Understanding Mould Illness: What It Is, How It Happens, and Why It Matters
By Alex Manos, MSc — Functional Medicine Practitioner
Mould is one of those topics that tends to get dismissed — by doctors, by landlords, even by the people living with it. We're conditioned to think of mould as an aesthetic nuisance: a dark patch behind the bathroom tiles, a musty smell in the attic. Something unpleasant, yes, but surely not a serious health threat.
The science tells a very different story.
Over the past two decades, a substantial and growing body of peer-reviewed research has established that prolonged exposure to indoor mould and its toxic byproducts — known as mycotoxins — can drive a cascade of biological events inside the human body, affecting the respiratory system, immune function, the nervous system, and far beyond.
In the UK, this issue is particularly pressing. Estimates suggest that anywhere from 4% to 27% of UK homes have some form of damp or mould problem — with privately rented properties worst affected. In 2023, Awaab's Law was introduced following the tragic death of a toddler from prolonged mould exposure, bringing the issue into sharp political and public focus. Yet for those of us working at the clinical frontline of chronic illness, mould-related illness has been a reality we've been navigating with patients for years.
This article is intended as a science-grounded introduction to the topic. It is not a diagnostic guide, and reading it does not replace working with a qualified clinician. Rather, it's designed to help you understand what mould actually is, how it gets into buildings and into the body, what distinguishes a passing exposure from a meaningful health threat, and why this subject deserves to be taken seriously.
If you're looking for a truly comprehensive, structured exploration of mould illness — from biology through to assessment and clinical recovery — that's exactly what our online course Mould Mastery is designed for.
Important Disclaimer
This article is for educational purposes only and does not constitute medical or clinical advice. Mould-related illness is a complex and individualised condition. If you suspect mould exposure is affecting your health, please consult a mould literate healthcare practitioner.
What Is Mould, and Why Does It Grow Indoors?
Mould: An Evolutionary Survivor
Mould is a type of fungus — and fungi are among the most evolutionarily successful organisms on the planet. There are an estimated 100,000 or more species of mould, most of which perform vital ecological roles: decomposing organic matter, cycling nutrients through ecosystems, and supporting soil health. They are, in many ways, essential to life as we know it.
But their very success as decomposers is precisely what makes certain species problematic in buildings. Mould reproduces by releasing microscopic spores into the air — spores that are, for all practical purposes, omnipresent. They float through outdoor air, drift through open windows, and hitch rides on clothing, pets, and building materials. Like seeds, they lie dormant and essentially inert — until they encounter the one thing they need to activate: moisture.
According to the World Health Organisation's landmark 2009 Guidelines for Indoor Air Quality, as little as 80% relative humidity in the presence of organic material is sufficient to support mould growth. At 90% relative humidity or above, conditions are ideal for active proliferation. Once a spore lands on a damp surface — a wall, a wooden beam, a ceiling tile, a piece of fabric — and that surface stays wet for 24 to 48 hours, mould begins to multiply and sporulate.
Which Moulds Are We Talking About?
While there are thousands of mould species, a handful are responsible for the vast majority of indoor air quality problems. According to the US Centers for Disease Control and Prevention (CDC), the most commonly identified indoor moulds include:
Cladosporium — often found on fabrics, wood, and HVAC systems
Penicillium — frequently found in water-damaged materials; also the origin of penicillin
Alternaria — a common trigger of allergic responses, particularly in damp bathrooms and under leaky sinks
Aspergillus — a diverse genus with species ranging from benign to highly toxigenic
Stachybotrys chartarum — the notorious 'black mould', associated with severe water damage and highly potent mycotoxin production
Campbell (2016), writing in Alternative Therapies in Health and Medicine, notes that while there are more than 100,000 species of mould in existence, approximately two dozen are of recognised clinical relevance to human health. Of these, those capable of producing mycotoxins — toxic secondary metabolites — are of greatest concern in the context of indoor air quality.
How Does Mould Take Hold in a Building?
