Humidifier Cause Mold? How Humidifiers Cause Mold & Mildew

In dry climates or drier seasons of the year, a humidifier increases the overall humidity in a room, reducing dry skin, cracked lips, eye irritation, and more.

However, many people who use humidifiers find they end up developing mold growth within their properties, so, the question is, “Do humidifiers cause mold?”, and in this article, we will be fully answering this and a whole lot more. So, for everything you need to know about using humidifiers and the potential for increased mold growth as a result, keep reading.

Can the use of a humidifier cause mold?

Humidifiers can worsen pre-existing mold growth, or be the cause of new growth if they are overused, used incorrectly, or not cleaned thoroughly. The humidistat on a humidifier should not exceed 55% or the warmth and moisture within a property will make it the perfect breeding ground for mold.

mold in humidifier

How can humidifiers cause mold growth?

The good news is that when used correctly, a humidifier will help to make you feel more comfortable in your home and will often not be the main cause of increased mold issues in a property.

The bad news is that using humidifiers can lead to higher risks of mold growth, and there are numerous ways that they can indeed either be the cause of or worsen an existing mold issue within a property.

Below are examples of some of the most common mistakes made by people using dehumidifiers.

Increased humidity

As you would expect, the whole purpose of a humidifier is to increase the overall level of airborne moisture within a room or for whole home humidifiers, the entire property. Whilst this can indeed make the environment more pleasant and comfortable for inhabitants, if not used wisely, can give mold exactly the kind of moisture levels it requires to live. High indoor humidity can definitely lead to mold issues.

Mold needs consistent humidity levels of over 55% in order to spread at its most rapid rate. This humidity level is high enough for moisture to begin condensing on surfaces, which can then be used by mold for hydration.

As an example of how easy it is to surpass this level, one of the least humid states in the US Arizona has an average humidity of 38.3%. In this circumstance, a humidifier should only be used to increase the moisture levels by 12%, to reach an ambient humidity of 50%. However, many people would surpass this and create an excessive amount of airborne moisture.

Lack of cleaning

Evaporation humidifiers work by passing air over a damp filter, this evaporates the liquid into the air which is then passed into the room it has been placed in.

Other humidifiers work by taking water past its boiling point so that steam is created and passed into the room.

In either example, as a result of high levels of moisture, and the potential for dust to enter the unit, mold can form within it. For this reason, it is vitally important that the units are inspected and cleaned on a regular basis to prevent mold formation.

Were mold to form in a humidifier, its spores could be propelled into the room it had been placed in, this causes mold growth in other areas of the property. Cleaning your humidifier regularly will lessen the chances of spores being spread through your home.

Faulty humidistat

A dysfunctional humidistat within a humidifier will cause an overproduction of moisture, increasing the ambient humidity levels to far higher than necessary. The unit working harder and for longer will also increase running costs considerably.

Without the use of an external hygrometer in the same room as the unit, it can be extremely difficult to tell if it is humidifying the room to the preset levels. This is why it is important to regularly check and maintain your humidifier.

Mineral-rich waters

Using standard tap or bottled water to fill your dehumidifier tank is a common error, as organic materials can be introduced during the bottling and water treatment processes.

These organic materials are in sufficient quantities to act as a source of nutrients for mold, so, after adding this kind of water to your humidifier, you have given mold two of its requirements (food and moisture), and you may find it begins to grow within the unit after only a short period of time.

Placement of the units

Where you place your unit makes a big difference as to how likely it is to cause or worsen an existing mold issue.

The ideal placement for a humidifier is in the most central room of a property, this allows it to evenly distribute moist air throughout as much of your home as possible.

Placing the unit too close to a wall, bed or any other surface will make condensation and moisture accumulation more likely. These high moisture areas of the property will give mold the exact circumstances it needs to thrive.

Should you only wish to humidify a single room, the unit should be placed as centrally as possible, away from any kind of furniture.

Room size

Room size should certainly be taken into consideration when placing your unit if the room is only small (a room used as an office for example), the humidity level will become very high very quickly if the unit is too large, or the settings are too high.

Having high humidity levels for a long period of time can lead to condensation formation, and of course with it, can cause a mold problem.

It is, therefore, best to use a calculator to decide which size of unit you require to make your property feel more comfortable, rather than buying a unit that is far bigger than required, this is a common reason why humidifier cause mold.

Below is a link to a calculator so you can find out exactly what size of unit you’ll need.

Humidifier Capacity CalculatorOpens in a new tab.

Water tanks left full

The water tank in a humidifier needs to be emptied when not in use, and ideally cleaned once each month to remove dust and dirt that can accumulate over time.

A water tank that is left for weeks or months at a time is likely to be the perfect environment for mold to grow in, as it has everything it needs.

Were this mold allowed to grow or go unnoticed, the humid air the unit would be passing out would also contain mold spores that could spread throughout your property, causing additional patches to grow.

Steam-producing humidifiers would likely kill the spores due to the heat created, however, mold spores could still escape from the unit, so it still stands that they should be cleaned just as frequently.

What kinds of molds grow in humidifiers?

There are a number of strains of mold that can grow in a humidifier, however, (and probably quite surprisingly), the most common “mold”, isn’t in fact mold at all, it is a bacteria.

Pink mold (as it is often referred to), is banded together with other strains as it is commonly found in the same environments. This form of bacteria is called Serratia marcescens, and whilst it is not a strain of mold or mildew, it can still cause similar health complaints and should be taken just as seriously.

