What Causes Air Conditioner Mould? How Do I Fix It?

What causes air conditioner mould

Air conditioners have made modern living more bearable, especially amidst globally rising temperatures brought about by climate change. Unfortunately, air conditioners are not immune to problems arising from regular use in a moist environment.

Among these problems is mould growth. Mould can spread unnoticed inside the AC unit for long periods if left unchecked. They cause health risks and illnesses, especially for people with respiratory issues.

In this article, you’ll learn:

  • The causes of mould growth in air conditioners
  • Why you should be concerned about it
  • Signs and symptoms of mould growth
  • Why air conditioner servicing is the best prevention
  • How to clean a mouldy AC unit

For all the facts about mould in your air conditioning system, read on!

What Causes Mould Growth in Air Conditioners?

Mould growth in air conditioning units is not caused by a single thing but rather by a combination of certain conditions. Since mould is a living organism, it requires certain conditions to grow: moisture, temperature, humidity, and nutrients (build-up in your AC unit).

Learn more about why air conditioning systems can grow mould below. 

1. Moisture 

Part of an air conditioner’s cooling process is the rise in the humidity of the air that circulates within a room. This causes air condensation once it passes through the AC unit and is one of the most critical ingredients for mould growth. 

One way of preventing this is using the air conditioner’s fan-only mode instead of the usual cooling mode. This will prevent the moisture inside the room from being blown back into the AC unit. Some modern air conditioners also come with improved moisture removal, often found in AC units with a SEER (Seasonal Energy Efficiency Ratio) of 14 and above. 

In addition, having the right size AC unit installed is also essential, as units that are too large for the room they are cooling will not be able to reach total capacity within the first few minutes of operation. This will prevent the system from removing humidity effectively due to a shorter on-cycle.

2. Temperature

Most mould species colonising inside air conditioners can only grow in specific temperatures, particularly above 4.4° C. However, simply reaching surface temperatures between 20°C and 30°C is generally considered optimal for mould growth. This makes it a little harder to prevent mould growth since the same temperature range is also ideal for human comfort. 

In addition, sudden temperature changes inside the room also affect mould growth. For instance, condensation produces moisture if warm air comes in and makes contact with cool surfaces, promoting mould growth.

3. Lack of Cleaning

Due to the nature of an air conditioner’s operation, it requires regular or routine maintenance and cleaning. Air filters will have to be swapped out for new ones (especially during the summer), and usual blockage points within the AC unit (like the condensate drain) should be checked regularly, depending on the frequency of use. 

While you can clean some parts of the air conditioner yourself, deep cleaning should only be done by a professional technician to ensure that no parts are damaged in the process.

4. Humidity 

Closely related to moisture, humidity is the root of moisture buildup within the air conditioning unit. Although the AC unit and the weather conditions can increase humidity, most modern air conditioners have a setting to limit humidity levels inside a room. However, humidity and temperature are not independently controllable through an AC unit, so a separate dehumidifier is usually a wise investment.

Indoor humidity in the range of 45% – 55% is generally considered ideal for mitigating mould growth and avoiding the spread of pathogens such as viruses and bacteria. However, some suggest that the relative humidity should not exceed 50% for the best results.

5. Dust & Blockages 

Organic material, no matter what it is, is a source of nutrients for mould. Whether these organic materials cause a blockage inside the air conditioner, their presence in your AC unit can promote mould growth.

Contaminants that fuel mould growth include atmospheric dust (which continuously falls inside the air conditioner apparatus and ductwork despite air filtration measures), dead skin cells, pollen, and other indoor air pollutants that make their way inside the room.

Blockages not involving pollutants may also cause the facilitation of mould growth. For example, clogged condensate drains will keep water in the interior of the air conditioner unit and eventually cause mould infestation. In addition, droplets of water that find their way into the air ducts and collect there whenever the air conditioner is not in use can also cause mould growth.

Why is Air Conditioner Mould Dangerous?

Regardless of where it grows, mould is dangerous, especially for people, as it is considered a hazard to people’s health. This is especially true for mould inside air conditioners since an air conditioner circulates air through your home, potentially spreading mould spores into every room. 

1. Mould in Air Conditioners Can Cause Illness

The danger posed by mould growth in air conditioners is higher among individuals who are sensitive to contaminants. This includes people who have allergies and those with autoimmune disorders, as well as babies, children, and older people. Exposure to the spores released by mould may cause inflammation of your airway and lungs, triggering pre-existing respiratory problems, skin allergies, and asthma.

The primary symptoms exhibited by people continually exposed to mould inside an air conditioner are fatigue, dizziness, nausea, wheezing, coughing, irritated throat, nasal congestion, and irritated eyes. In rare and extreme cases, the reactions can be life-threatening. 

Since the symptoms generally do not show themselves until your air conditioner has been infested with a significant amount of mould, most people are unaware of the mould situation until it’s out of control. This is where preventive maintenance and regular cleaning can help.

