Black Mold in Your AC Unit: Symptoms, Signs, and Prevention

Heather Ayer
Written by Heather Ayer
Updated April 19, 2023
Young family smiling in living room
Photo: Monkey Business / Getty Images


  • Black mold can develop in your AC unit due to excessive moisture and dust.

  • You might see black mold on the outside of your air vents before you see a bigger problem.

  • Black mold that circulates in your air ducts can impact your health.

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If you’ve wondered about tell-tale symptoms of black mold in your AC unit, you’re not alone. Mold can grow in your air ducts, but it’s fairly uncommon. However, it’s smart to test for mold in your vents if you suspect it may be there, as it can cause health problems for anyone with allergies, asthma, or other respiratory issues. Learn how to identify signs of black mold, if you should test for it yourself or hire a pro, and the cost of mold remediation.

What Is Black Mold?

Black mold in the corners of the house
Photo: nndanko / iStock / Getty Images Plus / Getty Images

Stachybotrys chartarum, or more commonly known as black mold, is a toxic type of fungus. As the name suggests, black mold is a dark blackish color but can also appear greenish-black or even gray. The texture of black mold is slimy and wet compared to other types of mold. 

So now you know what black mold is, but where does it come from? Black mold grows and builds off of moisture. Generally, you can find this on porous surfaces like wood, drywall, carpets, insulation, and you guessed it—air vents.  

If you don’t maintain your air vents, there will be excess dust and moisture for this mold to grow. When this mold grows, it releases spores containing toxic chemicals called mycotoxins into the air you breathe. These mold spores can cause health issues ranging from fatigue, nausea, headaches, and even long-term lung issues.

Signs of Mold in Air Ducts

The Centers for Disease Control and Prevention (CDC) states that there are no standards of measurement regarding how much mold is acceptable in a home. It's important to note that every household has some amount of mold. 

But if you still think you might have mold in your air ducts and aren’t quite sure, there are some signs to look for. Remember, these signs don’t always indicate mold and could have other causes. The following are some possible signs of mold growth within a home’s HVAC system:

Musty or Mildew-Like Odor

One of the most common ways you might find black mold is if you smell an odor coming through your vents when running your AC. Black mold has a musty, earthy smell, similar to what rotting leaves smell like. This smell will likely be noticeable if you have black mold, but it will surely increase when your AC is running as the cool air moves through your vents. 

While you can try to remove the musty smell in your home, the smell will keep coming back until you remove the mold. 

Black Dust Around the Air Handler or Vents

Black spots or dust around your air vents are good visual indicators of black mold. If you see these black spots, don’t touch them with your bare hands, as this could cause skin irritation and rashes. Instead, you can use a mixed solution of 1 part bleach to 3 parts water to wipe away the visible mold spots. Remember that this is not an overall solution to the mold problem, as the source is likely within the air ducts. Because of this, if you don’t remedy the whole problem, the mold spots will continue to grow back. 

Is Mold in Air Vents Dangerous for Those in the Home?

Black mold in your air conditioner unit can be dangerous, and you’ll need to remedy it as soon as possible because it can cause many black mold exposure symptoms. Your home’s HVAC system circulates the air, causing mold spores to spread throughout your home, even if they only grow in one small section. This means no room in your home is safe from the toxic chemicals found in mold spores. 

Even though you might not see black mold right away, a few significant physical symptoms signal a mold problem

Allergy Symptoms

Experiencing allergy symptoms during the spring is common, but having worse symptoms than usual or having them out of season is another early indicator of black mold in your air vents. Typical allergy symptoms can include itchy eyes or skin, more coughing and sneezing than usual, or difficulty breathing.

Frequent Headaches

Headaches are a common side effect of black mold, but it’s just as common to miss this early warning sign. These headaches might not seem severe or problematic at first, but over time, they will become more frequent and severe, and you might notice that they get better when you’re not at home. 


If you’re unusually fatigued with no real explanation for it, you might have a black mold problem. This is one of the most common health signs that someone might experience. If you notice you’re unusually tired, contact your doctor. Then if you have a black mold problem, contact an HVAC professional. 


For prolonged black mold spore exposure, nausea is a typical side effect. The toxins in the mold spores irritate the digestive system. Vomiting and abdominal pain can accompany nausea. Unlike headaches, these GI symptoms might not immediately disappear when you leave home. 