The WHO's 2009 guidelines identify several primary indicators of significant dampness and microbial growth in buildings:
Visible condensation on windows, walls, or ceilings
Visible mould growth, particularly dark or black discolouration
A persistent musty or earthy odour
Poorly maintained or contaminated air conditioning and ventilation systems
A history of water damage — whether from roof leaks, broken pipes, flooding, or rising damp
Pizzorno (2016), writing in Integrative Medicine, also identifies the complex internal chemistry of water-damaged buildings, noting that damp building materials can support the growth not only of moulds but also of bacteria, dust mites, and protozoa, while simultaneously releasing toxic chemicals from degrading materials themselves — including formaldehyde, phthalates, and a range of volatile organic compounds (VOCs).
In other words, a 'mouldy building' is rarely just a mould problem. It is a complex indoor ecosystem of biological and chemical agents — many of which, in sustained exposure, have meaningful physiological effects.
Mycotoxins: The Hidden Chemistry of Mould
To understand mould illness, you need to understand mycotoxins — because it is largely these compounds, rather than mould spores alone, that drive the more serious and systemic health effects associated with indoor mould exposure.
What Are Mycotoxins?
Mycotoxins are toxic secondary metabolites produced by moulds — chemical weapons, if you will, that fungi use to outcompete other microorganisms for resources. Campbell (2016) offers a vivid illustration of this: when you find a mouldy piece of bread in the back of the fridge, the coloured patches represent moulds and bacteria in competition for the food. The mycotoxins produced by the mould work to destroy the bacteria, leaving the food exclusively for the fungus.
This competitive chemistry is the inadvertent basis of one of medicine's greatest discoveries: Alexander Fleming's observation, in 1928, that the mould Penicillium notatum was destroying the bacteria in a contaminated petri dish ultimately led to the development of penicillin. Mycotoxins, it turns out, have been saving and ending human lives in equal measure throughout history.
Thousands of mycotoxins have now been identified. They are produced in large quantities under conditions of stress — particularly when moisture triggers rapid mould growth in organic materials — and they vary widely in their chemical structure, their biological targets, and their potency.
How Do Mycotoxins Get Into the Body?
There are three primary routes through which mycotoxins enter the human body in an indoor environment:
Inhalation — the most significant route in a water-damaged building; mycotoxin-laden spores and dust particles are breathed in and deposited throughout the respiratory tract
Ingestion — relevant primarily in the context of contaminated food, though settled dust can also be inadvertently ingested
Dermal absorption — skin contact with contaminated surfaces or airborne particles
Research from PMC (Ehsanifar et al., 2023) on mould, mycotoxins and brain disorders highlights that mould and its associated toxins can also enter the body via olfactory neurons — the nerve cells responsible for smell — which have direct communication pathways to the brain. This route of entry is of particular interest in understanding some of the neurological and cognitive effects associated with mould exposure.
Studies have confirmed that when Stachybotrys chartarum is present in an indoor environment, mycotoxins including trichothecenes can be detected in the blood serum of exposed individuals (Brasel et al., 2004, cited in Campbell, 2016). This is no longer a theoretical pathway — it is a measurable biological reality.
A Notable History
The history of mycotoxin-related illness is far older than many people realise. Campbell (2016) traces it back to ancient Assyrian texts from 600 BCE describing a 'noxious pustule in the ear of grain' — almost certainly a reference to ergot, the toxin-producing fungus Claviceps purpurea that grows on rye and other grains.
In medieval Europe, ergot poisoning — known as St Anthony's Fire — caused widespread epidemics of hallucinations, convulsions, gangrene of the extremities, and mass psychosis. Between the sixth and eighteenth centuries, there were 132 documented ergotism epidemics across Europe. As recently as 1944, approximately 100,000 people died in Russia after eating bread contaminated with the trichothecene mycotoxin from mouldy grain.
Closer to the present, research on Stachybotrys infestation in homes following Hurricane Katrina led the CDC to conclude that 'excessive exposure to mould-contaminated materials can cause health effects in susceptible persons regardless of the type of molds or the extent of contamination' — a statement that reflects just how far understanding has come.
How Mould Causes Disease: The Biological Mechanisms
Mould-related illness is not a single disease with a single mechanism. It is better understood as a cluster of overlapping biological disruptions, driven by a combination of immunological, toxic, and inflammatory pathways — the precise mix of which depends on the moulds involved, the duration and intensity of exposure, and the individual's genetic and immune profile.
