7 Simple Yet Effective Ways to Kill Humidifier Mold

humidifier on the table near to bouquet of flowers, in home interiorHumidifiers can be a godsend during the dry winter weather. They add valuable moisture to our homes (and workplaces), improving indoor comfort while preventing problems associated with dry air. By dispersing moisture throughout the house, humidifiers help relieve dry skin, reduce static electricity, and moisturize our nasal membranes. 

Unfortunately, if not adequately maintained, humidifiers can also create perfect breeding grounds for several microorganisms that can quickly become a health hazard. If the humidifier’s components aren’t cleaned and the water changed regularly, the resulting environment could promote mold and bacterial growth. As bacteria and mold multiplies inside the humidifier, traces of the two eventually find a way into the air we breathe. 

Mold in Humidifier Symptoms

One of the most common symptoms of impure air is humidifier lung. Also known as humidifier fever or more formally as hypersensitivity pneumonitis, humidifier lung is a condition of the lungs. It develops when one inhales moisture contaminated with certain strains of bacteria. Symptoms of the disease include;

  • Lung inflammation
  • Body aches
  • Fever
  • Chills
  • Cough
  • Malaise
  • Shortness of breath

The symptoms typically subside when you clean your humidifier and introduce clean, healthy air into your rooms. However, if the situation isn’t remedied, the patient may develop chronic symptoms. These might include weight loss, lung scarring, respiratory infection, and others. 

How to Kill/Prevent Mold in Humidifier 

Dealing with mold often involves killing the spores that are already developed and preventing the growth of new ones. We recommend the following tips;

1. Clean the humidifier regularly

Cleaning a humidifier

Regular cleaning of the humidifier is a simple process that should take less than 30 minutes, unless the unit is seriously attacked by mold. For the best results, proceed as follows;

    • Unplug the humidifier: Never clean a humidifier that’s plugged into the electrical outlet because you risk electrocution. Once you’ve unplugged it, take the unit outside where you can work in a well-ventilated area. Then, put on rubber gloves to protect your hands.
    • Empty the water reservoir: Pour out the water in the water tank. Also, check if the drain pan has any water and empty it too.
    • Disassemble the unit: In most cases, you should remove the water tank and filters with ease. Ensure to dismantle any other removable parts so they can be washed separately.
    • Soak the reservoir: Mix a solution containing equal parts of chlorine bleach and water and pour the solution into the reservoir. If the tank is not removable, take caution not to get any water inside the motor. Use a clean toothbrush to scrub away film and mineral buildup. Then, let the solution sit in the reservoir for 10 to 15 minutes.
    • Allow it to dry: Allow the parts to air-dry in the sun. You can wipe all the surfaces with a soft, clean cloth first or just set them in an open place in the sun to dry. 
    • Reassemble the humidifier: Once all the parts are dry, reassemble them, making sure to tighten the various parts as appropriate to prevent water spillage. If you intend to use the humidifier immediately, refill the reservoir with distilled or filtered water. Otherwise, wait until the day you need to use the humidifier. 

Additional Humidifier Cleaning Tips

Cleaning a filterless humidifierThe steps discussed above are for thorough-cleaning the humidifier, which should be done at least once every month. In between, you’ll need to perform general cleaning, at least every three days. 

General cleaning is easy. Simply turn off the humidifier and soak the reservoir in a 10% bleach solution for about 20 minutes. Then, wash both the inside and outside of the unit thoroughly with a soft, clean cloth. 

Aside from general cleaning, you may also need to clean the filter. Although ultrasonic humidifiers don’t have wicks, most warm-mist evaporative humidifiers do. 

Begin by checking if the wick filter is reusable. If it’s not, you’ll need to buy a new wick. However, if it’s reusable, you can clean it several times before replacing it. Make sure to check how many times you can clean it before replacing it. 

Cleaning the wick doesn’t require any chemicals or cleaning solutions. Instead, you only need to remove it from the humidifier and thoroughly rinse it in clear water. 

2. Invest in germicidal ultraviolet (UV) light units

Germicidal ultraviolet light (UVC) is a type of light that deactivates the DNA of bacteria, viruses, and other pathogens. By disabling their DNA, UVC light destroys the ability of these pathogens to multiply and cause disease. When the organism tries to replicate, it dies. 

This is yet another non-chemical approach to disinfecting humidifiers. UVC light disinfection is simple, inexpensive, and requires minimal maintenance. 

How it Works 

There are two broad ways germicidal ultraviolet can be used to purify indoor air;

  1. Coil sterilization
  2. Air sterilization.

– Coil sterilization

The way the technology works is simple. A shortwave purifier lamp inside a low-pressure tube is installed inside the return air duct in your humidifier to sterilize the handler coil. Coil sterilization is the most common of the two methods and can run for 24 hours continuously. Some of the sterilizers can even run continuously for seven days.

Sterilizing the handler coil treats the original problem because the chief source of contaminants in a humidifier is the coil and drain pan. Top of the range UV stick light bulbs can run for up to 9,000 hours or an entire year as long as the unit is well maintained. It’s advisable to change the bulbs every year. 

– Air sterilization

In this method, a germicidal UV light stick is installed in the return air duct and cycles on with the air handler blower. As such, the light sterilizes all the air moving through the HVAC system, killing mold, bacteria, and other harmful microorganisms. 

