Mould is a type of fungus found everywhere, both outdoors and indoors, playing a role in the breakdown of organic matter. It requires moisture and a food source to grow, which makes damp indoor environments like basements, bathrooms, and water-damaged walls perfect habitats for colonization. Common indoor types include Cladosporium, Aspergillus, and Penicillium species, which can thrive on building materials and household items. Exposure to mould is common, but the resulting health effects vary significantly from person to person.
How Mould Exposure Occurs
The primary mechanism by which mould affects human health is through the release of microscopic spores into the air. These fungal spores are lightweight and float easily, allowing them to be inhaled deep into the respiratory tract, often bypassing the body’s natural filtering mechanisms. Mould also releases tiny fragments and metabolites, which include volatile organic compounds that often create the characteristic musty odor. Some species produce toxic compounds called mycotoxins, which are carried on the spores and fragments. While inhalation is the most common route, exposure can also occur through direct skin contact or by ingesting contaminated food.
Immediate and Allergic Health Responses
For many people, mould exposure triggers an allergic reaction, which is an overreaction by the immune system to the inhaled spores. This response is often mediated by Immunoglobulin E (IgE) antibodies, a process known as Type I hypersensitivity.
Upon subsequent exposure, the mould spores bind to these IgE-coated mast cells, causing them to rapidly release inflammatory chemicals like histamine. This immediate release leads to the classic hay fever-like symptoms, including sneezing, a runny or stuffy nose, and itchy, watery eyes. These symptoms are part of the “early-phase” reaction and are essentially the body attempting to flush out the perceived threat.
Mould exposure can also trigger a delayed inflammatory response in the lower airways, worsening symptoms for individuals with pre-existing conditions like asthma. This reaction involves the migration of other immune cells, such as eosinophils, which sustain inflammation and can cause coughing, wheezing, and chest tightness. For some individuals, contact with mould can also cause dermal reactions, leading to the development of skin rashes and hives.
A more severe, though rarer, allergic reaction is hypersensitivity pneumonitis (HP), which is an inflammatory condition affecting the small air sacs and tissues of the lungs. HP is caused by the immune system reacting to inhaled mould particles, leading to inflammation and, with chronic exposure, potential scarring of the lung tissue, known as fibrosis. Unlike common allergic responses, HP is not mediated by IgE but involves a deeper, cellular immune response within the lung parenchyma.
Factors Influencing Severity and Risk
The health consequences of mould exposure are heavily influenced by the concentration of exposure and individual susceptibility. A clear dose-dependent relationship exists, where higher concentrations of visible mould growth in a home are linked to increased levels of total serum IgE, indicating greater allergic sensitization. Moreover, chronic exposure over a long duration is a prerequisite for more serious conditions like hypersensitivity pneumonitis.
Certain populations carry a substantially higher risk of experiencing severe or life-threatening effects. Individuals with compromised immune systems, such as those undergoing chemotherapy, organ transplant recipients, or people with HIV, are vulnerable to invasive mold infections. These infections, like invasive aspergillosis or mucormycosis, occur when mould spores are inhaled and the weakened immune system is unable to clear them, allowing the fungus to grow within the lungs and potentially spread to other organs.
The elderly and infants also represent high-risk groups due to differing stages of immune function. The aging process naturally weakens the immune system, making older adults more susceptible to infections and amplifying the effects of chronic respiratory ailments. Children and infants, whose immune and respiratory systems are still developing, are also more vulnerable to sensitization and the subsequent development or exacerbation of respiratory illnesses like asthma. Pre-existing conditions, particularly chronic lung diseases, also significantly amplify the severity of mould-related symptoms.