Why Dominant Indoor Actinobacteria Come from Human Carriage

by Catherine Fruechtenicht

Actinobacteria have emerged as a critical yet historically overlooked component of microbial contamination in water-damaged indoor environments. While mold has long dominated public and scientific attention, recent analyses suggest that actinobacteria—particularly Actinomyces and related actinomycetes—may play an equally significant role in building‑related illness. Their presence, behavior, and health implications warrant careful examination, particularly as new research reframes our understanding of “moldy” buildings and the individuals exposed to them.

Instead of moving the focus strictly to actinobacteria, as the “cause” of the “sick building”, it is, I would argue, MORE essential to look at the problem of the sick people living and breathing in the moldy building as the dominant source of the elevated actinobacteria and its persistence even after the mold is found and appropriately remediated. This, in turn, will explain how and why small-particle cleaning and routine deep cleaning post-remediation are among the most essential components in both restoring the home’s health and protecting the health of its occupants. The importance of this cleaning cannot be overstated. In addition to cleaning, I aim to demonstrate that maintaining hygiene is equally vital to reducing actinobacteria levels and proliferation in the home and among occupants during healing.

Actinobacteria in Water-Damaged Buildings: When the Humans Become the Source

In many water-damaged buildings, the dominant actinobacteria are from human carriage rather than the building itself, as documented in the Surviving Mold Actinobacteria Update and supported by broader indoor microbiome research showing that actinobacteria are common human microbes.

Actinobacteria are a diverse phylum of high-G + C‑, Gram-positive bacteria found in soil, dust, and the human microbiome. In the context of water-damaged buildings, they have traditionally been assumed to originate from environmental reservoirs such as wetted building materials or soil intrusion. However, emerging analyses challenge this assumption. Increasing evidence indicates that a substantial proportion of actinobacteria detected in “moldy” buildings originates from the occupants rather than the building itself. This insight reframes how microbial contamination develops and how exposure affects human health. This analysis of actinobacteria and the degree to which it causes the indoor microbiome upset makes more sense to me as someone who has experienced mold toxicity and as someone who lived through a massive remediation of our home, because our actinobacteria testing did not rise to problematic levels inside our house until we—the humans inhabiting it—were quite sick and mold levels were already high, which was the source of our sickness and immune system chaos in the first place.

Actinobacteria – The Nitty Gritty

Actinobacteria are well-established members of the normal human microbiota, colonizing skin, mucosal surfaces, and respiratory tissues. Humans continuously shed microbial material through skin cells, hair, clothing fibers, and respiratory aerosols. In any occupied indoor environment, this shedding contributes significantly to the microbial composition of settled dust. When moisture intrusion occurs, these human-derived organisms can proliferate, creating a microbial profile that reflects the occupants more than the building materials.

The Surviving Mold Actinobacteria Update provides key evidence supporting this interpretation. The report describes a Dominance Index that distinguishes actinobacteria associated with human carriage from those associated with soil habitats. In many water-damaged buildings, dust samples show a strong dominance of human‑habitat actinobacteria, indicating that the primary source of these bacteria is the occupants themselves rather than the building or outdoor soil. This finding challenges the assumption that actinobacteria in damp buildings arise mainly from environmental reservoirs.

Broader research on the indoor microbiome aligns with this conclusion. Actinobacteria are consistently found in indoor dust, even in buildings without visible mold contamination. Rintala’s review highlights their prevalence in indoor environments and their association with respiratory health effects. Because humans are major contributors to indoor microbial communities, their microbiota becomes a significant source of actinobacterial material. Moisture does not create these organisms; instead, it amplifies what humans have already introduced, and sick, especially mold-sick humans, can create A LOT of indoor actinobacteria.

Why Someone With Mold-Related Illness May Shed More Actinobacteria

When people talk about “mold toxicity,” they’re usually referring to chronic exposure to a water‑damaged building and the inflammatory problems that can follow. In that context, several well-understood biological processes can cause a person to shed more actinobacteria into their environment.

Here are the mechanisms that most researchers consider most relevant.

Chronic inflammation increases skin and mucosal shedding

Actinobacteria are abundant on:

Skin
Inside the nasal passages
In the Throat
Around the Mouth
Inside the Gut

When someone is inflamed or unwell, the body often:

Turns over skin cells faster
Produces more mucus
Sheds more epithelial cells
Exhales more microbial aerosols

Because actinobacteria live on these surfaces, more shedding = more actinobacteria released into the environment where those people live.

Immune dysregulation changes the microbiome

Chronic inflammatory states—whether triggered by mold exposure or something else—can disrupt the normal microbial balance.