2. Air Conditioners Can Spread Mould in the House 

A mould infestation inside your air conditioner can spread into the rest of the house through the fungal spores they release. These spores are microscopic particles that allow fungi like mould to reproduce, much like seeds are for plants. 

Mould spores are released either actively – through the intentional ejection by the mould into the environment – or passively. In the case of an air conditioner, the air vents can blow them into the house.

If your home has a relatively humid environment, then these spores will grow in other parts of the house, especially areas where it is moist (such as the bathroom). If you have noticed mould where it shouldn’t be growing, then your air conditioning may be causing the spread of mould spores.

3. Mould Can Block Air Conditioner Airflow 

Mould growth, if left unchecked, will spread so fast that it will infest the entire interior of your air conditioning unit, blocking airflow. This means an air conditioner’s components could be in perfect working conditions, but mould growth prevents it from reaching its peak cooling capabilities. It may even cause the AC unit to work extra hard to achieve the temperature setting, consequently inflating your energy consumption. 

This can eventually cause an air conditioner’s components to fail, leaving you with expensive repairs, replacements, or even having to replace the entire air conditioning unit itself. Removing extreme cases of mould infestation can be much more costly than buying a new unit, making it very important to monitor mould growth in your air conditioner.

Signs Your Air Conditioner Has Mould 

Much like other problems that plague air conditioners, there will usually be noticeable signs indicating mould growth inside the unit. We have listed the most common ones below:

1. Visible Mould Spores in Unit & Around Air Vents

Contrary to what most people believe, the large coloured growth visible to people is the fungal spores of the mould. The mould’s body is the secondary colour surrounding the spores. The colours of mould vary; it could be black, brown, grey, greenish, orange, pink, purple, yellow, red, white, or even a combination of these. 

Although mould colour indicates several factors, such as humidity level and light, the colours do not affect the spores’ toxicity. Regardless, if you see mould growth of any colour on your air conditioner, its vents, and the area surrounding it, it indicates mould growth that has probably spread through the entire interior of your air conditioner.

2.  Musty or Damp Smell when Air Conditioner is Running

One of the biggest signs of mould infestation is that your air conditioner smells bad when turned on. Mould produces what is known as Mycotoxin – naturally occurring toxins produced by fungi, including mould inside air conditioners. Scientifically called microbial volatile organic compounds (mVOCs), these compounds have powerful and unpleasant odours, often described as earthy, musty, or damp.

Since your air conditioner practically blows air from within the unit itself (where the mould also grows), people in the immediate vicinity will be able to notice the smell, with its potency depending on how bad the mould infestation is.

3. You Suffer From More Frequent Respiratory Issues 

The fungal spores released by fungi, such as mould growing inside air conditioners, have been proven harmful to people’s health, particularly their respiratory system. If you notice allergic reactions (such as wheezing, coughing, and sneezing), skin rashes, eye irritations, and asthma flaring up, it could indicate mould growth inside the AC unit.

Mild respiratory issues aside, fungal spores are also known to cause more severe respiratory illnesses such as toxic pneumonitis and hypersensitivity pneumonitis. For people with weaker immune systems, the damage to their health goes beyond respiratory issues and can cause chronic fatigue syndrome, kidney failure, and, in rare cases, cancer.

How Do I Remove Mould From My Air Conditioner?

Most residences will have a window or wall unit AC installed. These are relatively easy to disassemble for cleaning. But no matter the air conditioning or HVAC system type, wearing proper safety gear is essential. The fungal spores released by mould-infested air conditioners can make you very sick.

Ideally, you should have a professional aircon technician clean a mouldy aircon unit. However, the general steps are covered below:

  1. Turn off the air conditioning unit
  2. Wear the proper protective gear to prevent exposure to mould and mitigate any hazards to your health
  3. If the air conditioning unit can be moved, bring it to an area with good ventilation (such as outdoors) to prevent mould from circulating in an enclosed area.
  4. Open the wall unit by using the owner’s manual. Vacuum within the air conditioner itself to remove loose spores.
  5. Remove the air filters and wash them with water. If they are of the disposable kind, throw them away instead.
  6. Get a container filled with soapy water and wipe down the air conditioning unit itself. You may allow it to air dry and drip a little before wiping it with a clean and dry cloth.
  7. After it completely dries, install the filters and assemble the unit as intended.

You can use well-known mould preventatives such as tea tree oil, clove oil, a 50 / 50 mix of water and vinegar, or methylated spirits and water. Remember not to use bleach for mould removal.

How Do I Prevent Mould From Growing in my Air Conditioner?

Cleaning a mouldy air conditioner is good, but preventing mould growth is much better. Although some measures may cost a little more upfront, the long-term benefits and savings you get from simply preventing mould growth make it a much better decision. 

