Understanding Mold and Mildew: Clarifying Common Misconceptions

At McKee Environmental, we often encounter confusion regarding the terms mold and mildew. For example, customers will ask, “What is the difference between mold and mildew?” and “Is mildew toxic like mold?” Our in-house experts are here to bring clarity and understanding regarding mold and mildew, along with debunking a few frightening mold myths.

What does the word mildew actually mean?

Sometimes used in a general way, people refer to mold growth in damp areas as mildew. For example, in the shower or on a dewy windowsill. Interestingly, scientists who study microorganisms (mycologists) only refer to mildew in the context of plant disease; not in the context of fungus growth on materials or objects in your home or workplace.

What is mold?

Most often, people use the word mold for visible fungal growth indoors. It is a general term describing unwanted mold, irrespective of the type of mold that is present. Mold is a catch-all term, similar to how we might use the word pest to describe numerous types of unwanted pests in our homes. The US EPA (Environmental Protection Agency) considers all mold to be potentially allergenic, meanwhile, some molds may cause nonspecific health symptoms. Furthermore, the EPA states that no matter the type of mold growing indoors, it’s imperative to clean it up properly. That means determining the type of mold present is not necessary because we should treat all molds with the same respect to potential health risks and therefore remove it promptly.

Similarly to the animal kingdom, mold is a part of the fungal kingdom, scientifically classified by their genus and species. To better explain mold classifications, let’s first talk about the genus for dogs which is Canis. The genus Canis includes wolves, foxes, domesticated dogs, etc. Breaking it down further, the species for domestic dogs is Canis Familiaris and includes most all common domestic breeds. When discussing mold classifications, common genera of molds are Aspergillus, Penicillium, Cladosporium, Chaetomium, and Stachybotrys. Each of these genera has numerous species, all of which include distinct characteristics and potential environmental impacts.

A few common molds you may have heard of include:

  1. Stachybotrys: Stachybotrys, often abbreviated as Stachy, was incorrectly implicated as being a particularly toxic and “newly identified mold” in the late 1900s. This was later debunked due to a few studies by the microbiology community at large. As per the Centers for Disease Control (CDC), “Stachybotrys chartarum is a greenish-black mold. It can grow on material with a high cellulose content, such as fiberboard, gypsum board, and paper. Growth occurs when there is moisture from water damage, water leaks, condensation, water infiltration, or flooding. It requires constant moisture for its growth. At present, no test exists that proves an association between Stachybotrys chartarum and particular health symptoms.” The allergenic properties of Stachybotrys are unclear and there are no reports of infections in humans. However, like some other fungi, species of this organism may produce harmful mycotoxins. Although Stachybotrys is outdoors on decaying plant material, it is rare to detect it on outdoor air samples.
  2. Aspergillus/Penicillium Species: This group of fungi is the most common fungi isolated from the environment – indoors or out. Without speciation, they appear remarkably similar under the microscope and so are often categorized together in some analytical methods. Both are common in soil and on decaying plant material. This large group of fungi grows well indoors on a wide variety of substrates, is a common allergen, and may cause hypersensitivity pneumonitis. Scientists report that they may also cause extrinsic asthma and could be opportunistic pathogens. Many species produce mycotoxins which can adversely affect human health, but toxin production is dependent on a variety of factors including the species, the food source, competition with other organisms, and other environmental conditions.
  3. Cladosporium: One of the most common genera of fungi worldwide is Cladosporium. Find it in soil, plant debris, and on the leaf surfaces of living plants. The outdoor numbers are typically lower in the winter and often relatively high in the summer, especially in high-humidity regions. Furthermore, outdoor numbers often spike in the late afternoon and evening. Indoors, find Cladosporium growing on textiles, wood, sheetrock, moist windowsills, and in HVAC supply ducts or air vents. It can have allergenic properties and reports indicate it is a common cause of hypersensitivity pneumonitis.

Conclusion: All indoor molds come from the outdoors.

Understanding the use of layperson terms such as mold and mildew can improve communication between scientists and the layperson on such topics. At McKee Environmental, we focus on identifying and remedying indoor microbiological issues to help you have a healthier living or working environment.

Contact us for more information on indoor mold inspection, or if you suspect mold or mildew in your property. McKee Environmental experts are available to provide advice and guidance for obtaining and maintaining healthy indoor environments at home or on commercial properties.