I can’t count the number of times I’ve heard this question throughout my career. The real problem in answering it is that there is no consensus of what either of these terms actually mean. With the exception of Powdery Mildew (a plant parasite), neither are scientific terms, and both are largely defined by cultural assumptions of their meanings.
According to the American Industrial Hygiene Association, “The term “mold” is a colloquial term for a group of filamentous fungi that are common on food or wet materials.” (AIHA Publication Facts About Mold) Varying definitions for mildew can be found from innumerable sources, one being The National Institute for Occupational Safety and Health (NIOSH) which seems to make little to no differentiation between the two, stating, “Molds are sometimes referred to as mildew.” (NIOSH Publication Indoor Environmental Quality). Furthermore, the U.S. Department of Housing and Urban Development publication Healthy Home Issues: Moldstates, “The terms “mold” and “mildew” are non-technical names commonly used to refer to any fungus that is growing in the indoor environment (Burge and Otten, 1999).”
With all the aforementioned information in mind, it really doesn’t matter whether what one is observing inside a structure is what some would consider mold or others may consider mildew. Both terms are indicative of the presence of some type of fungi, and most fungi reproduce asexually through the process of sporulation (the process by which the hyphea or epiphytic mycelium form and release microscopic spores into the air). Therefore, the presence of molds or mildews are likely to cause irritation of allergy and asthma symptoms in susceptible individuals.
Often, well-meaning but misinformed individuals may suggest sampling of the suspected biological growth to determine if it is mold or mildew and, if it is indeed mold, to determine what type of mold is present. The proposed purpose is often to determine if the particular mold is potentially toxin producing or a potential health concern.
However, according to the National Institute of Environmental Health Sciences, “It is well established that allergic reactions to molds are the most commonly experienced health effects in humans.” (National Toxicology Program Fact Sheet). The ability or inability of a particular type of mold to produce toxins is not necessarily indicative of whether it is a health concern. The potential for allergic reactions from exposure to different genera and species of molds by occupants are akin to the potential for allergic reactions to different types of foods or other common substances. Allergy is more dependent upon the biological changes experienced during exposure by the occupant to the molds rather than the inherent characteristics of the specific types of mold(s) present.
Therefore, in my opinion, it is inconsequential whether a mold or mildew is toxin producing or even what type of molds are present in most cases as determining this fails to address the most common issue which people experience with mold—allergies. This is likely why the American Industrial Hygiene Association discourages testing of visible mold contamination without a specified purpose, stating, “If you know you have a mold problem, it is more important to spend time and resources solving the moisture problem and getting rid of the mold than to spend it on sampling.” (AIHA Publication Facts About Mold)
In summary, there are many assumptions surrounding the term mold and the term mildew that may not necessarily be entirely accurate. It is important that the public has accurate information to make a well-informed decision based on facts rather than common misconceptions.