Other benefits of Humidifier UV light Sterilization

Other than killing mold, germicidal ultraviolet light bulbs;

  • Reduce colds and flues
  • Reduce smells
  • Remove VOCs
  • Reduce clogging by preventing algae growth
  • Maintain a cleaner coil, thus improving HVAC efficiency


Though extremely effective at killing bacteria and mold, germicidal UV light comes with a few drawbacks. For example, the light can destroy non-UV stabilized plastics in the air handler, even if only the items in the line of sight are affected. 

Additionally, UV light may damage the drain pipe in 3-5 years and eat the flex duct in about two years. For these reasons, germicidal UV light purification should be used with caution. 

3. Avoid common humidifier mistakes

humidistat vs hygrometerAnother way to kill or prevent humidifier mold is to avoid common mistakes people make when using the units. Here are two mistakes that are particularly familiar among most humidifier users;

  • Ignoring humidity levels

In simple terms, humidity is the level of aerated water in your indoor air. Though you need this aerated water, too much of it can cause mold growth. During summer, you need to maintain relative humidity below 60% while in the winter, the recommended value is 30%. 

Whenever the humidity in your home is significantly higher than these recommended values, you risk creating a conducive environment for mold growth. After a while, you’ll start seeing mold growing on the walls, ceiling, and other damp areas.

  • Poor wick filter maintenance

The wick filter serves a critical purpose of trapping dirt and other solid particles in water so that these particles don’t find a way into your home. The particles usually stick to the filter and stay there until the filter is washed or replaced. 

It is therefore vital that you maintain the filter accordingly to prevent dirt buildup. If it’s a reusable filter, make sure to wash it at least twice a week or as recommended by the manufacturer. Once the filter reaches its end of life (as stated by the manufacturer), replace it with a new one. Failure to wash and replace the wick filter puts you at risk of mold and bacterial infections. 

4. Add Selected Elements to Prevent Mold Growth?

There are a couple of things you can add to your humidifier to prevent mold growth. The following are a few options to consider;

    • Vinegar: Adding a cup of white vinegar to the water in your humidifier helps prevent mold growth. The vinegar helps to kill existing mold spores thus preventing the generation of new spores. The best part is that being an antibacterial, white vinegar also helps to kill bacteria in humidifier water.
    • Bleach: Made of sodium hypochlorite, bleach is another solution you can add to your humidifier to prevent mold growth. The recommended ratio is half a cup of bleach to one gallon of water. Do this every week for the best outcome.
    • Humidifier tablets: Yes, there are humidifier tablets designed specifically to help kill humidifier mold and bacteria. They also prevent the buildup of lime as well as eliminate any bad smells. Be sure to pick the right tablets.
    • Others: Finally, there are a few other options including hydrogen peroxide and tea tree oil. Hydrogen peroxide is an antibacterial that easily kills microbes, including mold. Tea tree oil, meanwhile, is an anti-inflammatory anti-fungal, and antiviral with antibacterial properties. The oil is, however, not very easy to access.

Other Humidifier Maintenance Strategies

Aside from the above three tips, the following are other steps you can take to prevent humidifier mold and bacteria buildup in your humidifier;

5. Used distilled/demineralized water

bowl of water on top of registerTap water has minerals added to it that help with killing germs. During humidification, these minerals are dispersed into the air along with the water vapor. The mineral deposits inside the water tank, and aid bacterial growth. Distilled water is, therefore, your best bet. If you must use tap water, make sure to distill it first. 

6. Change the water regularly

If you’re using a portable humidifier, it’s best to refill the unit before use and empty the water when not in use. That’s because even the cleanest water contains minor traces of bacteria. Besides, the water is likely to contact a few bacteria from the air. 

Letting the water sit in the humidifier for a few days would allow the bacteria to multiply. Draining the tank after every use and refilling with fresh water before the next use helps to break the breeding cycle. 

7. Rinse out the tray before every use

Aside from the water tank, the basin (or tray) also needs rinsing before every use. Before you turn the humidifier on for the day, ensure that the water tray is empty and rinse out any water. This is the only way to dispose of any mold and bacteria that might have been growing there. It’s also recommended that you disinfect the tray every few days, perhaps once a week.

Bonus Point: Use natural cleaning solutions

Adding vineger to kill humidifier moldThis is particularly important for people with asthma and other respiratory allergies. Although chemical cleaning solutions do a great job at killing bacteria and clearing up mold, the solutions tend to leave behind rests and a smell that may trigger allergic symptoms. 

Natural cleaning solutions are a great alternative because they too kill bacteria and mold, but without leaving behind any irritant smells. And natural white vinegar is one such natural cleaning solution.

Could it be time for a new humidifier?

If all the tips discussed above don’t work, it might be time to shop for a new humidifier. Suppose the mold keeps returning within just a few days of thoroughly cleaning the humidifier. In that case, it might be a sign that the sediment buildup in the tank or other parts of the humidifier is just too much. At that point, the best solution is to replace the old humidifier with a new unit. 

Key Takeaway

In a nutshell, humidifiers are an excellent solution to the harmful effects of dry air. However, humidifiers also come with a few challenges, chief among them is humidifier mold. You have to clean and maintain the humidifier to remove existing mold as well as prevent the growth of new mold and bacteria. 


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Melanie Mavery is an aspiring HVAC technician who is fascinated by the trends and opportunities in the HVAC industry. She spends most of her day writing content on home improvement topics and outreaching to prospects.  She's always looking for ways to support HVACs!