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Why Does Mold Form?

Mold under sink

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.

Ian Cull

Ian Cull is a nationally recognized expert in the field of indoor air quality. He is the Chief Science Officer of Indoor Science, a company he started in 2004. He speaks around the world on air quality topics and is a training provider of the Indoor Air Quality Association. Mr. Cull is a Licensed Professional Engineer (PE) and Certified Industrial Hygienist (CIH). His degree is in Environmental Engineering from the University of Illinois - Urbana Champaign. Mr. Cull has developed 50 air quality related courses for the IAQA University and is the author of the book, “Fundamentals of Mold Remediation”. In his words… “Besides being passionate about indoor air quality, I enjoy cycling, music, the Chicago Bulls, and having fun with my three kids.”

2 thoughts on “Why Does Mold Form?

    Do you do outdoor testing ? We have cedar siding on our home and it gets black mold . I have cleaned it off for years with various products but comes back . Not so bad this year but how does one get red of it and how much ?


    We are mainly indoor air quality professionals, so the mold growth on the outside of your home would be a little outside of our expertise. The same things are causing growth on the outside as mold growth inside: moisture, food, and correct temperatures. To stop the mold you need to get rid of one or more of these conditions. With exterior siding, there is really just 1 thing that you might be able to do, and that it to control the food source for mold. This can be done by cleaning the siding a few times a year, there may also be mold-resistant treatments for the siding that is available.