Mold in Air Ducts: Signs, Removal, and Prevention

Ceiling mold close to air duct

The condition of your home’s HVAC system directly affects your indoor air quality. If there’s mold present, you and your household may suffer adverse effects, so it’s vital to get rid of mold as soon as you can. However, it’s not always obvious when air ducts have mold, or what to do when it happens. This guide covers mold in air ducts and its signs, removal, and prevention to help you as a homeowner keep your HVAC in good shape.

Mold in Air Ducts

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The ductworks of an air conditioning system are a perfect breeding ground for mold to grow. They’re dark, warm, moist, almost never disturbed, and full of debris, such as dirt or dead skin cells. Almost any amount of moisture can lead to mold, and one small amount can multiply into a colony frighteningly quickly. In particular, a black mold infestation in your air ducts can spread and cause serious health problems.

When it comes to dealing with mold in your air ducts, black or not, there are three vital steps to follow; recognizing signs, removal, and prevention. You need to be able to recognize mold in order to treat it, and if you don’t take preventative measures, it might come back. 

Signs of Mold in Air Ducts

When you have mold in your air ducts, it should be fairly obvious if you know where to look. Signs will be in various places around your home, and you may even be able to tell by the symptoms you and your household present. Below are a few signs that your ductworks have been infested by some form of mold.

Black Dust

Look for black dust or spots near your AC vents. While these can be other things, such as rust or insulation, it’s also a sign of the dreaded black mold. If you see them, do not touch them. Clean up the visible spots and do a thorough inspection of your HVAC and duct system. If you don’t take care of the problem at the root, they’ll come back, and do a lot more damage.

Musty Odors

Mold has a distinct smell, which will come through your air vents if it’s present in your ducts. Different types of mold may smell different; for example, black mold is similar to rotting leaves. The smell will be stronger when your AC is running. If you notice any strange smells coming from the vents, check your duct system for mold.

Increased Moisture

This one is a bit of a misnomer. Increased moisture isn’t exactly a symptom of mold, but you should still be looking out for it. Mold needs moisture to grow, and your duct system is the perfect breeding ground. It only takes about 24 hours for mold growth to develop after moisture exposure in your air ducts. If you notice that your ducts have been exposed to moisture, even just condensation from your AC, you may have mold soon if you don’t already.

Visible Mold

Of course, a sign that you have mold in your air ducts is mold. Look for visible mold on your HVAC system’s filter, your air vents, air handler and on the outside of your ducts. If it’s present in any of these places, it’s a fairly obvious indicator that you have mold inside your duct system.

Flu-Like Symptoms

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Lastly, you should pay attention to your own body if you think your ducts have mold. Mold causes allergy or flu-like symptoms in the human body, so if you develop them, and can’t find any particular reason for it, you should look to your duct system. Symptoms of mold exposure include headaches, fatigue, sore throats, a cough, and other typical allergy or flu symptoms.

Removing Mold in Air Ducts

Once you know you have a mold problem in your air ducts, you need to remove it. This, however, is easier said than done. Without the help of a professional mold remediation service, you may struggle to combat your mold issue. In fact, expert help may be your only option if the mold has spread over a large area. If it hasn’t, there are some DIY options you can try.

Here’s a breakdown of the steps you should take when removing mold:

Step 1: Choose your mold killer. Store-bought remedies work fine, but you can also make your own. For non-porous surfaces, mix one part bleach to 16 parts water. For porous surfaces, use one tablespoon of detergent and half a cup of baking soda mixed with one cup of water.

If you’d prefer to avoid using chemicals, you can try a UV light treatment to get rid of your mold. UV light is a common treatment used on air conditioners and AC ducts. However, it’s not easy. You need a special kind of UV lamp, which can be dangerous if used incorrectly. Therefore, UV mold treatments are best left to professionals.

Step 2: Once you have your mold killer, you can begin the actual removal process. The first step is to turn off your HVAC system.

Step 3: Wear protective clothing, such as gloves, safety glasses, long sleeves and pants, and an N95 mask.

Step 4: It’s time to start the mold remediation process. You can either apply your cleaning chemical by scrubbing the ducts manually or use a mold fogger.

