Do Humidifiers Cause Mold – Spores, Growths And Colonies

High humidity is often associated with damp problems in our homes and it is common to wonder if using a humidifier could potentially exacerbate the problem and cause mold to grow.

High relative humidity caused by an inappropriate preset level or a faulty humidistat in a humidifier can cause mold in a room. Whereas, dirty filters, using old reservoir water, or mineral water, clogs and scales can lead to mold growth inside a humidifier, with the spores then dispersed into the room. 

This article explains molds and how humidity affects their growth. We then discuss how humidifiers work and if they favor or restrict mold buildup. Towards the end, we also answer some FAQ on the topic. 

What Is Mold?

Molds are multicellular organisms classified as fungi. They are complex organisms, measuring 3 to 40 microns, and are essentially everywhere.

While the concentration of mold spores varies in air and can be as high as 50,000 counts per cubic meter, their colonies are also found on almost all surfaces, ranging from paper, wood, and tiles to dust and paints.  

Molds produce allergens and toxic substances that can harm people and pets via inhalation and touching of spores. Other than that, food contamination is also one of the primary sources of mold exposure. This can lead to various health implications like mild to severe allergic reactions, sneezing, skin rash, wheezing, fatigue, dyspnea (difficulty breathing), runny nose, asthma attacks, etc.  

Here are some of the common mold found indoors and their health effects: 

Mold Health Effects 
Cladosporium These don’t typically cause disease in humans, but long-term exposure can lead to allergies, asthma, and eye infections. 
Penicillium Penicillium spores can alter human DNA and produce toxic substances like mycotoxins. The hazardous effects permanently damage neurological, psychological, and immune systems. 
Aspergillus Aspergillus spores can cause allergic reactions and infect the lungs and other vital organs. 
Acremonium These are particularly toxic for immunocompromised individuals and can cause superficial infections.  
Fusarium When ingested, some species of Fusarium impairs intestinal health and immune responses.  

As of yet, there are no EPA-accepted levels of mold spores, and according to the Center for Disease Control and Prevention (CDC), no amount of mold is safe in a commercial building.  

How Does Humidity Affect Mold? 

Mold cannot grow without moisture; therefore, high humidity plays an essential role in their buildup. Unfortunately, this does not imply the reduction or inactivation of mold spores in dry weather or dehumidified air; instead, the spores simply remain dormant until adequate humidity levels are provided.  

Relative humidity (RH) is basically the percentage of moisture in the environment. Typically, an RH of over 55% at 65 degrees or more is required to propagate spores and for mold colonies to sprout from them.  

Therefore, indoor RH levels should be maintained between 30% and 50% (as recommended by the EPA). 

How Humidifiers Affect Mold Growth

A humidifiers works by drawing dry air inside the unit via a rotating fan. Ultrasonic humidifiers use a metal membrane vibrating at a high frequency to propel tiny water molecules (mist) into the circulating air. While, evaporators use a heating element to vaporize water, and the vapors are then used for humidification.  

Based on the type of appliance and settings, a warm or cold mist is produced. 

  • The humid air is dispensed into the room.  
  • Once relative humidity in the room reaches the set level, humidification temporarily stops to prevent over-humidification.  

Through periodic humidification, humidifiers increase and maintain RH within a healthy range.  

The controls on modern humidifiers allow us to set our comfortable humidity level. Most units can achieve and maintain a relative humidity of up to 80%. However, we know that RH above 55% favors mold growth, and therefore, when set at a high level, humidifiers can lead to mold build-up on surfaces and encourage spore production. 

Apart from setting the humidity to above 55%, there are 4 other reasons why humidifiers contribute to mold problem: 

1. Faulty Humidistat 

Humidifiers have a sensor called a humidistat that senses the relative humidity levels of the room and, based on that, regulates a humidifier. If the humidistat starts acting up, this process does not occur correctly, and the unit can mistakenly over-humidify.

At high levels, this can easily cause mold growth.  

2. Mineral Water or Hard Water

Humidifiers require water to work. If mineral water (or unfiltered tap water) is added to the tank, the calcium, magnesium, and salts contained in it are left behind while the water is used up in humidification.

