Mould can be dangerous, particularly for people with asthma, allergies, or weakened immune systems. For most healthy adults, short-term exposure to small amounts causes mild irritation at worst. But prolonged exposure in a damp building is a different story, potentially triggering respiratory illness, chronic fatigue, and cognitive problems that can take weeks or months to resolve after the source is removed.
The risk depends on three things: how much mould is present, how long you’re exposed, and how vulnerable your body is. Here’s what the science actually shows.
How Mould Affects Your Body
Mould causes harm through two distinct pathways, and understanding the difference matters because the symptoms and severity look quite different.
The first is an allergic response. Your immune system recognizes mould spores as foreign invaders and overreacts, producing antibodies that trigger inflammation. This is the same mechanism behind hay fever and dust mite allergies. Symptoms include itchy, red eyes, a blocked or runny nose, sneezing, coughing, and wheezing. For people with asthma, mould spores can trigger full asthma episodes. A related but more serious allergic condition called hypersensitivity pneumonitis causes deeper lung inflammation, with flu-like symptoms including fever, chills, and shortness of breath.
The second pathway involves irritation and toxicity from mould byproducts. As mould grows, it releases volatile organic compounds (the chemicals responsible for that musty smell) and, in some species, mycotoxins. These substances can irritate airways and mucous membranes even in people who aren’t technically allergic. The most studied mycotoxins interfere with protein synthesis in your cells, disrupt intestinal barrier function, suppress immune defences, and in extreme cases of chronic ingestion (primarily through contaminated food rather than indoor air), have been linked to liver and kidney damage.
The “Toxic Black Mould” Question
Stachybotrys chartarum, the species commonly called “toxic black mould,” gets outsized attention. It produces mycotoxins and tends to grow on water-damaged materials with high cellulose content, like wet drywall and ceiling tiles. But the CDC’s position is straightforward: it is not necessary to determine what type of mould is growing in your home. All mould should be treated the same with respect to potential health risks and removal.
A widely reported link between Stachybotrys and bleeding in infants’ lungs has not been proven despite years of investigation. That doesn’t mean Stachybotrys is harmless. It means the evidence doesn’t support treating it as uniquely dangerous compared to other indoor moulds. Many common species, including Aspergillus, Penicillium, and Cladosporium, produce their own mycotoxins and allergens. The colour of mould tells you almost nothing about its danger level.
Symptoms of Prolonged Exposure
Early symptoms of living or working in a mouldy building tend to be mild and reversible: irritated eyes, nasal congestion, sneezing, coughing. Many people mistake these for a lingering cold. A key hallmark is that symptoms improve or disappear entirely within a day or two of leaving the building, then return when you go back. This pattern, sometimes called sick building syndrome, is one of the strongest clinical clues.
With continued exposure, symptoms can escalate. Recurrent sinus infections and bronchitis are common. Some people develop new-onset asthma or asthma-like breathing difficulties, including a burning sensation in the airways and a persistent cough. Headaches, nausea, and unexplained fatigue often follow. Some individuals develop heightened sensitivity to chemicals and scents they previously tolerated without issue, including the ability to detect mouldy odours on other people’s clothing.
More controversial but increasingly studied are neurological effects. People exposed to mouldy environments report anxiety, depression, difficulty concentrating, and “brain fog.” Several small studies found that neurologists could not distinguish between patients with repeated mould exposure and patients with mild to moderate traumatic brain injury based on cognitive testing alone. The proposed explanation is that inhaled mould spores activate the same immune signalling cascade that your body uses to fight infections. This immune activation produces fatigue, pain, memory problems, and mood changes that mirror the “sickness behaviour” you feel during a bad flu. Small mould fragments may also reach the brain directly through nasal pathways.
The link between mould and respiratory problems is well established. The link to neurological and cognitive symptoms is still debated, but the pattern of reported symptoms is consistent enough that researchers are actively working to identify the underlying mechanisms.
Who Faces the Greatest Risk
Mould is not equally dangerous to everyone. People with existing asthma or mould allergies are the most obviously affected, with symptoms that can flare quickly in a damp environment. But the most life-threatening risks belong to people with weakened immune systems. Organ transplant recipients, people undergoing chemotherapy, those with uncontrolled HIV, and patients on high-dose immunosuppressive medications can develop invasive mould infections, where fungi penetrate blood vessels, deep tissues, or organs. The two most common types are mucormycosis and aspergillosis. These infections are rare but cause severe illness and death without early treatment.
Infants and young children are also more vulnerable because their immune systems are still developing and they breathe faster relative to their body size, inhaling more spores per kilogram of body weight. Older adults face higher risk for similar reasons: immune function naturally declines with age, and chronic conditions can compound the effects of mould exposure.
Even among healthy adults, sensitivity varies widely. Some people live in mildly mouldy homes for years with no noticeable symptoms. Others develop problems within weeks. There is currently no standardized lab test that reliably diagnoses mould-related illness. Diagnosis remains clinical, based on your history of exposure, your symptom pattern, and whether symptoms improve when you leave the affected building.
Structural Damage to Your Home
Beyond health, mould threatens the building itself. Mould feeds on organic materials: wood, paper, carpet, insulation, and even everyday dust and dirt that accumulates in damp areas. In wood-framed buildings, unchecked mould growth can weaken floors and walls by slowly digesting structural timber. Drywall, which has a paper facing, is especially vulnerable. By the time you see mould on a wall surface, it has often already spread through the material behind it.
This structural degradation creates a compounding problem. Weakened materials hold more moisture, which accelerates further mould growth, which causes more damage. Catching it early makes the difference between a manageable cleanup and a major renovation.
When You Can Clean It Yourself
The EPA draws a clear line: if the mouldy area is smaller than about 10 square feet (roughly a 3-by-3-foot patch), most people can handle cleanup themselves using detergent and water, proper ventilation, and protective gear including gloves, goggles, and an N95 mask. Porous materials like carpet, ceiling tiles, and drywall that have been saturated usually need to be removed and replaced rather than cleaned.
If mould covers more than 10 square feet, or if it resulted from sewage backup or significant flooding, professional remediation is recommended. OSHA classifies jobs between 10 and 30 square feet as mid-sized and those between 30 and 100 square feet as large, each requiring progressively more containment and protective measures. If you can smell mould but can’t see it, the growth may be hidden behind walls or under flooring, and professional assessment is worth the cost.
Preventing Mould Growth
The WHO’s primary recommendation is simple: prevent persistent dampness. Mould needs moisture to grow, and controlling moisture is more effective than any cleaning product or air purifier. Keep indoor humidity below 50% (a basic hygrometer costs a few dollars). Fix leaks in roofs, walls, and plumbing promptly. Ventilate bathrooms, kitchens, and laundry areas to the outside. If condensation regularly forms on windows, the room needs better airflow or dehumidification.
After any water event, whether a burst pipe, a roof leak, or a flood, drying the affected area within 24 to 48 hours is critical. Mould colonizes wet materials fast. Pulling up wet carpet, running fans, and using dehumidifiers in the first day or two can prevent a problem that would otherwise require professional intervention weeks later.