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Indoor Vs. Outdoor Mold: Why Is Indoor Mold Problematic for Health?

by Catherine

What is the difference between indoor and outdoor mold? Mold is everywhere outside and does not seem to make me sick, but I worked in a water-damaged building and became almost bedridden. Can you please help me to understand why indoor mold is so problematic to our health, but outdoor mold only seems to cause “seasonal allergies”?

That was the reader question I received last week. It is such a valid, complex and wonderful question that I wanted to answer it here on the blog, so that everyone can (hopefully) learn a little about mold from my answer.

Now, I am not a Certified Industrial Hygienist, an Environmental Health Scientist or a Building Biologist, so my answer to this reader is not “reference book material” or may not be completely scientifically accurate, so please be aware of that fact on the front end. My answer and knowledge of mold comes from my experience and my 20 books of handwritten notes from our mold ordeal with answers to this and many other questions in layman’s terms as I lived our nightmare and consulted some of best licensed professionals on mold, mold remediation, and health care professionals in the business at that time.

That being said, here is my reply:

Thank you for your question. I may seem long-winded, but I want to explore a few angles to this answer.

One important reason indoor mold has become so problematic to our health has to do with our recent building history.  In the 70s and 80s when air-conditioning became widespread and we had an energy crisis, builders began sealing home HVAC systems.  While sealing an HVAC system makes it more “energy efficient,” it can also cause the negative effect of air being recirculated within a home, if the system and indoor air are not also properly filtered.  Sealed HVAC systems also can trap moisture, which raises indoor humidity to levels that are conducive for mold growth.

 In addition, the actual building materials that are used trap moisture and are loaded with nutrients that mold loves. In the outdoors, nature solves this problem itself, because, once decomposed, the mold’s food turns to soil, but inside, the mold can proliferate. Mold loves cellulose—paper, gypsum, cardboard, fiberglass, blown in insulation. Mold is 10 times more likely to grow on cellulose than wood.  Also the more recent use of antifungals and anitmicrobials used during the manufacturing of these building materials—to make them mold-free and durable—is actually water soluble. That means that if water penetrates them, it breaks down their barrier to molds and bacteria, rendering them ineffective. Then, only the mycotoxin-producing molds are the ones that are stimulated and encouraged to survive—the most dangerous to your health! The production of mycotoxins is actually the mold’s evolutionary defense mechanism against these added chemicals to protect the species.

Now to look at indoor/outdoor mold from a more biological perspective:

The first and one of the most interesting things, in my opinion, that I came to understand about outdoor mold spores is the size factor. Most commonly occurring outdoor molds have larger spore sizes than those that are found in water-damaged buildings. There are, of course, exceptions to every rule. For example, some large outdoor spores of Cladesporium, the most common outdoor mold, are up to 15-25 microns, whereas two molds commonly found in mold-contaminated buildings, penicillium and aspergillus, are typically 1 micron or smaller.

Spore size is extremely important, because bigger particles are easier for the body to guard against and get rid of. This, I am told, is why outdoor molds cause upper respiratory seasonal allergies. The body does not always completely absorb them, because ithas natural defenses to both capture and expel them on a more surface level. They can cause sneezing, watery eyes, congestion, phlegm, but, for the most part, the body can get rid of them and recover.

Mold spores that proliferate indoors are smaller and are found at larger concentrations. These spores get inhaled through the mouth or nose and are able to penetrate the body’s barriers—sinus, lung, blood, brain—much easier, but are also much harder for the body to expunge. Some scientists and doctors say that they are “sticky,” because they attach to mucous-lined areas and colonize in places inside the body. As a matter-of-fact, on the SinusitisWellness.com site, you can view actual photographs of mold colonies inside the human sinus. Scary stuff.

One building biologist we worked with really brought the point home for me by telling me that in order to actually see indoor mold growth with the naked eye, there had to exist at least 144 billion colonies per square foot. Our issue was in our HVAC system, so the fact that we could see black mold inside the units and those units were blowing air throughout our entire home and mold spores with that air almost made me nauseous to think about. I will say for the sake of taking another opportunity to preach about it, though, is this is why containment, HEPA vacuums and air scrubbers are so important during remediation. Any disturbance of these colonies causes a colossal release of mold spores and mycotoxins that must be dealt with.

Another difference between outdoor and indoor molds is their temperature preference. The most common outdoor molds typically like colder temperatures. Looking back at Cladesporium as our example, it can grow at temperatures down to freezing. Molds that are typically found to cause health issues indoors are ones that like warmer temperatures. This is exactly why indoor molds are those also associated with the presence of pathogens, bacteria and virus. When found outdoors, these molds are on rotting detritus or compost. The higher temperatures we enjoy indoors draw these dangerous molds there too.

 So, in conclusion, when you have modern building materials, water intrusion, and warmer temperatures indoors, you cultivate high concentrations of the most dangerous molds to human health. Couple that with the fact that most Americans spend 93% of their time indoors, our diets and lifestyles are compromising our immune and digestive systems, and you have a recipe for disaster.  In my opinion, it is no wonder that so many people are being made sick by the poor indoor air quality and mold.

Well, I sure hope I shed some light on that reader’s question. Do you ever wonder why mold is “suddenly” making so many people sick? Is it because we are now aware of it, or because we are more susceptible to it making us sick? What do you think?

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Matt - 2:03 am

Hello and thank you so much for you taking the time to write such a well written article – I must have scoured close to a hundred pages but this one has stood out.

I was originally a cleaner and was exposed to odd bits of mold over time on windowsills and silicone failures. Eventually when my rental property needed mold remediation due to poor fan systems in the bathroom, I became more involved after I read lots to ensure safety before tackling. I’ve since done a variety of larger mold jobs but have now got an outdoor mildew one and wasn’t sure how this compared to indoor mold. This article is fantastic and sums up all the science I’ve read with logical points and has given me more confidence going it to it – full of excellent points.

I encourage everyone to read as much as possible before undertaking any similar job also and to treat it as seriously as possible but try not stress about it at the same time. If it is an environment you live and from my experience of my rental property it is pyschologically better for you to move or to have professionals completely take care of it so you are not reminded. Mold can be remediated effectively but the reminder can still linger and carry a mental burden.

I hope for your continued recovery and thanks for sharing your story.
All the best, Matt

Catherine - 4:06 pm

Thank you so much for writing, Matt. I appreciate you taking the time. I am so happy the article was helpful.


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