The Indoor Microbiome and Toxic Mold: Is Your House Making You Sick?

man-tiredThe ancestral health movement is built on the premise that there’s a mismatch between the human genome and our current milieu. Humans evolved to live as hunter-gatherers in natural environments, not as office workers in large, polluted cities. This mismatch theory underlies pretty much everything we talk about in the evolutionary health community, and a lot of focus is placed on how we can adjust our lifestyle – including sleep, sun exposure, diet, exercise, and so forth – so it more closely resembles that of our physically fit ancestors. More recently, it has become clear that it’s not just the rapid and profound changes in the environment we can see with our naked eyes that we should take into consideration when we set out to improve our health, but also recent changes in the invisible, microbial world in which we live.

We’re connected to the rest of the environment through the trillions of microorganisms that live in and on our bodies

Over the last decade, we’ve learned a tremendous amount about the microorganisms that live in and on us, and it has become abundantly clear that the human body shouldn’t be viewed as an independent organism that’s primarily made up of eukaryotic cells, but rather as a superorganism that’s composed of trillions of microorganisms that co-exist side by side with a human host. We aren’t separated from the rest of the environment, but rather a part of it – connected to everything else through the microbial cloud that we bring with us everywhere we go.

In the ancestral health community, the microbiome has been given a lot of attention lately. Much focus has been placed on the impact diet, pharmaceutical use, hygiene, exercise, and many other components of our lifestyle have on the composition of our microbiome, and on pretty much every paleo blog out there you can find articles promoting the consumption of fermented foods, resistant starches, and other pro- and prebiotics.

I too have focused a lot on these things, and for good reason. The foods we eat, the drugs we take, the exercise program we adhere to, and so forth can have a profound impact on the composition of our microbiome, and consequently, our general health. However, what has become increasingly clear to me is that these things, which can all be included under the “lifestyle” umbrella, are only one part of a larger puzzle. Another big part of the puzzle is the microbes that surround us: the organisms in the air we breathe, the bacteria we pick up from animals and other humans, and the collection of microorganisms that live in our house.

The recent loss of microbial diversity from the human microbiota has largely been attributed to an overuse of broad-spectrum antibiotics and other drugs, widespread consumption of highly processed foods, and low intakes of dietary fiber. While I certainly agree that these things play a major role and can largely account for why so many people in industrialized nations walk around with a degraded microbiota, I think the rapid and dramatic change to the microbial systems in our environment may in some instances be even more important.

It’s not just the microbes that live in and on our bodies that we have learned a lot more about lately, but also the microbes that surround us. Leaps of progress have been made in terms of mapping the microbial ecosystems found in our homes and the buildings we work, and also in the rest of the world around us. Gaining a better understanding of this larger microbial cloud is important for a number of reasons, one of which being that we are (as mentioned) a part of our environment. We’re continually exposed to microorganisms associated with animals, other humans, the air we breathe, and plants; an exposure that contributes to shaping our own microbiome.

I think for a lot of people, the idea that the human body is a superorganism that’s connected to and shaped, in part, by the larger microbial world, sounds strange and maybe a bit “crazy”. It did to me at first too. I just didn’t believe that the microbes in my environment had much of an effect on my health and well-being or that indoor air quality or toxic mold were anything to really be concerned about. When compared to diet, physical activity, etc., these things were certainly not worth paying much attention to, I thought. I now know better.

There have been dramatic changes to humans’ microbial environment over the last 10.000 years

Needless to say, the world around us has changed dramatically in a very short period of time. For most of the evolutionary history of our genus (Homo), a hunter-gatherer lifestyle was the norm for our species, and grocery stores, iPhones, computers, junk food, and all of the other things we now take for granted were obviously nowhere to be found.

What is sometimes forgotten is that it’s just not the world we can see with our naked eyes that has changed a lot since the Paleolithic era some 10.000 years ago, but also the invisible, microbial world around us (1, 2). If you’ve been following the research in this area, you’ve undoubtedly come across the Old Friends Hypothesis, which basically states that the increasing prevalence of chronic inflammatory disorders in industrialized nations can partly be attributed to inadequate exposure to microorganisms that co-evolved with our ancestors for millions of years.

By washing, cooking, and otherwise removing bacteria from the food we eat, moving into large apartment complexes, cleaning our clothes, bodies, and homes with various cleaning agents, and otherwise disconnecting ourselves from nature, we’ve lost touch with some old microbial friends that we depend on for a well-functioning immune system. There’s solid data to show that inadequate exposure to microorganisms from the natural environment can contribute to gut dysfunction, immune dysregulation, and chronic inflammation (1, 2, 3).

