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The Difference between Mold and Mildew

The Difference between Mold and Mildew

July 9, 2024

There is much literature on the differences between mold and mildew. But I get this question during most of my site assessments. So, it is worth discussing. Scientists consider mold and mildew as fungi. 

Fungi are one of the five life forms on earth: animals, plants, fungi, bacteria, and, some say, viruses. They differ in what they look like, how they grow, and the way they process nutrients. 

Mold colors can be black, red, green, or white. The same mold can have different colors based on the environment and substrate. The color of mildew is generally only white to a very light grey and covers areas in a blanket appearance. Molds have many looks. They can appear as spots, clumps, thick streaks, or large areas. The colors can be single or multicolored.  Mature colonies can almost look like animal fur. 

A common way to identify mildew is that it grows flat to the surface. It looks like drywall dust, dirt, or a film. The growth structures cling tightly to the surface, adopting a flat shape.  Both molds and mildew can grow on living plants, wood, paper, or leather. They appear frequently on surfaces with a glossy finish, such as furniture. During my property assessments, I often see mildew growing on overhead floor joists in the basement. It looks like white drywall dust.

Many people I work with say they can smell a moldy odor in their homes.  Both mold and mildew produce these odors. Remember, when you smell it, mold is present in the home. There is enough moisture for it to grow.  

The odor is from the off-gassing during their growth processes.  If there is not enough moisture for them to grow, then they go dormant, and the off-gassing stops. I have been in heavily contaminated buildings. They have no odor because they are dry. On a side note, many think light is needed for mold or mildew. But fungus can grow indefinitely in the dark.

In horticulture, mildews fall into the fungal order of Erysiphales, which is a yeast. It is a chronic problem for gardeners. This occurs in parts of the country with hot and humid summers. High humidity is required to hydrate mildew colonies and materials, allowing them to grow. Both mold and mildew secrete enzymes that break down organic surfaces so they can absorb nutrients.

Mold and mildew need the same ideal conditions for growth. The optimal temperature for molds is 70 to 90 degrees Fahrenheit, and for mildew, 77 to 88 degrees. The optimal relative humidity (RH) conditions for both are 70 to 93 percent.  

When the RH drops below 70 percent, the metabolism slows, and when it reaches 62 percent, it stops. This is one reason that if you live in a hot and humid part of the country, you should never have the windows in the basement open in the summer. The fall, winter, or spring is fine for open windows from a fungal growth point of view. But never in the summer. 

I always recommend to clients that they install a humidistat in their basement to monitor the RH. Mount it on an interior wall, not an exterior foundation wall, and not near the HVAC unit.  

Because molds and mildew are both fungi, the mycotoxins that are produced by mold and mildew cannot be separated.  Most of the technical literature discusses mycotoxins that are produced from mold. 

As I have written before, some molds produce certain mycotoxins, and several types of molds can make some mycotoxins. Although it is known that mildew can produce airborne mycotoxins, it is difficult to evaluate the kind of mycotoxins it creates.

The methods of cleanup between mold and mildew can be a little different. Because mildew does not penetrate into the substrate as mold does, for a hard surface such as wood, a common method of removal is soap and water. 

Mold penetrates deep into a substrate. So, you may need chemicals like hydrogen peroxide or sodium hypochlorite. If the contaminant is mold, it is recommended that soft fabrics with mold are discarded. But, for fabrics with mildew, published literature says it can be cleaned. 

The University of Florida IFAS extension published a paper (FCS3042) that described how to clean fabrics for mildew. It was published in 2001, so the technical way to clean fabrics has changed since then. 

I do not necessarily agree with all of the statements made, but this is a good starting point for you.  On hard surfaces (i.e. varnished surfaces), I recommend only using soap and water.  Remember, you don’t care about killing mold or mildew. You are focused on removing the mycotoxins that are associated with the contaminant. 

Spraying something on fungi without cleaning does not resolve the presence of mycotoxins. By the way, the Environmental Health Center in Dallas, Texas, published a page. It says that Carbon Dioxide dry cleaning can clean mold, mycotoxins, and bacteria.

They cite the Institute of Inspection Cleaning and Restoration Certification (IICRC) S500. It is a water damage standard. However, IICRC’s standard for mold remediation is described in IICRC S520.  

There are many sources of information, and they can be confusing. The important point discussed here is that it is common to see white dust on your overhead floor joists in your basement. It may be mildew or mold. You would need to have it tested to confirm. 

Mildew does contain mycotoxins that can affect your health, so if you see more than 32 square feet, you may want a professional to look at it. Removing it may be as simple as soap and water. But it may release microbes. So, consult a professional before you begin. Don’t forget that you will need to evaluate why it grew there in the first place, or it will come back. 

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