Some Basic Science about “Toxic Molds”

As many of my readers know not only am I a Professional Chemist but my original field of training included Biology and I am also a Registered Professional Biologist. If you read deeply into my resume you will discover a handful of certifications in mold investigation, assessment and remediation. At one point in my career almost half my time was spent in occupational health and safety testing. A couple years ago I wrote a post for the Huffington Post on Toxic Molds. Well on the weekend another “toxic mould” story came up in the news and I thought it was time to dust off that old story as it is once again relevant. The following is an update of that older mold post with some new references thrown in to address new learnings since the original post went out.

Let’s start with some mold basics. Mold (or mould if you prefer) is a non-scientific term for a varied group of fungi. Molds are literally everywhere. Molds existed on the planet long before humans and will likely exist long after the last humans are gone. Humans evolved in a world heavily populated by molds. What does this mean? Well that that we, as a species, have mostly evolved to live side-by-side with molds and to filter out their spores. That is lucky because virtually every breath we take, indoors or out, brings us in contact with mold spores.

In order to grow, mold only needs warmth, moisture and food (often called “the mold triangle“…the mold version of “the fire triangle“). Molds will thrive at temperatures over 5 degrees C (and under about 45 degrees C) and humidity over about 50 per cent. Molds have evolved to live on pretty much anything organic in nature so can grow almost anywhere. To make it worse some molds, like Penicillium or Cladosporium, can tolerate colder temperatures. This is why you tend to find these two molds growing on rotting veggies in your fridge or on the cold grout in your windows in winter time.

Molds absolutely love places that are wet and dusty. Most mold spores travel with the wind and deposit (stick) to places that are wet. That means that a well-designed air-conditioning system will filter out mold spores but that means the drop trays in your air conditioning system should be regularly cleaned because that is where the spores went. This will also mean that indoor air concentrations of mold spores should be lower than outdoor concentrations. If the opposite is true that means you likely have mold growing in your house that needs to be cleaned up. A big warning for people with flooded homes is that molds also grow pretty quickly. Within 24-48 hours of water intrusion mold will start growing, but on the bright side if you eliminate the water the molds will stop growing and dry up.

Given its ubiquity, you might wonder why one would spend so much time testing for mold? Well in the last 20 years an industry has built up around the idea of “toxic molds.” This industry preys on our fears and ignorance with mold being described as “black gold” in some circles. The reality is there is no such thing as “toxic mold.” There are some mold species that are “toxigenic,” that is they produce “mycotoxins.” Mycotoxins are metabolites produced by molds that are capable of harming other living organisms. Molds evolved these metabolites as part of their strategy to battle bacteria (and each other). Molds have spent the last billion years in an ongoing arms race against bacteria; their primary competition for living space and food. One of the most famous of these mycotoxins is a compound we call penicillin. Penicillin is produced by the mold Penicillium (one of the supposedly “toxic molds”) and is essentially harmless to non-allergic humans in the concentrations encountered in our day-to-day lives.

Certainly, there are people who can be deathly allergic to penicillin but even these people are exposed to the mold Penicillium on a daily basis with no ill-effect. As for allergies, approximately five per cent of individuals have some allergic airway response to elevated mold spore concentrations. That is, these people will get runny noses, itchy eyes and some wheezing when encountering high concentrations of mold spores. But let’s put that number into perspective, about 10 per cent of people are allergic to household pets.

Now I am not saying that mold is good for you as that is clearly not the case. Molds can and do produce spores that can act as human allergens. I can personally attest that at high enough concentrations mold spores can even induce headaches in people who are not directly allergic to mold. In addition I have to include this important proviso, individuals with illnesses that decrease their immune response (immunosuppressed individuals) should be especially careful to reduce their exposure to molds as molds can cause them serious harm. From a physical perspective, molds can also damage and weaken structures. But on a day-to-day basis, molds and mold spores are not a significant risk to a healthy individual.

As for ingesting mold. Issues with mold have been known since biblical times and everyone knows that you should not eat moldy food as it can make you sick. Moreover, it is not unheard of for horses to actually die from eating moldy hay. But for people to die from eating mold is incredibly rare.

The question you are probably asking is: if mold is so harmless why has this industry grown so big? It has been argued that our current generation of mold panic can be directly linked to U.S. Centers for Disease Control (CDC) studies in 1994 and 1997. At that time, the CDC incorrectly linked lung damage in children to the presence of Stachybotrys chartarum mold. In 2000, this linkage was retracted by the CDC. Unfortunately, by then the damage was done and a few very lucrative lawsuits later, the “toxic mold” industry was born.

So what is the truth about “toxic mold”? The fact that is understood now, that was not fully recognized in the 1990s, is that it is not the mold in your house that is making you sick. Rather it is living in conditions where mold can thrive that actually causes illnesses. As explained by the World Health Organization in 2009

Sufficient epidemiological evidence is available…to show that the occupants of damp or mouldy buildings, both houses and public buildings, are at increased risk of respiratory symptoms, respiratory infections and exacerbation of asthma. Some evidence suggests increased risks of allergic rhinitis and asthma. Although few intervention studies were available, their results show that remediation of dampness can reduce adverse health outcomes.

As for the mycotoxins, the research is also clear:

Current scientific evidence does not support the proposition that human health has been adversely affected by inhaled mycotoxins in home, school, or office environments (Hardin Kelman and Saxon, 2003)


Currently, there is no supportive evidence to imply that inhaling mold or mycotoxins in indoor environments is responsible for any serious health effects other than transient irritation and allergies in immunocompetent individuals (Fung and Clark, 2004).

So what are the take-home messages about “toxic molds”? It is not “toxic mold” that is making people sick, it is living in conditions conducive to mold growth that is bad for human health. If you are living in a house with high humidity and low temperatures then you are going to get sick irrespective of the presence or absence of “toxic molds”. As such mold can serve as a useful indicator. If you see mold growing in your house it is time to deal with the conditions that are likely to make you sick sometime in the future.

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4 Responses to Some Basic Science about “Toxic Molds”

  1. John says:

    Excellent post. You wanna do gluten next? 😉


  2. wryheat2 says:

    I seek your permission to repost your article on molds. Send answert


  3. wryheat2 says:

    May I have your permission to reblog your article on toxic molds on my site Please reply to


  4. Pingback: Some Basic Science about “Toxic Molds” | wryheat

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