Over time, the residue forms scales that clog the tank and accumulate in other humidifier components.  

The minerals provide nutrition to mold and facilitate its growth inside the tank and outlet of the humidifier. Once this mold begins to produce spores, the humidifier can disperse them into the room with the moisture, exacerbating the mold problem.  

3. Dirty Filter 

Humidifiers have a filter that captures dust and other large airborne particles, and protects the unit from contamination and damage. Once the pores are saturated, the filter needs to be cleaned or replaced (depending on the model and filter condition).  

When not maintained adequately, the filter fails to prevent contaminants from entering the system. As a result, dust and debris may accumulate in the unit, causing clogs and germ buildup. 

Besides filtration, filters also serve as a wick in evaporative humidifiers, which implies that it exposes reservoir water to the air moving through the fan. If the filter is not cleaned and tank water is not drained and replaced frequently, the moisture can lead to mold growth on the filter.    

4. Old Reservoir Water 

Generally, humidifiers have a specific tank capacity and hence, a particular runtime in which the water is fully consumed. Even if a humidifier is not operated continuously, you must drain the reservoir water, clean the tank, and refill clean water every two days (when using the appliance again).  

If you don’t change the water, it becomes a breeding ground for molds and bacteria.  

The mold spores that grow inside the unit are then propagated into the room. Using clean, demineralized, distilled water, cleaning, descaling, disinfecting the humidifier regularly, and replacing any malfunctioning mechanical parts of the unit can prevent the problem. 

Types of Humidifier That Help Prevent Mold 

Both cool and warm mist humidifiers can cause germs if not cleaned and maintained well. However, warm mist humidifiers are less likely to cause mold and bacterial growth and exposure to the room than the units producing cool mist. 

This is because the heating element in warm mist humidifiers boils the water (hence the evaporation), killing many mold spores. However, dirty reservoirs and clogged filters can still cause germs in warm mist humidifiers. 

Best Anti-Mold Humidifiers 

While humidifiers cannot typically kill mold, some modern units are equipped with different antimicrobial mechanisms and air filters that can restrict mold growth. Here are three examples:

MistAire Evaporative Humidifier 

The MistAire Eva-4-Speed Evaporative Humidifier by Pure Enrichment comes with a pre-installed filter that is coated with germicidal material. This filter removes mold spores, bacteria, and allergens like pollen, dust, and dander.  

Afloia 2-in-1 Air Purifier and Humidifier 

One way to enable humidification with antimicrobial features is to combine humidifiers with air purifiers. Afloia 2-in-1 Air Purifier and Humidifier is one such device.

It comes with an ideal three-tier filtration system comprising a pre-filter, a HEPA filter, and an activated carbon filter. The True HEPA filter captures microscopic particles, germs, and pathogens, including bacteria and mold spores.

AZEUS Ultrasonic Cool Mist Humidifier 

AZEUS Ultrasonic Cool Mist Humidifier is one of the best anti-mold humidifiers. It comes with microporous filters that can remove toxic airborne contaminants and germs.  

Can a Humidifier Make Mold Worse? 

Humidifiers can contribute excess humidity to the environment, causing mold growth in the immediate room. Mold spores that accumulate and grow inside the humidifier can also be emitted into the air and dispersed around the home.  

How Does Mold In a Humidifier Look? 

In a humidifier, mold may appear on the filter, vent, fan blades, or inside the water tank. Although typically mold colonies look black or green spots, you may also see white or pink colors. Note that mold growth in a humidifier is not always noticeable.   

Also read about how to identify black mold in your humidifier here.

How To Prevent Mold When Using a Humidifier? 

Maintaining humidity levels below 55%, keeping the humidifier clean, and using clean, distilled water in the unit is crucial to prevent mold growth during humidification. The anti-mildew activity of essential oils like clove and eucalyptus suggests that adding a few drops in compatible humidifier tanks may help inhibit mold. 

Russell Singleton

Russell holds a Bachelor of Science (Environmental and Marine Geoscience) with Class I Honors. He is currently completing his doctorate in science and is passionate about all earth processes, especially isotope geochemistry and paleohydrology.

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