Signs of Black Mold In Air Vents (A/C Unit Symptoms?)

Discover the warning signs of black mold growing in your air vents. In this guide, you’ll discover:

  • Signs of toxic mold in vents
  • Common allergic symptoms
  • Tips to clean and prevent AC unit mold growth
  • And more!

One of the most disturbing things that you can encounter as a homeowner or tenant is the possibility of black mold in vents, also known as Stachybotrys chartarum.

During home inspections, I frequently discovered mold growth on air vents AND inside the indoor air handler. And since mold needs moisture, darkness, and food to survive and thrive — the HVAC system can be a perfect breeding ground.

Uncover the mystery behind mold in air vents with this article!

What Is Black Mold?

Stachybotrys chartarum is the species of mold which is generally known as black mold in the home industry.

Even though not all black mold is S. chartarum, this mold species is most commonly referred to as ‘toxic black mold’ since it can release harmful mycotoxins into the air.

S. chartarum is found in homes that have high humidity, water leaks, extreme temperature changes (unconditioned air) and zero sunlight — such as a dark basement corner that is saturated in water.

Mold loves to grow on cellulose-rich building materials such as drywall and wallpaper from areas with water damage. This mold species requires constant water saturation for it to grow and thrive.

Stachybotrys chartarum sometimes releases mycotoxins (microscopic particles) into the air that may cause health problems and is associated with sick building syndrome.

Is It Black Mold On My Vent?

S. chartarum can sometimes grow on air vent covers or inside the furnace or A/C, but it is relatively rare.

Air vents aren’t usually conducive to S. chartarum since there isn’t a lot of cellulose material and it usually isn’t saturated with water. 

However, it is possible, and the only way to confirm the presence of black mold (or any other type of mold) is with laboratory testing.

RELATED: Pictures of Mold in Air Ducts

Signs of Mold In Air Vents

  • Black Dust On The Air Vents
  • Visible Mold Inside The Air Handler
  • Musty Odor Around The Home
  • Brain Fog Or Forgetfulness
  • Fatigue Or Getting Tired Easily
  • Allergic Symptoms And Itchiness
  • Upper Respiratory Tract Symptoms

A musty smell emanating from the HVAC system is a telltale sign of mold in the A/C system — especially if the musty smell gets stronger when the system turns on.

Black mold can have a distinctive smell that is more pungent than common molds — an earthy fragrance such as that of soil.

Some visual clues to mold are blotches or black circles on the air vent registers. It is important to note that mold growth on air vents is a common sight and is usually the result of condensation or moisture forming on the vent. There is also a good chance that the dark area on the vent is mildew or dark-colored dust.

However, if you see black dust on the surface of an air vent that can’t be easily cleaned—it may be S. chartarum. If mold is inside your air duct system, the spores can get blown to the vents, and eventually end up as black dust.

Also, mold in vents can grow because dust accumulates on the surface — which is a food source. Cleaning the visible mold on vents is recommended because the mold may release spores into the air, possibly causing harmful health issues.

Health Symptoms

The most common allergic symptoms of black mold in the AC unit include itchy eyes, skin, and a stuffy nose. Mold can also make your eyes and skin inflamed with a reddish color.

I once was exposed to toxic mold after coming back from a vacation. The refrigerator turned off, and mold grew everywhere in the fridge. While cleaning it out, I had this itchy sensation all over my skin.

When you combine allergic symptoms with visible mold on air vents as well as a strong musty odor—these are all toxic signs of mold exposure.

Studies have also shown that mold exposure can lead to upper respiratory tract problems. Things like coughing, wheezing, and even hypersensitivity pneumonitis in vulnerable people. If you have asthma, mold can make it worse and even trigger asthma.

Brain fog is also a known health effect of breathing in mold spores or mycotoxins coming from the HVAC system or elsewhere. A recent study pointed out that the symptom of brain fog was just as common as symptoms of the respiratory tract due to mold.

If you have inexplicable tiredness, it may be a symptom of mold in the ductwork. If you also noticed fatigue when you turn on the HVAC system, it may be from contaminated air ducts.

Causes of Black Mold In Vents

The most common cause of mold on air vents is simply due to moisture buildup on the vent cover.

When the cool or hot air from the HVAC blows onto the register, and meets a different temperature, condensation (moisture) can form on the vent.

Mold tends to grow where there is water present and a food source. The food source on air vents is typically household dust.

Hidden Mold Growth

There also may be hidden mold growth inside of the air ducts or the HVAC unit and the black dust on the air vents are spores or mold dust (dead mold cells).

If you have a duct disconnection somewhere in the attic or crawl space, your HVAC system may be pulling humid air from unconditioned spaces — raising the humidity and likelihood of mold growth. Anytime your indoor humidity goes above 60%, there is a high likelihood of mold growth.