Install a UV Air Purifier

Installing an Ultraviolet air purifier utilises a special light frequency that has been proven to kill mould and other harmful microbes. The light is installed on the indoor air handler part of the air conditioner and emits UV light 24 hours a day. These come with a specially designed viewing hole that allows you to verify if the UV light is on. It is vital to ensure that the UV light is turned off before installation to avoid damage to your eyes.

It should also be noted that a UV air purifier should not be installed when mould already exists inside the air conditioner or HVAC system. The AC unit should be cleaned first and free from mould before installation. The location where the UV air purifier will be installed is also important, as it should have as much surface area coverage as possible to maximise effectiveness.

Have Your Air Conditioner Professionally Cleaned 

Nothing beats a professional cleaning done by a licensed professional or an air conditioner HVAC specialist. These people work in the industry and know how to identify mould infestations and the best way of cleaning them from your AC unit to prevent them from spreading again. Although their services are never cheap, you can never go wrong with seeking expert cleaning help.

Having your AC unit professionally cleaned also gives you peace of mind since the technicians will often use tools explicitly meant to remove mould and keep it gone for as long as possible.

Regularly Clean or Change your Air Filter 

Air filters are responsible for catching and stopping all those unwanted particles and pollutants from damaging the system and rereleasing into the house. More often than not, organic materials will get caught up in the filters. These organic materials will accumulate if left unchecked, eventually serving as a nutrient source for mould growth.

As such, cleaning your air filters regularly will prevent this from happening. The frequency of cleaning will depend on how often and long you use the air conditioner, as well as the nature of the environment you live in. Areas with a lot of dust and pollutants (such as near main roads) will require more frequent cleaning.

If your air conditioner uses disposable filters, consult your local air conditioning specialist on how often they should be replaced. These filters will usually come with a recommendation on how long they should be used before needing replacement. Still, this replacement interval will also vary on factors similar to reusable air filters.

Wipe down the AC Unit Every Week 

This may seem very basic, but you’d be surprised at how effectively this can prevent mould growth. Remember that mould grows in humid environments with a lot of moisture. You significantly reduce the likelihood of mould growth by simply wiping down the air conditioner around and a little inside through the air vents. You can even go the extra mile and wipe down its surroundings. 

It does not necessarily have to be every week, especially for busy individuals. But generally speaking, the more often it is wiped down, the less likely it is for mould to infest the unit.

While you may want to treat your air conditioner with a mould preventative, many chemicals can damage your AC unit, so it’s best to leave this to your air-con technician during your annual service.

Mouldy Air Conditioner FAQs

Is Mould Worse in Summer or Winter?

The risk of mould growth is higher during summer due to warmer temperatures and rains. The days in summer are hot and humid, which encourages mould growth. The temperature difference worsens this – condensation can occur when hot air from outside enters the cold interior, creating moisture and promoting mould growth.

Do Air Conditioner Ducts Need to Be Cleaned?

Yes, air conditioner ducts need to be cleaned occasionally, especially if your home has a fireplace or stove that circulates fine debris, ash and dust. Having your ducts professionally cleaned every 1 to 3 years is ideal for keeping them clean and preventing mould growth.

Is Mould in Air Conditioners Common?

Yes, mould in air conditioners tends to be very common, given how easy it is to achieve the ‘ingredients’ required to cause mould growth. It goes undetected for quite a long while in some households. Proper knowledge in detecting the signs is, therefore, vital.

What Does Air Conditioner Mould Smell Like?

Air conditioner mould smells like regular mould – earthy, musty, or damp. This smell is caused by the compounds produced by the mould, which differ from the fungal spores it produces. The scent is generally distinct enough for people to know it is mould.

What are the Symptoms of Mould Exposure?

Symptoms of mould exposure are usually allergic or respiratory responses, such as asthma, coughing, and wheezing. Non-respiratory symptoms include nausea, fatigue, throat irritation, skin rashes, eye irritation, kidney failure and cancer.

How Soon Can Black Mould Affect You?

Symptoms from black mould exposure usually manifest within two to nine hours of exposure. This time can vary depending on how long an individual was exposed and how intense the exposure was. Symptoms generally last anywhere from one to three days, including shortness of breath and coughing, among others.

Should I Get My Mouldy Air Conditioner Professionally Cleaned?

Professional mould removal depends on how much mould is present or how bad the mould infestation is. However, these things are generally unknown until a professional inspects the mouldy air conditioner.

The reality is there is only so much an individual can do to eliminate mould without the knowledge and expertise of professionals. Bad cases of mould growth will almost always have to be dealt with by an air conditioning professional for safety. 


This article is published in good faith and for general informational purposes only. It does not constitute an estimate or quote for any specific service. Platinum Power & Air does not make any warranties about the ongoing completeness, reliability and accuracy of this information. This article is not intended to replace consultation with a licensed technician. 

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