Brain Fog 

Brain fog is a unique side effect of breathing in black mold spores over time. This may or may not accompany other symptoms like headaches or difficulty breathing, so if you’re noticing you have persistent brain fog with no other medical cause, it could be due to black mold spores. 

Difficulty Breathing 

Along with other allergy symptoms, black mold can cause difficulty breathing. The mold spores can trigger asthma-like reactions and cause coughing, sneezing, and chest tightness. 

What Causes Mold to Grow in a Home’s HVAC System

Man inspecting air vent in home
Photo: Serenethos / Getty Images

Regardless of whether you have black mold in your air ducts, it’s important to know how this invasive fungus can invade your home. Mold can happen for a few reasons, including:

  • Your AC unit is oversized: Having an oversized AC might cool your home faster (at the expense of your electricity bill), but it won’t be able to remove humidity from your home in the way it should. This moist environment is the perfect breeding ground for black mold.  

  • The AC is set at a very low temperature: Having your AC set to arctic temps might feel nice, but the super cold air mixing with the warm room air can cause moisture to form inside and around the vents. If this doesn’t dry out, this is a place where black mold can grow. 

  • There are leaks in the duct system: If you have leaky ductwork, the mixture of warm and cool air can cause moisture and create the perfect place for black mold to hide. 

  • Humidity: Black mold loves humidity, and in the summer, there’s plenty of it. Humidity levels higher than 55% can support black mold growth. If running your AC isn’t enough to keep the humidity down in your home, consider installing a dehumidifier to help prevent mold growth.

  • Poor ventilation: Poor AC ventilation can cause moisture to build up in and around your air vents. This can be caused by clogged air filters or a dirty condenser coil. 

How Do I Get Rid of Mold in My Air Ducts?

You can’t always know for sure if you have mold in your air ducts (unless you have visual proof) without first testing for it. Then, you can move on to the remediation stage.

You can use several cleaning methods to rid yourself of mold, but a professional is your best bet for safety and to ensure proper removal. 

Follow the steps below to keep your home mold-free and healthy for you and your family.

1. Identify the Source of the Mold

Identifying the source of the issue can be difficult, but it’s much more prevalent in homes with water leaks. If you are experiencing a mold problem, identify and resolve any issues related to leaking water, like an air duct leak. Professionals offer mold inspections and know what to look for when identifying black mold and other mold types.

2. Hire a Professional to Test

There are many DIY mold test kits on the market. But If you suspect you have mold anywhere, including in your air ducts, it's best to leave the detection and elimination up to a professional for your safety.

3. Conduct Mold Remediation

If mold testing indicates you have mold in your air ducts, you may need mold remediation services. A mold remediation specialist will know the proper protocol, including protective clothing, adequate removal tools, and how to remove the mold without exposing you or anyone in your family. The tester should retest after removal to ensure the remediation service has been successful. 

The CDC advises asking the professional you hire about the mold removal guidelines they follow. Look for them to reference the American Conference of Governmental Industrial Hygienists (ACGIH) guidelines or other applicable guidelines from professional or government organizations.

How Can I Prevent Mold Problems in My Air Ducts?

The best way to avoid the health problems associated with black mold and the hassle of having to remove it is to prevent it from growing in the first place. To help keep mold at bay, follow these preventative steps:

  • Replace the air conditioning filter every month

  • Reduce condensation within the ducts by insulating them

  • Invest in a dehumidifier and use it in musty or damp areas that could collect mold

  • Clean the drip pans regularly

  • Check for any leaks around the pipes and condensation areas, then seal areas so moisture cannot collect

  • Consider installing HVAC UV lights for mold prevention

If you think you have black mold (or any mold) in your air ducts, your best bet is to hire a local air duct cleaner. Air duct cleaning will help ensure there are fewer places for mold spores to remain. Cleaning shouldn't require replacing your air ducts unless there's damage or it's inaccessible.

The cost of an air duct cleaning job can run between $300 and $500 and takes several hours to complete. This can also be a preventative step to ensure your vents stay mold-free, but talk to a professional to determine if it’s necessary.

Stephanie Shaykin contributed to this piece.

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