Pizzorno (2016) identifies the following primary mechanisms by which damp building exposure causes physiological harm:
1. Immune Stimulation and Hypersensitivity
Mould spores and their associated proteins act as antigens — foreign substances that trigger an immune response. In susceptible individuals, repeated exposure sensitises the immune system, leading to increasingly pronounced reactions over time. Kraft, Buchenauer and Polte (2021), in a comprehensive review published in the International Journal of Molecular Sciences, note that the four most clinically significant genera — Alternaria, Cladosporium, Aspergillus, and Penicillium — are well established as triggers of IgE-mediated allergic responses and hypersensitivity reactions.
What makes this particularly insidious is that once sensitisation occurs, even relatively low levels of subsequent exposure can provoke a disproportionate immune response. The body, in effect, remembers the threat and reacts accordingly — even when the original source of exposure has been removed.
2. Immune Suppression and Dysregulation
Paradoxically, mycotoxins also have well-documented immune-suppressive properties. Campbell (2016) notes that immune suppression and modulation are among the established effects of indoor mycotoxin exposure. Research shows that mycotoxins can impair the function of T cells and B cells — key components of both innate and adaptive immunity — leaving exposed individuals more vulnerable to secondary infections.
Kraft et al. (2021) elaborate on the molecular mechanisms, noting that mycotoxin exposure triggers the production of reactive oxygen species (ROS) and suppresses the Nrf2 antioxidant defence system while simultaneously activating NF-κB — a master regulator of inflammatory gene expression. The net result is a state of chronic, low-grade inflammation combined with impaired immune competence: a difficult combination to resolve.
3. Inflammation
Inflammation is perhaps the most central mechanism in mould illness. The complex cocktail of agents found in water-damaged buildings — including mould spores, mycotoxins, bacterial endotoxins, beta-glucans (structural components of fungal cell walls), and volatile organic compounds — each contributes to inflammatory signalling in the body.
A comprehensive review published in PMC (Shoemaker et al., 2013) identified inflammation, oxidative stress, toxicity, infection, allergy, and irritant effects as the primary mechanisms of illness resulting from water-damaged building exposure. Importantly, the review noted that these factors are likely to act synergistically — meaning that the combined effect of multiple agents is greater than any individual component alone.
Beta-glucans deserve particular mention here: these water-insoluble structural components of fungal cell walls are inherently pro-inflammatory, even independently of mycotoxin content. They stimulate pattern recognition receptors on immune cells and drive cytokine production — contributing to the inflammatory burden of indoor mould exposure even in buildings where classical 'toxic moulds' are absent.
4. Neurotoxicity
One of the more surprising and clinically important dimensions of mould illness is its neurological component. A 2023 review by Ehsanifar et al. in the Journal of Integrative Neuroscience summarised evidence showing that mycotoxins induce effects in the brain analogous to those seen in neurological disorders: oxidative stress, neuroinflammation, and disruption of normal neurological function.
Specific mycotoxins have been shown to alter the blood-brain barrier, damage astrocytes and oligodendrocytes, and induce expression of pro-inflammatory genes within the central nervous system. Gliotoxins produced by Aspergillus fumigatus have been associated with tremors and CNS injury. Trichothecenes from Stachybotrys chartarum are classified as neurotoxic in Campbell's (2016) summary of mould-mycotoxin effects.
In a large study of patients exposed to water-damaged buildings, neurological dysfunction — including short-term memory loss, inability to walk a straight line with eyes closed, and similar indicators — was identified in 70% of cases (Thrasher et al., 2004). This is not a marginal finding.
5. Organ Toxicity
Certain mycotoxins have well-characterised direct effects on specific organs:
Aflatoxin B1 (produced by Aspergillus flavus) is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC) — the highest risk classification — with particular relevance to liver cancer
Ochratoxin A (produced by Aspergillus niger and related species) is nephrotoxic, carcinogenic, teratogenic, and immune-suppressive, and is classified by IARC as a possible human carcinogen (Group 2B)
Fumonisins (produced by Fusarium species) are associated with oesophageal cancer and have been linked to neural tube defects in experimental animal studies
Trichothecenes (from Stachybotrys and related species) have been linked to pulmonary haemorrhage in infants and are classified as both neurotoxic and immune-suppressive
These are not obscure laboratory curiosities. As Campbell (2016) emphasises, they are metabolites produced by moulds that commonly grow in homes and workplaces — and they are detectable in the bodies of people living and working in contaminated buildings.
