Alternaria is a genus of dark-pigmented, filamentous fungi, commonly known as mold. It is one of the most frequently encountered molds both outdoors and indoors, thriving by decomposing organic matter. The release of its airborne spores is the main concern, as they act as a biological contaminant affecting human health. Understanding the nature of this widespread mold and its associated health risks is important for managing indoor air quality and personal well-being.
Common Habitats and Morphology
Alternaria obtains nutrients from dead or decaying organic matter, and it is readily found in soil, compost, and on various types of vegetation. The fungus is a known plant pathogen, causing leaf spots and rot on thousands of plant species, which contributes significantly to its presence in agricultural environments. Outdoors, its spores are most prevalent during warm, dry, and windy conditions, typically peaking in the late summer and early autumn in temperate climates.
The spores are easily aerosolized and can travel long distances, allowing them to enter indoor spaces through windows, doors, and ventilation systems. Once inside, Alternaria can colonize damp areas, particularly where water damage or persistent moisture is present, such as in bathrooms, near leaky windows, on carpets, or within HVAC systems. Macroscopically, colonies often appear dark, ranging from gray to olive-brown or black, and have a velvety or fuzzy texture.
The conidia of Alternaria are distinctive, appearing large, dark, and often club-shaped or ovoid. These spores frequently form in chain-like arrangements. Their relatively small size makes them highly respirable, allowing them to remain suspended in the air for extended periods. Upon inhalation, they can subsequently penetrate deep into the human respiratory tract.
Health Risks: Allergies and Mycotoxins
Exposure to Alternaria spores represents a significant health concern, primarily due to its potent allergenic properties. The mold is recognized as a major aeroallergen, with sensitization strongly associated with the development and worsening of various allergic diseases. The most common reactions involve the upper respiratory system, presenting as allergic rhinitis, characterized by symptoms like sneezing, nasal congestion, and a runny nose.
The allergenic proteins within the spores, such as the dominant allergen Alt a 1, trigger an immune response that can also cause conjunctivitis, resulting in itchy and watery eyes. There is a strong link between Alternaria exposure and asthma, particularly severe asthma exacerbations in children. Because the spores are light and small, they can bypass the upper airway defenses. This allows them to reach the bronchi and alveoli deep within the lungs, leading to inflammation and potentially life-threatening asthma attacks.
Beyond allergic reactions, Alternaria species are known to produce secondary metabolites called mycotoxins. Specific toxins include alternariol and tenuazonic acid, which have been studied for their potential cytotoxic and genotoxic properties. While these toxins are often associated with the contamination of crops and food products, research into the human health effects of inhaling Alternaria mycotoxins indoors is still ongoing. Mycotoxin exposure is distinct from an allergic response, as it involves chemical toxicity to cells rather than an immune system overreaction to proteins.
Strategies for Exposure Reduction
Minimizing exposure to Alternaria spores requires controlling moisture, which is the most effective indoor strategy. Since the mold requires a minimum water activity level to thrive, indoor humidity levels should be maintained below 50%. This range is often achievable through the use of dehumidifiers and air conditioning, especially during humid seasons.
Improving ventilation is important, particularly in moisture-prone areas like kitchens and bathrooms, by using exhaust fans to vent humid air outdoors. Promptly repairing any water leaks, whether from plumbing, windows, or the roof, eliminates the primary moisture source that supports indoor mold growth. For existing mold, proper cleaning with a water and detergent solution or a commercial mold remover is necessary, ensuring the area is completely dried afterward.
Air filtration is a useful tool for reducing airborne spores, which can be accomplished by using high-efficiency particulate air (HEPA) filters in vacuum cleaners and HVAC systems. Individuals with known Alternaria allergies should be mindful of outdoor activities, as spore counts peak in the summer and fall. Limiting time outdoors when mold counts are high, and wearing a protective mask when performing yard work, such as raking leaves or handling compost, can significantly reduce spore inhalation.