Do Humidifiers Cause Mold? An In-Depth Analysis

Humidifiers can be a helpful tool to soothe dry sinuses and respiratory related issues. However, while they may be helpful in managing indoor air quality (IAQ), especially during dry seasons, they can also engender an ideal environment for mold.

Due to how a humidifier functions, it effectively creates an incubator for germs, bacteria, and mold spores. This is especially troublesome for dehumidifiers with misting functions that aerate the surrounding area with moist, mold laden water. Depending on where you live, the use of tap water can also be an issue because of the various organic material and total dissolved solids found in the water; the use of distilled water is recommended.

Like many other home appliances, humidifiers can become harmful over-time. This is especially true in the case of mold growth as a result of using a humidifier. A humidifier is troublesome because of increased humidity levels, neglect, and improper use.

What is Humidity?

To fully comprehend what humidity is, you first need to realize that there is water in the air all around you in the form of a gas called water vapor. Humidity is then described as the presence of water vapor in the atmosphere.

A weather forecaster will talk about humidity in terms of being either absolute or relative.

Absolute Humidity: The mass of water vapor divided by the mass of dry air in a certain volume of air at a specific temperature. It is expressed in moisture per cubic meter of air (g/ m3)

fan humidifier blowing

Relative Humidity: The ratio of the current absolute humidity to the highest possible absolute humidity. For instance, a measurement of 100 percent relative humidity means that the air is completely saturated with water vapor.

Humans are very sensitive to humidity levels as the skin relies on the air to get rid of moisture. Sweating is part of the sympathetic nervous system (autonomous) which helps to orchestrate the body’s reaction to stress, like increased body temperature.

To better outline this, water damage restoration company Water Mold Fire details that humidity levels have a major impact on the growth of mold.

Humidity’s Impact on Mold

It’s important to first note that mold will only grows in the presence of water, an organic (carbon-containing) nutrient source, and oxygen. As a result, mold thrives in climates that have elevated levels of relative humidity.

You might start to see the relationship here.

If the air has 100 percent relative humidity, the sweat from our body isn’t able to evaporate. Meaning, you feel hotter than the actual temperature.

This same concept applies to other surfaces, not just human skin. For instance, when moisture laden air comes into contact with cold surfaces like walls and floors, the water vapor condenses and moisture then penetrates these surfaces. This same scenario is problematic for cold pipes where-in moisture can develop and build over time.

As we’ve learned, moisture/water is a key component for mold growth.

The EPA notes that indoor relative humidity should be kept below 60 percent; Ideally within the range of 30-50 percent to help avoid these situations and the resulting growth of mold.

How to Measure Humidity Levels

Most indoor thermometers will output a variety of useful metrics like temperature and humidity levels. However, the most basic way to test humidity levels is with a hygrometer. The most common type of hygrometer is a psychrometer.

A psychrometer is essentially two thermometers. One is a dry bulb thermometer that measures ambient temperature (much like a regular household thermometer). The second is a wet bulb thermometer that measures temperature by covering the wet bulb with a water-laden cloth.

Depending on the design of the psychrometer it is either swung or remains static until the wet bulb has cooled. Humidity level can then be ascertained by taking the difference between the dry and wet temperatures.

plant humidifier home

This process works based on evaporation. Meaning, dry air absorbs more moisture than wet air. The smaller the change in wet bulb temperature the more humid the air is.

By design, a humidifier adds moisture to the air. Manufacturers of humidifiers identified this as problematic as it’s implied that the user knows when to use the humidifier however, they may not know when to turn it off.

As a result, many equip their models with a built-in hygrometer or ‘humidistat’ that maintains optimal humidity levels in your home automatically.

Types of Humidifiers

In terms of minimizing mold growth, it’s important to understand the types of humidifiers and how they work as some are better than others at inhibiting its onset.


Steam humidifiers work based on the humidistat concept iterated above. Meaning, when the humidifier senses that the relative humidity level has dropped it turns on.

A steam humidifier essentially heats water that’s contained in a reservoir and converts it into steam which then raises the humidity levels in a space.

Evaporative Humidifier

An evaporative humidifier works by drawing air through an intake. The air then contacts a moist wick filter absorbing water through the reservoir. The water vapor laden air is then exhausted into the surrounding space.

lighted humidifier

Ultrasonic Humidifier

An ultrasonic humidifier features a small metal plate or piezoelectric transducer that oscillates at an ultrasonic frequency to break water particles. The oscillating vapor isn’t able to remain on the surface of the plate and instead is exhausted as water vapor.

Impeller Humidifier

Instead of using a wick filter, an impeller humidifier uses a pump that rotates a disc. As the disc rotates it flings the water through a diffuser. The diffuser works to break the water particles into a fine mist.

Tap Water or Distilled Water in a Humidifier?

All of these humidifiers make use of water to add moisture to the air. While their method of doing so differs, their goal is the same: to take liquid water from a reservoir and add it to the air.

The problem then becomes the water itself. We noted above that one of the biggest issues with humidifiers is that people don’t use them properly. All water purifiers will tell you to change the water reservoir regularly and to use distilled water.

Depending on where you live and your source of water, it can be rich in minerals and total dissolved solids. When it’s dispersed into the air through either of the methods, the result is a white residue or dust. You might be familiar with limescale build-up on a faucet or other surfaces where hard-water has evaporated from.

As we’ve learned above, mold metabolizes carbon containing compounds like calcium carbonate (limescale). Meaning mold has all it needs to grow: nutrients, water, and oxygen. That’s why you shouldn’t use tap water and why the water reservoir needs to be changed regularly.

As a result, it’s recommended that you use distilled water because it has far less minerals than that of tap water.

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