I have personally inspected over 100 homes that had mold growth in their furnace or A/C air handler when I took off the front cover. Most HVAC covers are easy to take off and just a few sheet metal screws need to be removed.

RELATED: How To Prevent Mold in HVAC Systems

Must Be Lab Tested To Confirm

If you have suspected black mold on your air vents, the only way to confirm that it is mold or Stachybotrys chartarum is to get it lab tested.

The cheapest option is to buy a DIY mold test kit online or at a hardware store. You will simply swab the mold, insert it into the package, and mail it to the laboratory.

There are also air tests that you can buy where it is usually placed by an air vent to collect air samples. Homeowners can also contact mold remediation companies or even home inspectors to do the mold testing for you — but you can expect to pay more.

How To Clean Black Mold

The simplest way to clean mold on air vents is to simply remove the vent and clean it with dish soap, water, and a brush.

If the vent cover is really moldy, dusty, and rusted, it may be easier to just buy new ones, they are pretty cheap.

Air vent covers are easy to remove. It’s usually just two screws that hold it in place. After removing the vent, thoroughly wash it and let it air dry.

I also recommend coating the air vent with a sealing spray paint such as Kilz which will help prevent new mold growth.

RELATED: How To Get Rid of Mold in Basement

Prevention Tips

The best way to prevent mold growth on your air vents is to regularly change your furnace filter. Not changing the filter every month will usually lead to a buildup of dust in the A/C system which becomes a food source for mold.

I recently did a home inspection where there was mold growing on some of the air vents, and I noticed that the filter hasn’t been changed in over a year!

UV Lights

UV lights for HVAC systems are a device that shines disinfecting ultraviolet light over the evaporator coil, blower fan, and HVAC plenum.

The UV light bulb is on 24-hours a day (or when the furnace is on) with a special short-wave light frequency called UV-C.

UV-C has been proven to kill mold, mildew, and even viruses by disrupting their DNA and inhibiting their ability to reproduce. Hospitals have been using ultraviolet light for years to disinfect operating rooms and other infected areas.

RELATED: Benefits of UV Lights in HVAC Systems

A Properly Draining A/C

An A/C or furnace that isn’t properly draining water can become a source for mold growth. 

The air conditioner needs to drain moisture (condensate) away from the unit. If there is standing water in the air handler because the drain pan or condensate piping isn’t sloped correctly, then this may feed mold growth in the unit.

Insulated Air Ducts

Air ducts that aren’t insulated, have air leaks, or just in poor condition can lead to mold growth due to moisture buildup or dust getting into the system.

The most likely places to have moisture problems are in unconditioned spaces such as attics, crawlspaces, and garages due to large temperature differences. These uninsulated ducts can contribute a significant amount of moisture into the air.

We recommend insulating the air ducts in unconditioned areas to prevent excessive moisture buildup which may be contributing to mold growth.

Plumbing And Rainwater Leaks

Plumbing or rainwater leaks can contribute to mold growth in the HVAC system because it raises the humidity levels inside the home.

The most frequent cause of water or moisture into the home is from poor exterior grading — which means that the ground slopes towards the home rather than away.

The downspouts also need to drain at least several feet away from the home — the easiest way to fix this problem is to install downspout extensions.

When To Call A Pro

If you suspect that there is mold growth deep inside your HVAC system, or if you are experiencing serious allergic symptoms, we recommend calling in a professional mold remediation company or HVAC contractor to inspect your system for mold.

If there is an earthy or musty smell coming from your A/C system when it is turned on, please call in a professional to inspect your home.

At minimum, someone should visually inspect all vent covers with a flashlight and take off the cover of the furnace or A/C unit.

Frequently Asked Questions

Can Mold Inside The Vents Be A Health Hazard?

Mold in air vents (or anywhere else) can make you sick if enough mycotoxins or mold is sent into the air and breathed in. Allergic symptoms such as sneezing, runny nose, and itchy eyes are all common signs of mold exposure.

What Is Best Way To Get Rid of It?

  1. Remove the air vent from the ceiling or wall (usually just two screws hold it in place).
  2. Wet the vent register with water.
  3. Use a brush and dish soap to scrub off the mold.
  4. Let it air dry.
  5. Spray it with Kilz stainblocking spray paint to help prevent mold growth.

How To Verify I Have Black Mold?

The only 100% sure way to know it’s black mold is to get it tested. But here are some of the main indications:

  1. Visible black or dark blotches or black dust on the vent registers.
  2. Musty odor gets stronger when A/C is turned on.
  3. Allergic symptoms like itchy eyes and sneezing.
  4. Brain fog.
  5. Visible mold growth when cover taken off furnace or A/C unit.

Article References

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