When the immune system is dysregulated:

Some actinobacteria expand because they face less competition
Others become more active or more easily aerosolized
Commensal species can shift toward opportunistic behavior

This doesn’t mean the person is “producing” new bacteria. This indicates that their existing microbiome has become unbalanced, and actinobacteria often flourish under such conditions.

Respiratory symptoms increase aerosolization

People exposed to moldy buildings often report:

Coughing
Post-nasal drip
Sinus irritation
Mouth breathing
Sneezing

Each of these dramatically increases the number of bacteria expelled into the air. Actinobacteria are common in the upper airway; therefore, respiratory irritation is associated with increased airborne actinobacteria.

This perspective also reframes the metaphor of “becoming the moldy building.” The microbial ecology of a water-damaged structure becomes an amplified reflection of the humans who inhabit it. The building does not merely contaminate the occupants; the occupants help create the microbial environment that ultimately affects their health.

When Actinobacteria Cause Problems Inside a Home

Health implications arise when human-derived actinobacteria proliferate under damp conditions. NIOSH investigations have documented associations between actinomycetes and respiratory symptoms among occupants of water-damaged buildings, including reduced lung function and asthma-like symptoms. If humans are the primary source of these organisms, exposure becomes a feedback loop: occupants seed the environment, moisture amplifies the microorganisms, and occupants re-inhale higher concentrations of their own shed microbiota. It is a vicious circle that grows if 1) the moisture problem is not solved, 2) the mold source is not removed, 3) the home is not thoroughly cleaned post-remediation, and 4) the humans and pets that live there do not practice proper hygiene and cleaning. All of these things must go hand in hand if the home and everyone in it are to recover. It may sound difficult, but I did it (on a budget, I might add), and it is not hard. It just takes consistency, the right products, and a willingness to do it right.

What Can You Do?

Recognizing humans as a significant source of actinobacteria in water-damaged buildings has practical implications. Remediation must focus not only on removing visible mold but also on controlling moisture, reducing dust reservoirs, and improving ventilation. Environmental assessments should consider the microbial signatures of occupants, as their microbiota may be the dominant contributors to the indoor actinobacterial load. In this light, I recommend that mold testing be conducted by professionals such as Building Biologists, Mold Dogs, or mold-literate IEPs, and that tests such as the ERMI or, preferably, the EMMA Combo be used to determine whether remediation is needed and to what extent. These professionals can also assist with a remediation plan and strategy.

If you are sick, you need to identify the source or sources of moisture, remediate it, and strategically and safely remove the mold and moldy/wet building materials. Then, you need to clean the small particles and perform clearance testing until your levels are acceptable. After that, you need to employ weekly, targeted cleaning methods using products such as EC3 products from Micro Balance that effectively reduce mold and mycotoxin levels in the home. I have used these products even before officially working with the company and have continued to use them because they work and because no one in our home reacts to them no matter how much or how often I clean. I never recommend using HOCL or other “kill” products, because a “hospital” clean is not healthy for your home. A restored, healthy indoor microbiome with normal ecology is most health-affirming for the body. (Note: I am only writing about what I do. I do not intend to create controversy with this statement, but HOCl is a highly unstable molecule. Using it on surfaces and as a constant topical cleaner increases resistance and often provides little to no decontamination if the product is no longer in an active form, which, unless you are utilizing a HOCL generator, it likely is. It can also create a “mold-like” scent on many surfaces, which, for me personally, triggers fear, PTSD, and looping behaviors around cleaning. No one in the nontoxic cleaning community ever talks about the problematic nature of these products, but I am willing to.) This cleaning should include HEPA vacuuming with a bagged HEPA vacuum and, if necessary, fogging with a ULV fogger (NOT a thermal fogger or biocide which can both make your environment much more toxic) over the course of many months. Finally, you will need to use new products, such as the EC3 Head-to-Toe Cleanser to wash your body and hair, and the Nasal Rinse Bundle to rinse your nasal passages daily, to further reduce the respiratory load of actinobacteria in your environment.

In our situation, I was fortunate to discuss our home with Dr. Jack Thrasher, a renowned toxicologist and mycotoxin researcher. I will never forget his advice to me as we began our remediation process: “This is the easy part. It is your willingness to clean and to continue cleaning after the remediation that will reclaim this home. You will need to clean thoroughly for at least 6 months to return this home to homeostasis. You cannot use toxic products, though. You need natural products and lots of elbow grease.” He was so right. We did the remediation and kept cleaning. I started to feel better slowly. Every time made a difference, and I did not give up. I hope this information helps you and encourages you to keep fighting and cleaning. Your home depends on it, and your health is worth the hassle.