluids and whatnot.

The problem is that these normal maintenance services don’t typically cover cleaning the AC Units. This is a separate service.

There is no exact time that the Air Conditioner system should be cleaned but it just be checked at least once a year and cleaned as needed. Cleaning it can also rid the home of musty odors.

Cleaning the air handler includes all components like the coils and blower.

The ac units ducts should also be cleaned as needed however depending on what the air ducts are made out of, it may not always be possible.


Black mold also won’t typically grow in the air conditioning ducts because they are typically made of fiberglass or metal.

But that doesn’t mean it can’t be on the dust and debris that goes through the Air Conditioner system…

How Black Mold Can Be Transported Through Ductwork

While black mold growth is uncommon in an air conditioner, it can still be transported via the air ducts.

Stachybotrys spores can be airborne.

Mold in your air can cause all sorts of mold spores. Especially when it's black mold. Inhaling mold spores can cause a chain reaction especially if somebody has an auto-immune disorder.

If there is some type of water damage within the home the spores can also land in settled dust and be transferred from one part of the home.

And then what happens?

They settle on other building materials and contents like clothes or furniture.

You have to remember Stachybotrys spores are so tiny that they can only be seen with a microscope. That means you can be ingesting it without even realizing it.

The best way to ensure black mold is not being transported through the air conditioning system is to ensure you don’t have black mold within a home at all.

Black mold grows when you have a significant water leak.

So, if the air conditioning is leaking or anything else for that matter, it needs to be corrected immediately.

Can Black Mold Grow In Wall Unit Air Conditioners?

Black Mold will not typically grow in window or wall air conditioners for the same reason it won’t grow in central air conditioning units. Stachybotrys will also not typically grow on mini-split units.

Wall units are made of metal and plastic.

And just like the air handler, mold can still grow on the unit. You can prevent mold by cleaning the unit often.


Mold In an AC Unit

Black mold is no laughing matter. If can get people very sick and can be hard to remediate if it spreads through the air conditioner.

The reason that it can be so hard to remediate is that you can’t actually see the spores so EVERYTHING needs to be cleaned.

Mold remediation sometimes involved a process called micro-cleaning where every single thing in the home is cleaned. It also involves cleaning mold in your air.

This is the only sure tell sign that all black mold spores are removed from the home if it goes through the Air Conditioner system.

Meet the author: Brad Fishbein is an ACAC council-certified Microbial Investigator. In the fall of 2012, he became a Licensed Mold Assessor in the State of Florida through the Department of Business & Professional Regulation. Brad has helped homeowners with over 5,000 successfully completed Mold Inspections since 2009.

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