The second most common strain is called Aspergillus and is often mistaken for black mold, as it appears in dark colorations and has a velvet-like texture.

And finally, we have penicillium strains, coming in blue-green, grey, white, or even turquoise colors and presenting in a powdery texture.

How to identify mold growth in a humidifier

The most common sign of mold growth in a humidifier may not be something you see, in fact, you are more likely to smell mold growth before you finally spot it growing somewhere.

As soon as you turn on your humidifier, you would likely smell a musty odor that may remind you of an old attic or basement. Upon further investigation, you may see small dots or circular clusters of growth in either dark brown, black, or blue colors.

Are the molds dangerous?

Yes, molds (and bacteria), found within a humidifier can be hazardous to both humans and animals, as they can produce mycotoxins as a defense mechanism.

These toxins can cause a myriad of health complaints in otherwise healthy individuals such as:

  • Headaches
  • Nausea
  • Brain fog
  • Fatigue
  • Muscle weakness

And for those with suppressed immune systems or allergies, more serious conditions such as breathlessness, asthma attacks, skin irritation, and potentially serious infections can be caused by exposure to mold and its spores.

In healthy individuals, a small amount of mold exposure may not lead to health complaints if swiftly dealt with, but in those with immune system issues, even short-term exposure can be harmful.

How to prevent mold in humidifiers

We’ve explored what kind of molds grow in humidifiers, whether they are dangerous, and how they can possibly be spread through the home, so now let’s look at some of the most effective ways that you can prevent this problem from happening entirely.

Clean the unit

The idea that you should clean your humidifier (especially the tank), may seem obvious, but they are often left for weeks or even months at a time without being inspected.

To make sure mold never has the opportunity to start growing within your humidifier, remove the water tank and clean it with detergent, warm water, and an abrasive sponge to remove any dust or grime that mold could potentially use as a source of nutrients.

Clean any other areas of the unit where it is safe to do so. For more information on how to clean your unit, check the user manual that came with it, visit the manufacturer’s website, or contact them directly.

Choose the setting carefully

More is not always better, and if you are concerned about the potential for mold growth due to excessive humidity levels, you should certainly be paying attention to how high you have your units set.

In order to make an environment feel comfortable, the humidity level is often suggested to be set between 40-60% during the summer months, and between 30-50% during the winter months.

Bear in mind that the summer setting suggestion of 40-60% humidity would put you in the zone where mold grows most rapidly, so keeping your property clean will be even more important during these months.

There is no need to go beyond these suggested settings, and should you do so, you are giving mold a far greater chance to breed successfully. If you can reduce the level just a little and keep your unit set to between 30-50% humidity throughout the year, you should have very little issue with mold growth whilst remaining comfortable in your property.

Whilst you use a humidifier, it is useful to keep check of the humidity levels within each room of your property to ensure that no one room has higher ambient humidity than another, which could develop into a mold or mildew issue.

Use distilled water

We have explained that bottled water and tap water can both contain organic compounds that mold can use as a source of sustenance. It is therefore advised that instead of using these sources, you use distilled water instead.

Distilled water can be purchased from department stores and has been boiled to remove any impurities. These impurities are what mold would otherwise be able to feed off, so by removing them, it will not have the option to live in the humidifier’s water tank.


What can you put in a humidifier to parent mold?

Half a teaspoon of hydrogen peroxide placed into the water tank of a humidifier will be sufficient to kill any mold that tries to grow. A few drops of tea tree oil will also help to prevent mold, as it has anti-fungal properties, and introduces a pleasant aroma to the room.

Is it normal for humidifiers to get moldy?

It is very common for humidifiers to become moldy, this is due to the warm and moist environments they provide. Unless they are cleaned on a regular basis, dust accumulation and organic materials within the water can build in the water tank and provide the nutrients mold uses for sustenance.

Can humidifiers cause damp?

Yes, humidifies can cause damp if they are used too frequently, for too long, if the settings are set too high, or if the room they are in is too small for the size of the unit. Excessive humidity in a room for prolonged periods of time will lead to condensation and eventually damp issues.

Is a humidifier or dehumidifier better for mold?

Dehumidifiers are more effective than humidifiers to control mold growth, as drawing moisture out of a room helps prevent initial mold growth, and dries it out if it has already developed. Humidifiers may worsen mold issues, as increasing moisture in a room can increase the rate at which mold breeds.

What humidifier kills bacteria and mold?

As manufacturers of humidifiers are well aware of the common occurrence of mold within the units, several models have been developed to kill mold and bacteria before and as it develops.

Some of the more modern units use ultraviolet light to kill mold spores on contact. However, slightly more old-fashioned humidifiers heat water past boiling point to release steam. In this case, the high temperatures will kill mold, its spores, and bacteria.

Below are a few examples of humidifiers that claim to be able to effectively kill mold and bacteria.

Safety 1st Stay Clean HumidifierOpens in a new tab. (US)

Pure Enrichment® MistAireOpens in a new tab. (US)

LEVOIT 3L HumidifierOpens in a new tab. (UK)

LtYioe HumidifierOpens in a new tab. (UK)


Humidifiers can be helpful in making otherwise dry environments feel far more comfortable, but if not used correctly, they can worsen pre-existing mold growth by providing it with the moisture levels it requires. Keeping the unit clean and not setting the humidistat higher than 50% should keep your room comfortable without risking mold growth.

Chris Walker

Chris Walker has struggled for several years with mold after buying his own property. After finding the solutions to several issues around his home, he decided to create this site in order to answer as many questions about mold and mildew as possible to help others dealing with the same problems.

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