A mold fogger is a device that distributes your chosen mold killer through the ducts as a fine mist for easy and even application. You can rent or buy one from most hardware stores. If you choose this route, make sure to follow the product’s labeled directions.

Step 5: If the mold proves to be too much for you, contact a professional mold removal service so they can tackle the problem for you.

Step 6: When you’re sure the mold is gone, clean your ducts. This will get rid of any leftover mold, along with the residue of whatever cleaner you or your professional used. It will also help prevent mold in the future.

How to Prevent Mold in Air Ducts

Dirty AC air filter
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Once you’ve gotten rid of the existing mold in your air ducts, you need to prevent it from coming back. And even if you’ve never had mold, it’s still important to follow prevention practices so that you never have to go through the process of removing it. To that end, there are a few things you can do. Here’s a short list of some methods for mold prevention that can help you keep your air ducts clean:

  • Change your air filters regularly
  • Fix any leaks as soon as you find them
  • Insulate your air ducts
  • Clean your HVAC’s condensation drip pans
  • Control the humidity levels in your home, especially around your air vents

In addition, you should apply an EPA-approved mold inhibitor, also called a biocide or antimicrobial solution, after you’re done with any mold treatment. A mold inhibitor will suppress mold growth in your HVAC and prevent the mold from coming back. Even if you haven’t been treating for mold, you can still use it as a precautionary measure.

What Causes Mold in Air Ducts?

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The cause of mold growth in your air ducts is very simple, and can be boiled down to a single word; moisture. Any source of moisture can lead to mold, including some you may not think of. Since it can take as little as 24 hours for mold growth to form following moisture exposure, it’s vital to keep your ducts dry so that you can avoid health issues for you and your household, as well as expensive fixes. Some causes of moisture in your ducts include, but are not limited to, the following:

  • Condensation
  • Animal urine
  • Roof leaks
  • AC leaks

FAQ About Mold in Air Ducts

Does mold in the vents blow into the house?

Debris from your vents, including mold and mildew, usually adhere to the surface of the vent and don’t leave it. However, even if you don’t have to worry about the mold itself blowing into your house, the spores still can. They’re airborne, so they can travel through the HVAC system and cause serious health problems if they get into your indoor air.

Why does mold keep coming back?

If you have a recurring mold issue, it’s likely you’re not removing all of it with whatever treatment you’re using. If you haven’t already, hire a professional to take care of it. Have them do a thorough examination of your entire HVAC system to make sure you get every trace of mold. If the problem still persists, inspect your home and see if there’s any other possible sources that could be bringing the mold back.

What is the difference between mold and mildew?

Mold and mildew have a few differences. For one, they have different textures; mold is slimy or fuzzy, and is raised on whatever surface it’s on, while mildew is powdery and grows flat. In addition, mold comes in a wide variety of colors, while mildew is generally only white or gray. Finally, while both are bad for people, mold is much more harmful than mildew.

Why is black mold such a big deal?

Black mold is such a big concern because it’s one of, and possibly the most, dangerous out of them all. Also called Stachybotrys or toxic mold, its symptoms include the usual allergy or flu-like effects, but it can also induce much more serious health issues. In severe cases, memory loss or organ failure can occur, and it can even be deadly in worst-case scenarios.

Black mold can also have long-term deleterious effects. Children living in homes with a black mold infestation can develop asthma, and seniors may experience pneumonia. Lastly, black mold can also cause fungal infections. This mold is a major cause for concern, so if you believe your home is infested with it, don’t hesitate to call a professional.

Ditch the Mold

Having mold in your air ducts can be scary, but it’s easier to deal with than you might think. Being able to recognize the signs, treat the problem, and prevent it from happening again will go a long way toward improving your indoor air and quality of life.

If you have mold in your air ducts, contact your local mold removal professionals. For an air duct cleaning service or routine HVAC maintenance, contact your local HVAC pros. Leave it to experts to rid your ducts of harmful mold and debris, and service your HVAC system so that you don’t have to worry about it.

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Austin Geiger

Austin Geiger is a writer who's passionate about home care. He enjoys writing about home maintenance practices, as well as projects to turn an outdoor space into a backyard paradise.