This abstract from a paper by one of the leading researchers in this field, Dr. Graham Rook, summarizes the core principles of the old friends hypothesis:

There is much to be gained from examining human diseases within the expanding framework of Darwinian medicine. This is particularly true of those conditions that change in frequency as populations develop from the human “environment of evolutionary adaptedness” to the living conditions of the rich industrialized countries. This development entails major changes in lifestyle, leading to reductions in contact with environmental microorganisms and helminths that have evolved a physiologic role as drivers of immunoregulatory circuits. It is suggested that a deficit in immunoregulation in rich countries is contributing not only to increases in the incidence of allergic disorders but also to increases in other chronic inflammatory conditions that are exacerbated by a failure to terminate inappropriate inflammatory reponses. These include autoimmunity, neuroinflammatory disorders, atherosclerosis, depression associated with raised inflammatory cytokines, and some cancers. (1)

When we made the transition from living in “nature” to living in closed buildings, we unknowingly ventured into a very different microbial world…

The microbiome of the built environment

It’s not just your body that harbours a complex microbiome; your house has a microbiome too. Recent research suggests that the microbiome of a building is shaped, in large part, by the microbiota of its residents (4). Every time you enter and spend time in a building, you set a microbial stamp on the surroundings: You leave microbes on doorknobs and furniture and breathe out a mist of bacteria.

What is important to remember it that this transfer can occur the other way as well: You are exposed to microorganisms that are a part of the microbiota of the built environment. Most of these microbes are completely harmless, and some, in particular those that were initially shed by other humans/animals, can even benefit your health in the sense that they may find an available niche in your body and become a part of your microbial rainforest. For example, some bugs may make their way pass the gastric barrier in the tube that runs through your body, set up shop in your colon, and assist you in the digestion of food.

Recent research suggests that a house with a more bacteria-rich environment is a healthier one. For example, a recent study that profiled 104 infants inside their homes found that the babies exposed to house dust with the greatest bacterial diversity before age 1 were the least likely to have asthma symptoms as 3-year-olds (5).

Unfortunately, not all of the microbes you encounter in the buildings you enter are so friendly. Some can actually do you a lot of harm. It’s not just inadequate exposure to “beneficial” microbes that is a major problem for contemporary humans, but also too much exposure to microorganisms that we rarely or ever encountered throughout most of our evolution. We all know that infectious diseases are a major public health problem in many developing countries. What is less well-known is that exposure to harmful organisms found in the buildings we live in and in the air we breathe can cause of a wide range of chronic health conditions that are typically viewed as non-infectious.

It’s well established that we humans can develop an unhealthy, degraded microbiota if we lead an unhealthy lifestyle or are not exposed to biodiversity from the natural environment. The same can happen to buildings. Inadequate ventilation or water damage can set the stage for the development of an imbalanced home microbiota characterized by low biodiversity and overgrowth of fungal pathogens. That’s why it’s so important to maintain proper humidity levels in our homes, stay away from fabrics, paint, detergents, etc. that release harmful gasses, and make sure we bring nature into our homes, such as by opening the windows or decorating with some houseplants.

One of the key terms to keep in mind in this discussion is the Sick Building Syndrome, which is defined as follows:

The Sick Building Syndrome (SBS) refers to a number of ailments that occur as a result of exposure to harmful chemical toxins at a home or work building.

The main conditions for experiencing Sick Building Syndrome are spending long periods of time in well-sealed, poorly ventilated buildings that contain indoor air toxins.

Some of the toxins that may be present in “sick” buildings include synthetic fibers in furniture and often formaldehyde used in manufacturing, dust mites, mold and mildew, cigarette smoke, VOCs, carpet and gasses released from fabric, to name only a few. (6)

Mold illness

There’s convincing research data showing that “sick buildings”, particularly those contaminated with toxic mold, can wreak havoc on your health and well-being (7, 8, 9, 10, 11, 12). As you can see from the quote above, the list of toxic compounds in modern buildings that can make you sick extend far beyond those produced by microorganisms. That said, an overgrowth of toxic mold and/or other microorganisms may be especially problematic.

When compared to the dense microbial ecosystem found deep in your gut, the built environment is a relatively sterile place. This is because there’s generally not a lot of water or nutrients present, which microorganisms need to survive and grow. However, this can quickly change, for example if the building is subjected to water damage.

