Why Does Mold Form?

Apr 4, 2019

Inevitably, you have seen mold grow at some point in your life. Whether on a loaf of bread that you let sit around too long, on the fruit you let spoil in the fridge, or in the basement of your Grandma’s house that always flooded, mold grows routinely around us in our daily lives. In a previous blog post, we included some common pictures of what mold looks like. You may be wondering, how does an organism seem to grow on all these drastically different materials, and if it can grow on food, in the shower, on walls and floors; what’s stopping it from taking over everything?!

Let me start by saying mold is a fungus which grows branching structures called hyphae. Molds are made up of microbes from many different taxonomic groups, but they are all a part of the Kingdom Fungi. Generally speaking, mold has a few basic requirements for growth. Mold needs water, food, and the correct temperature range to grow.


Moisture can come from any type of water you can think of: basement flooding, the forest floor after a rainstorm, elevated relative humidity in the swamps of the southeast US to name a few. You may not even think about it, but even a lot of the food we eat contains moisture that can support mold. The largest moisture concern in the home is liquid water, for example from a burst pipe or a drain backup. However, mold can grow without any liquid water present, only using the moisture content in the air as a water source. , Mold needs relative humidity to be above 60%, however, many types require it to exceed 70%, 80%, or higher.


When you find mold growing on your old bread or produce, it is pretty easy to understand what that mold is using for a food source. But when mold is growing in your shower, what does it “eat” then? Mold can get nutrients from a huge range of sources. Anything from the food in your fridge to the skin cells in the dust under your sofa can provide mold with the nutrients it needs to live. Different mold types may prefer different nutrient sources. For example, the paper facing of drywall is a popular food source with many different mold types. However, the grime on the tile on your shower will only support a limited number of mold types. Each mold has unique digestive enzymes which results in some molds being commonly found on the same surface. For example, Stachybotrys is commonly found on drywall paper facing, Penicillium is commonly found on fruit, and Cladosporium is commonly found on dust.


Just like plants and animals, fungi have adapted to live in a specific range of temperatures. Some fungi grow in extreme high and low temperatures, but those types are not likely to affect your home. The mold found indoors typically likes to grow in temperatures ranging from 70-90°F. Most molds found indoors don’t infect human tissue because they can not grow at the 98.6°F body temperature of humans. There are some exceptions, such as Aspergillus fumigatus, which can infect lung tissue in suspectable individuals with compromised immune systems causing Aspergillosis.

Controlling Moisture = Controlling Mold

If you remove any one of these pieces from the equation, mold won’t be able to grow. Due to the ubiquity of organic material that mold can use as a food source, and the fact that indoor temperatures are typically in mold’s preferable range,  that leaves moisture as the easiest of the three to control. If you think about shelter, one of the primary goals is to keep us dry.

If you have had a moisture or mold issue in your home, we can perform an inspection and sampling to help you determine the cause, and give you recommendations on next step actions to address the mold and moisture.