6. Epigenetic Modification
Perhaps most concerning from a long-term health perspective is the growing evidence that mould and dampness exposure can cause epigenetic changes — alterations in how genes are expressed, without changing the DNA sequence itself. Pizzorno (2016) notes that research is now supporting the hypothesis that mould and dampness exposure upregulates multiple inflammatory genes through epigenetic mechanisms. This may help explain why some individuals remain unwell even after leaving a contaminated environment, and why sensitivity to other allergens — including house dust mites and pollen — can increase following mould exposure.
Normal Exposure vs. Problematic Exposure: Where Is the Line?
This is one of the most important and frequently misunderstood aspects of the topic — and one that creates genuine confusion for clinicians, patients, and policymakers alike.
Everyday Exposure Is Unavoidable
Mould spores are a normal part of the outdoor and indoor environment. The human immune system has co-evolved with fungi over hundreds of thousands of years and is well equipped to handle ordinary, low-level encounters with mould spores. Walking through autumn leaves, opening an old book, or briefly entering a damp cellar does not constitute a health hazard for the vast majority of people.
The critical issue is not exposure per se — it is sustained, concentrated exposure in a water-damaged building, particularly where mould has actively colonised building materials and begun producing mycotoxins, VOCs, and other biological agents.
What Makes Exposure Clinically Significant?
Several factors determine whether a mould exposure crosses from incidental to clinically meaningful:
Duration of exposure — days or weeks are very different from months or years of living or working in a water-damaged environment: Duration
Concentration of mould and mycotoxins — active colonisation of building materials produces far greater quantities of toxic agents than ambient environmental spores: Concentration
Species involved — not all moulds produce mycotoxins; those that do (particularly Stachybotrys, Aspergillus, Penicillium, and Chaetomium) carry significantly greater health implications: Species involved
Individual susceptibility — genetic factors, pre-existing immune conditions, nutritional status, gut microbiome health, and prior sensitisation all affect how severely an individual responds: Individual susceptibility
Kraft et al. (2021) make a particularly important observation here: while healthy individuals can develop hypersensitivity reactions and inflammatory responses from mould exposure, those with pre-existing immune dysregulation — including autoimmune conditions, allergic disease, or compromised immunity — appear to be disproportionately affected. The exposure, in their case, exacerbates underlying pathology in ways that can be difficult to attribute without a thorough clinical history.
The Challenge of Individual Variation
As the WHO itself acknowledged in its 2009 guidelines, the causative agents of adverse health effects in damp buildings 'have not been identified conclusively' — not because the effects are not real, but because the agents are so diverse and clinical effects appear to be highly dependent on individual susceptibility and sensitivity (including your state of health at the time of exposure).
This is perhaps the most important sentence in the entire mould illness debate. It explains why two people can live in the same water-damaged flat and have entirely different health outcomes. It explains why mould-related illness is so frequently dismissed: the absence of a clear, universal dose-response relationship makes it look, superficially, like the problem is 'just in someone's head.' It also explains why a thorough, individualised clinical approach is essential to working with this population effectively.
The Scale of the Problem: How Common Is Indoor Mould Exposure?
The statistics on indoor mould and damp are sobering — particularly in the UK context.
Global Prevalence
The WHO's 2009 indoor air quality guidelines estimated that between 10% and 50% of indoor environments across Europe, North America, Australia, India, and Japan had clinically significant mould problems — with higher rates in coastal areas and river valleys.
A 2022 Health Homes Barometer report, based on research by RAND, estimated that 14% of Western Europeans were living in homes with damp and mould. Research cited in Pizzorno (2016) found that a meta-analysis of studies showed a 30–50% increase in asthma and asthma-related health problems among those living in damp or mouldy buildings — a finding of enormous public health significance.
The UK Situation
In the UK, the picture is particularly stark. According to the UK Health Security Agency, around 2 million people — representing 3–4% of households — are currently living in homes with significant damp and mould. However, this figure almost certainly understates the problem. Self-reported data from the English Housing Survey suggests that nearly 27% of UK households have experienced some form of damp, condensation, or mould problem.
