What is toxic mold? A clear guide to mycotoxins, health risks, and what to do next"

You've probably heard the term toxic mold and wondered whether it actually applies to your situation. Here's the short version: certain molds produce toxic chemicals called mycotoxins. The mold itself isn't technically "toxic." But the chemicals it releases are, and they can harm the immune system. Researchers have identified over 300 mycotoxins to date, and the World Health Organization recognizes indoor dampness and mould as a cause of respiratory symptoms, allergies, asthma, and immune system disruption. The most studied mycotoxin-producing species include Stachybotrys chartarum, Aspergillus versicolor, and Chaetomium globosum.

What is toxic mold?

Most online advice about "toxic mold" treats mold as a binary: dangerous or harmless. The reality is more specific. And more useful.

Certain mold species produce mycotoxins when they grow on damp materials. These are secondary metabolites, which means the mold doesn't need them to survive. It creates them under specific conditions, particularly when moisture levels are high. According to Nielsen (2003), mold can start growing at a water activity near 0.8, but it doesn't produce significant quantities of mycotoxins until that number hits 0.95 or higher. In plain English: mold needs seriously wet conditions to become truly toxic.

Not all mold is dangerous. Thousands of species exist, and most are harmless. The ones most linked to health problems in water-damaged buildings include Stachybotrys chartarum (the species people call "black mold"), Aspergillus versicolor, Chaetomium globosum, and certain Penicillium species. These thrive on common building materials like drywall paper and ceiling tiles once water damage has set in.

It's more than just spores

When people think of mold, they picture fuzzy patches on a wall. But in a water-damaged building, you're dealing with much more than visible growth.

Research shows that water-damaged buildings release a complex mix of mold fragments, bacteria, mycotoxins, and volatile organic compounds (VOCs). Microscopic mold fragments can be hundreds of times more numerous than whole spores in indoor air, and each fragment can trigger an immune response. The WHO calls microbial pollution a key element of indoor air pollution, caused by hundreds of species of bacteria and fungi growing indoors when sufficient moisture is available.

So "toxic mold" isn't just about one species or one type of particle. It's about the entire inflammatory environment a water-damaged building creates.

Why it matters

How toxic mold affects your health

The health effects of mold exposure go well beyond sneezing and congestion.

A systematic review and meta-analysis by Fisk et al. found that residential dampness and mold correlate with a 30-50% increase in respiratory and asthma-related health outcomes. That number only tells part of the story.

For an estimated 24% of the population, mold exposure triggers something deeper than allergic reactions. These individuals carry a variation in their HLA-DR gene that prevents their immune system from properly recognizing and clearing biotoxins. When that clearance fails, toxins recirculate. Inflammation doesn't shut off. The result is a condition researchers and healthcare providers call Chronic Inflammatory Response Syndrome (CIRS), a multi-system, multi-symptom illness that can affect most areas of the body simultaneously — including the brain, hormones, and immune function.

The neurological side is especially worth understanding. Research published in Frontiers in Pharmacology (2025) links mycotoxin exposure to neuropsychiatric symptoms including cognitive impairment and mood disorders. A separate review in the Journal of Integrative Neuroscience found that mold and mycotoxin exposure can damage the central nervous system through neuroinflammation and disruption of the blood-brain barrier.

One person on Reddit described what this feels like day-to-day:

"For the last 5 years, especially during the last 2, I have noticed increased anxiety, depression, mood swings, sweating, racing thoughts and honestly pure mental anguish. Also, joint pains/stiffness, aggression, headaches, fatigue, muscle weakness, irregular periods, heart palpitations, temperature regulation issues... I genuinely felt a sense of impending doom every second I was in that house." (source)

That kind of multi-system symptom pattern is consistent with what researchers describe in mold-related illness. It's not random. And it's not "all in your head."

Not just an allergy

This is the distinction most doctors miss.

Mold allergy is an IgE-mediated immune response. Your body overreacts to mold spores, and you get sneezing, congestion, and watery eyes. Leave the exposure, and symptoms typically improve.

Mycotoxin-related illness works differently. It involves both allergic and non-allergic mechanisms, including activation of the innate immune system. In genetically susceptible people, this triggers chronic systemic inflammation that can persist even after the person leaves the moldy environment. It's also possible to have both — a mold allergy alongside the chronic innate immune activation associated with mycotoxin-related illness.

That's why someone might move out of a water-damaged apartment and still feel terrible months later. The toxins are trapped. The inflammation has become self-perpetuating. Healthcare providers refer to this chronic inflammatory response as CIRS, and it requires more than just removing the exposure. If you're interested in understanding the full treatment approach, MoldCo's CIRS treatment guide walks through the evidence-based protocol.

The 1-in-4 genetic factor

Why does one person in an office get seriously ill from mold while everyone else feels fine?

The answer often comes down to genetics. An estimated 1 in 4 people carry HLA-DR gene variations that make them poor eliminators of mycotoxins. (That estimate is based on HLA typing frequencies observed in case studies; larger population studies are still needed.) Their immune system can't properly "tag" these toxins for removal, so the toxins keep circulating and inflammation never receives the signal to stop.