Some experts have suggested that as many as 50% of buildings could have a toxic mold problem (7). Even though you can’t necessarily see or smell the problem doesn’t mean it’s not there, as mold can lurk behind walls and behind your ceiling. Humans are very good at building things, but unfortunately we often forget that nature has its own mechanisms for breaking down and recycling everything around us.

For those with a compromised immune system, unhealthy microbiota (certain beneficial bacteria can help the body detoxify mycotoxins), and/or a genotype that make them particularly susceptible to biotoxins, the mycotoxins produced by mold are particularly troublesome and may cause a long list of health problems, including structural brain injury, chronic fatigue, muscle weakness, brain fog, gut dysfunction, and low libido (7, 8, 9, 10, 11, 12). Individuals who are exposed to molds in their house, workplace, etc. often also develop a sensitivity to mycotoxins found in foods such as cheese and wine.

One of the reasons I’m so interested in this topic is that I long lived in a “sick” house. As my immune system and microbiome were already somewhat compromised when I moved in, I was particularly susceptible to the toxins produced by the organisms (e.g., mold) in my home environment.

If your home is making you sick, you can eat an extremely healthy diet, sleep 8 hours every night, and otherwise do everything “right”, it won’t matter, you’re not going to overcome your chronic health problems. Just imagine that you’re breathing in mold toxins day in and day out for years. It will make you sick, fatigued, and unhappy, especially if you’re among the unfortunate group of people who are particularly susceptible to the mycotoxins produced by these critters.

Mold illness – and other disorders caused by exposure to pathogenic microorganisms in your home, office, etc. and/or the compounds they produce – is a hidden epidemic. If you feel like you’ve tried “everything”, including diet and lifestyle changes and restoration of microbiome diversity, to overcome your chronic health problems, but without success, you should definitely consider the possibility that microorganisms in your home, school, or work place could be making you  ill.


When discussing how to achieve a healthy microbiota and fit, well-functioning body, it’s not enough to talk about diet, fermented foods, prebiotics, antibiotics, and so forth; we should also consider how the microorganisms in our milieu impact us. We have to keep in mind that it’s not just the environment we can see with our naked eyes that has changed dramatically over the last 10.000 years, but also the microbial world in which we live. We evolved to live in nature, not in polluted cities, mold-contaminated homes, and poorly ventilated office buildings.

The gene-environment mismatch theory that underlies everything we talk about in the ancestral health community has to be expanded to not only include the environment we can see with our naked eyes, but also the invisible, microbial world.

Adopting an ancestral diet and lifestyle can do wonders for your health and well-being. That said, a lot of people find that their health problems persist even though they eat an extremely healthy diet, exercise on a regular basis, make sure they sleep 8 hours every night, and otherwise follow a very healthy lifestyle. For these people, the problem may very well be that they work or live in an environment that’s making them sick.

Some may find that non-intrusive measures, such as improving the ventilation of their home, getting a dog, bringing in some houseplants, and focusing on improving the health of their own microbiota, are all that is needed to build a better home microbiome. Others, particularly those who live in mold-contaminated buildings, may have to carry out mold remediation, or in some instances, move.

Now I want to hear from you: Do you pay any attention to the air quality and microbiome in your home? Have you – or someone you know – suffered from mold illness or other disorders associated with exposure to harmful microorganisms found in buildings?

Picture: Creative Commons picture by Henti Smith, Some rights reserved.


  1. MichelleP says:

    Thank you for this timely article and for bringing up a topic that is very relevant to me. I have been struggling with persistent mild fevers, rather constant sinus symptoms and weight gain for over a year and a half now, in addition to increased migraines. I added more houseplants to my home this winter to “bring the outdoors in” and keep the windows open as much as possible now. I’ve found that I feel best in “fresh air” – closed work environments and being cooped up all winter are tough for me. I remember as a child I couldn’t stand being in air conditioning …and when migraines strike me I feel the need for fresh air. I’ve been seeing an allergist now, and the interesting thing, while nothing new, is that to help you out – they give you shots of the things you’ve developed allergies to, to build up your resistance, like this article mentions. While more natural resistance build up would certainly be better, this is helping me for now. I try to get out in the woods or the garden as much as possible to live more naturally and improve my health.


  1. […] Your home has a microbiome too, which interacts with the one you carry with you. […]

  2. […] building can faciliate human health, a “sick” building that harbours an imbalanced microbiota can prove very hazardous. These effects are likely intensified in fitness centers, tanning salons, and other high-traffic […]

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