Private renters are the worst-affected group: in 2022–23, 10% of private rented properties had damp identified by a professional surveyor. Among households with children present, 39% reported damp or mould — a deeply concerning statistic given the particular vulnerability of developing immune systems.
The NHS has estimated it could save £38 million per year on treating mould-related illness if damp problems in housing were adequately addressed — a figure that, while significant, almost certainly underestimates the true cost to health services when systemic and chronic conditions are factored in.
The death of two-year-old Awaab Ishak in Rochdale in 2020, following prolonged exposure to black mould in social housing, prompted a seismic shift in public and political awareness — and the introduction of Awaab's Law in 2023, compelling social landlords to address damp and mould within defined timeframes. It was a tragic catalyst, but a necessary one.
Mould in the Workplace
It is not only homes that are affected. Research cited in Pizzorno (2016) found that in a study of 1,300 office workers, a remarkable 67% of cases of adult-onset asthma had begun after working in a water-damaged office building. Contaminated HVAC systems, in particular, can spread mould spores throughout entire office buildings, exposing potentially hundreds of occupants simultaneously.
What Does Mould Illness Actually Look Like?
This is where the complexity of mould illness really becomes apparent. Unlike an acute infection with a clear pathogen and a predictable course, mould-related illness tends to be diffuse, multi-system, and easily mistaken for other conditions.
Campbell (2016) describes the symptoms of indoor mould exposure as 'general and non-specific,' noting that affected patients are frequently moved from specialist to specialist, accumulating diagnoses — chronic fatigue syndrome, fibromyalgia, treatment-resistant Lyme disease — that describe their symptoms without identifying the underlying driver.
Respiratory Presentations
The respiratory effects of mould exposure are the best-established and most widely accepted in mainstream medicine. The WHO and CDC both recognise the following as conditions causally linked to dampness and mould:
Allergic rhinitis — chronic runny nose, sneezing, nasal congestion
Asthma — both new-onset and exacerbation of existing asthma
Recurrent respiratory infections and bronchitis
Persistent cough and wheeze
Hypersensitivity pneumonitis — an inflammatory lung condition driven by repeated allergen exposure
Allergic fungal sinusitis and chronic rhinosinusitis
Pizzorno (2016) stresses that every patient with any chronic respiratory condition — particularly adult-onset asthma — must be thoroughly evaluated for mould and damp building exposure. The research base on this is, by his assessment, 'voluminous and clear.'
Neurological and Cognitive Presentations
Less established in mainstream medicine — but increasingly supported by the research literature — are the neurological effects of mycotoxin exposure. Campbell (2016) identifies the following among the most commonly reported symptoms of indoor mould exposure:
Cognitive difficulties — 'brain fog,' difficulties with concentration and recall
Short-term memory loss
Mood changes, including anxiety and low mood
Unusual headaches
Tremors
Sleep disturbance
The 2023 review by Ehsanifar et al. in the Journal of Integrative Neuroscience found that mycotoxins induce toxicological effects consistent with those seen in neurodegenerative and neuroinflammatory conditions — including oxidative stress and chronic neuroinflammation. The olfactory pathway, which connects the nasal passages directly to the brain, is identified as a particularly significant route by which mycotoxins reach and affect neurological tissue.
Systemic Presentations
Beyond respiratory and neurological symptoms, mould-related illness has been associated with a broad range of systemic complaints:
Profound fatigue and post-exertional malaise
Widespread muscle and joint pain
Palpitations and cardiovascular symptoms
Blurred vision and visual disturbances
Gastrointestinal disturbance
Skin reactions, including eczema
The wide variety of presentations is not a reason to be dismissive — it reflects the fact that mycotoxins are capable of affecting multiple organ systems simultaneously. The review of over 800 water-damaged buildings and more than 2,000 residents cited by Saghir and Bancroft (2025) found that mycotoxins produced systemic effects across respiratory, nervous, hepatic, and renal systems — as well as developmental toxicity in the case of foetal exposure.
Why Mould Illness Is So Often Missed
Given the scale of the evidence, it is reasonable to ask: why is mould illness still so frequently overlooked in conventional clinical practice?
Several factors conspire to make this a difficult diagnosis to reach:
Non-specific symptoms: The multi-system nature of mould illness means its presentation overlaps with dozens of other conditions — from fibromyalgia to chronic fatigue syndrome to anxiety disorders. Without a detailed environmental history, the environmental trigger is rarely considered.
