Are Indoor Molds Really a Health Hazard?

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Indoor Molds on Walls

Indoor molds on the ceiling

It is always debated as to whether indoor molds can actually cause health problems.

To able to answer this question, it is important to understand what indoor molds are and what they can and cannot do. Molds belong to a group (kingdom) of organisms called Fungi. This kingdom consists of not just molds but also yeasts, and mushrooms.

For the purpose of this discussion we will restrict ourselves to molds. Based on their lifestyles, we can categorize molds into 3 groups. These are:

  1. Symbiotic molds- This group lives in other organisms, for example the roots of a plants, in a mutually beneficial relationship. Exposure to this group is unlikely and it has no know health effects to humans and therefore we will not discuss it further.
  2. Parasitic molds – This group live on or in, and obtain nutrients from a host organism. Some members of this group are specialized in causing plant diseases while others specialize in causing human and animals diseases. Therefore, the fungal infections humans and animals occasionally get are caused by this group.
  3. Saprophytic molds – This group utilize dead organic material as food, and commonly cause its decay. Saprophytic molds play an essential role in the break down of cellulose though this makes them a nuisance because they destroy even material that is valuable to us. Indoor molds are in this category. Some molds that cause disease in plants can adopt a saprophytic lifestyle if a living plant host is not available.

Can Indoor Molds Cause Infection?

Indoor molds, with the exception of a few members do not cause infection. The few that potentially can cause infections are said to be opportunistic since they are only capable of causing infections to individuals with weak immune system.

In short the molds that grow in our homes and offices are very highly unlikely to cause infections to healthy people. However, the spores of molds (regardless of the group), if inhaled, may cause allergic reactions to susceptible individuals. Indoor molds are therefore a health hazard if airborne spores are inhaled.

Whether one would react to these spores or not will depend on the amount of spores inhaled and individual’s sensitivity to mold. Currently there is no set exposure limit for mold. That means at this time we cannot talk of safe or unsafe amount of spores.

There are still people, including scientists, who don’t believe indoor molds are a health hazard. I personally react to certain strains of Penicillium when I am working with them in the lab. I get instant nose running if I accidentally inhale the spores of these indoor molds.

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Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at
Dr Jackson Kung'u

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