This explains the frustrating pattern: everyone in the building breathes the same air, but only one or two people get seriously ill. In a household the picture can look different — because family members share genetics, a susceptible parent may pass the HLA variation to one or more children, so multiple family members often get sick while others stay healthy. In a workplace with greater genetic diversity, the 1-in-4 pattern is more visible. Either way, it's not weakness. It's genetics.

And here's what makes that knowledge useful: if you know about this genetic factor, you know what to test for. Understanding your susceptibility changes the entire approach to getting answers. MoldCo's HLA gene test can identify whether you carry the susceptibility variation.

Wondering whether your environment could be affecting your health? MoldCo's free symptom questionnaire can help you find out.

What to do if you suspect toxic mold

If you suspect toxic mold in your home or workplace, here are three clear steps.

1. Test your environment with DNA-based dust testing.

Skip air samples. They capture a snapshot of whatever happens to be floating at that moment, and results vary based on airflow, HVAC activity, and time of day.

Instead, look for a DNA-based dust test like the HERTSMI-2 (Health Effects Roster of Type-Specific Formers of Mycotoxins and Inflammagens, 2nd version). It analyzes the DNA of the 5 mold species most linked to water-damaged buildings. You collect a dust sample, mail it to a lab, and get a score that tells you whether your home is likely safe. MoldCo's HERTSMI-2 home test kit ($199) is one option that ships to your door with instructions and prepaid return shipping. Results come back in 1-2 weeks. You can learn more about interpreting ERMI and HERTSMI-2 scores in our detailed guide.

2. Check your body's inflammatory response with blood biomarker panels.

Environmental testing tells you what's in your home. Blood testing tells you what it's doing to your body. Ask your healthcare provider about biomarker panels that measure inflammation markers like MMP-9, TGF-beta1, and MSH, which are the markers most linked to mold-related illness. If your current provider isn't familiar with these markers, that itself is useful information about whether they can help you with this specific condition. MoldCo's Starter Health Panel ($99) measures these 3 core markers through a standard LabCorp blood draw.

3. Work with a provider trained in evidence-based biotoxin protocols.

If your environmental and blood tests suggest mold-related illness, the next step is working with a healthcare provider who understands the condition. Not all providers are trained in this area, and a general practitioner may not recognize the pattern. When evaluating a provider, ask specific questions: Do they follow a peer-reviewed biotoxin protocol? Can they interpret CIRS biomarkers like MMP-9 and TGF-beta1? Do they understand the role of HLA-DR genetic testing? A provider who can answer these questions confidently is more likely to give you an accurate diagnosis and effective treatment plan. MoldCo connects you with providers trained in the Shoemaker Protocol, the most peer-reviewed framework for treating this condition.

FAQ

Is all mold toxic?

No. Of the hundreds of thousands of mold species, only a small fraction produce mycotoxins. Common indoor molds like Cladosporium are generally not considered toxic. The species most linked to health problems in water-damaged buildings are Stachybotrys chartarum, Aspergillus versicolor, Chaetomium globosum, and certain Penicillium species.

What does toxic mold look like?

There's no single appearance. While Stachybotrys chartarum ("black mold") is dark greenish-black and often slimy, other species like Aspergillus, Alternaria, and Chaetomium can also appear black. And some mycotoxin-producing molds appear white, green, or yellow. You can't determine whether a mold produces mycotoxins just by looking at it. DNA-based testing like HERTSMI-2 is the reliable way to identify problematic species.

Can toxic mold make you sick even if you can't see it?

Yes. Mold growing inside walls, under flooring, or in HVAC systems releases microscopic fragments and mycotoxins into the air, which then mix and settle with house dust, all without any visible growth. DNA-based dust testing can detect these species even when the growth itself is concealed.

I smell musty odors — does that mean I have toxic mold?

Not necessarily, but it's worth investigating. A musty or earthy smell is often a byproduct of active microbial growth, which may include mold. It doesn't confirm the presence of mycotoxin-producing species on its own, but it's a strong signal that something is growing. A DNA-based dust test like HERTSMI-2 can determine whether the mold species linked to health problems are present in your environment.

How is toxic mold illness different from a mold allergy?

Mold allergy is an IgE-mediated immune response: sneezing, watery eyes, congestion. It typically resolves when you leave the exposure. Toxic mold related illness involves chronic systemic inflammation triggered by mycotoxins, affecting multiple parts of the body, including the brain, hormones, and immune system. In genetically susceptible people (an estimated 24%), symptoms can persist even after leaving the moldy environment because the biotoxins remain trapped in the body.

What tests can confirm toxic mold exposure?

Two categories. First, environmental testing using DNA-based dust tests like HERTSMI-2 or ERMI to identify problematic mold species in your home. Second, blood biomarker panels measuring inflammatory markers (MMP-9, TGF-beta1, MSH) to assess your body's response to exposure. You can read more in our mold illness testing guide. MoldCo doesn't recommend urine mycotoxin tests because they lack validated reference ranges and can't distinguish dietary exposure from inhaled exposure.

Understanding what toxic mold is marks the beginning of getting answers. If you're experiencing chronic symptoms and suspect your environment, there are clear next steps. MoldCo can help you start your evaluation and move from uncertainty to a plan.

Any health-related claims made on this site have not been evaluated by the Food and Drug Administration (FDA). The information provided on this site is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. MoldCo assumes no responsibility or liability for any errors or omissions in the content of the references, nor for any actions taken in reliance thereon.