Individual variability: As the WHO has acknowledged, the absence of a consistent, universal dose-response relationship makes it easy to dismiss the connection between exposure and illness — particularly when housemates or colleagues in the same environment appear unaffected.
Under-investigation: Standard clinical investigations rarely include assessment for mycotoxin exposure. Without a referral pathway or a clinician who thinks to ask the question, the diagnosis is simply never considered.
Lack of awareness: Mould illness remains outside most conventional medical training curricula. Many general practitioners are unfamiliar with the research literature, or understandably cautious about a field that — until recently — sat largely in the domain of functional and integrative medicine.
Building complexity: Mould in buildings is often invisible: hidden behind walls, under floors, or within HVAC systems. Occupants may smell mustiness and see no visible growth — yet be living or working in a highly contaminated environment.
This is not a criticism of conventional medicine — it is an observation about the gap that exists when a condition is complex, multi-factorial, and sits at the intersection of environmental science and clinical medicine. Bridging that gap requires a different kind of clinical lens: one that takes environmental exposure seriously, thinks systemically, and is prepared to look for triggers rather than simply treating symptoms.
The Bottom Line
Mould illness is real. The science is clear, the mechanisms are established, and the population affected is larger than most people realise — particularly in the UK, where ageing housing stock, fuel poverty, and inadequate ventilation create ideal conditions for sustained mould growth in homes and workplaces.
What remains complex is the clinical picture: understanding who is most susceptible, how to assess individual exposure, what investigations are meaningful, how different mycotoxins affect different biological systems, and — critically — how to support recovery once exposure has been identified. These are not questions with simple, universal answers. They require depth of knowledge, clinical nuance, and a framework that integrates the environmental, immunological, and systemic dimensions of the condition.
That is precisely the level of depth that Mould Mastery is designed to provide. Whether you're a health-conscious individual who suspects mould may be an unrecognised driver of your symptoms, or a clinician looking to expand your understanding of this increasingly important field, the course offers a structured, evidence-grounded journey through the science and clinical landscape of mould illness.
Ready to Go Deeper?
Mould Mastery is a comprehensive online course built for anyone who wants to truly understand mould illness — from the biology of mycotoxins through to assessment, clinical patterns, and the path back to health. Developed by Alex Manos, MSc, a Functional Medicine Practitioner, and Ryan Carter, nutritional therapist.
Visit Mould Mastery to learn more and enrol.
Key References
Campbell, A.W. (2016). Molds and Mycotoxins: A Brief Review. Alternative Therapies in Health and Medicine, 22(4), 8–11.
Ehsanifar, M., Rajati, R., Gholami, A., Reiss, J.P. (2023). Mold and Mycotoxin Exposure and Brain Disorders. Journal of Integrative Neuroscience, 22(6), 137.
Kraft, S., Buchenauer, L., Polte, T. (2021). Mold, Mycotoxins and a Dysregulated Immune System: A Combination of Concern? International Journal of Molecular Sciences, 22(22), 12269.
Pizzorno, J. (2016). Is Mold Toxicity Really a Problem for Our Patients? Part I — Respiratory Conditions. Integrative Medicine, 15(2), 6–10.
Saghir, S.A., Bancroft, J., Ansari, R.A. (2025). Molds and mycotoxins indoors II: Toxicological perspective. Archives of Clinical Toxicology, 7(1), 8–40.
Shoemaker, R.C. et al. (2013). A Review of the Mechanism of Injury and Treatment Approaches for Illness Resulting from Exposure to Water-Damaged Buildings, Mold, and Mycotoxins. The Scientific World Journal, 2013, 767482.
Thrasher, J.D. et al. (2004). Effects of toxic exposure to molds and mycotoxins in building-related illnesses. PubMed, 15143852.
World Health Organisation (2009). WHO Guidelines for Indoor Air Quality: Dampness and Mould. Copenhagen: WHO.
Clark, S.N. et al. (2023). The Burden of Respiratory Disease from Formaldehyde, Damp and Mould in English Housing. Environments, 10(8), 136.
UK Health Security Agency. The Burden of Disease Caused by Damp and Mould in English Housing. Research Portal, 2023.
