Alternaria alternata is a widespread mold found outdoors in soil, decaying plants, and air, and indoors in damp or poorly ventilated spaces. It belongs to one of the most common fungal genera on earth, with A. alternata being the single most abundant species. For most people, it matters for two reasons: it’s one of the most potent triggers of mold-related allergies and asthma, and it contaminates a wide range of foods with toxins that are drawing increasing scrutiny from food safety authorities.
Where It Grows
A. alternata thrives wherever organic material and moisture meet. Outdoors, it colonizes dead leaves, rotting wood, compost, and soil. It’s also a major plant pathogen, causing dark spots and rot on tomatoes, apples, citrus fruits, grapes, strawberries, peppers, cucumbers, melons, cereals, sunflower seeds, and oil crops. It can infect these plants in the field and continue spreading after harvest during storage.
Indoors, A. alternata needs relatively high humidity to establish itself. Studies show that humidity levels of 84% and above promote both fungal growth and toxin production. You’ll find it in carpets, bedding, bathroom walls, and anywhere with persistent dampness or poor ventilation. Homes with faulty insulation, outdated ventilation, or pest infestations (particularly cockroaches or large numbers of cats) tend to have higher levels. When enough mold accumulates in an enclosed space, it contributes to what’s sometimes called sick building syndrome.
What the Spores Look Like and How They Spread
Under a microscope, A. alternata produces dark, club-shaped spores (conidia) that typically measure 22 to 31 micrometers long and 11 to 14 micrometers wide, roughly one-quarter the width of a human hair. The spores have distinctive internal divisions called septa that give them a segmented appearance, and they form in branching chains at the tips of the fungal filaments.
These spores are lightweight and release easily into the air, especially during warm, dry weather and thunderstorms. Average outdoor concentrations during summer can range from 11,000 to 25,000 spores per cubic meter. On thunderstorm days, counts surge above 7,000 spores per cubic meter even in localized measurements, a level researchers describe as “very high.” Strong winds and rain turbulence break spore chains apart and loft them into the atmosphere, which is why mold-sensitive people sometimes experience flare-ups during summer storms.
How It Triggers Allergies and Asthma
A. alternata is one of the most clinically significant allergenic molds. The key player is a protein the fungus secretes called Alt a 1, which triggers an antibody-driven allergic response in the vast majority of people sensitized to Alternaria. When inhaled, Alt a 1 lands on the cells lining your airways and activates immune receptors on their surface. These receptors kick off a signaling cascade that recruits inflammatory cells, including white blood cells that drive swelling, mucus production, and airway narrowing.
This process makes Alternaria sensitivity not just a cause of hay fever-like symptoms (sneezing, runny nose, itchy eyes) but a recognized risk factor for developing asthma. Research has linked it to severe and potentially fatal asthma more than almost any other fungus. Lab studies confirm the connection: when researchers engineered fungal spores to produce roughly 2.5 times the normal amount of Alt a 1, the immune response was dramatically higher. Spores engineered to lack the protein triggered a much weaker reaction, confirming that Alt a 1 is the central driver.
Testing for Alternaria Allergy
If you suspect a mold allergy, a blood test can measure the level of allergy-specific antibodies (IgE) your body produces in response to A. alternata. Results are reported in classes:
- Below 0.35 kU/L: No significant allergy detected
- 0.35–0.70 kU/L: Low level
- 0.71–3.50 kU/L: Moderate level
- 3.51–17.50 kU/L: High level
- Above 17.50 kU/L: Very high level
One important caveat: higher antibody levels don’t always predict worse symptoms. Some people with moderate lab results react severely to exposure, while others with high levels have milder symptoms. Skin prick tests, where a tiny amount of Alternaria extract is introduced into the skin, are often used alongside blood tests. A negative result on either test doesn’t completely rule out a clinical allergy.
Mycotoxins in Food
Beyond airborne allergies, A. alternata produces several toxins that contaminate food crops. The most studied are alternariol, alternariol monomethyl ether, and tenuazonic acid. These compounds survive food processing and have raised enough concern that the European Food Safety Authority has conducted formal risk assessments on four of them.
Alternariol, first identified in 1953, is the best characterized. It damages DNA through indirect mechanisms and has been linked in epidemiological studies to higher rates of esophageal cancer in regions where contaminated grain is a dietary staple. Its chemical structure resembles estrogen closely enough to bind to and activate estrogen receptors in human cells, giving it endocrine-disrupting properties. Computational toxicology models have also flagged potential effects on blood vessels, kidneys, and blood cell production, though these predictions still need confirmation in human studies.
These toxins also appear to affect the gut. Research shows they can alter the balance of bacteria that normally colonize the digestive system, reducing the viability of beneficial strains. They also suppress inflammatory responses in the immune cells lining the gut, which could impair the body’s ability to fight off other infections. Germany’s food safety authority has set a specific limit of 500 micrograms per kilogram for tenuazonic acid in sorghum and millet-based infant foods, one of the few regulatory thresholds that exist so far for Alternaria toxins.
Skin and Tissue Infections
In rare cases, A. alternata causes direct infections in humans, almost exclusively in people with weakened immune systems. The most common form is cutaneous alternariosis, a type of skin infection where the fungus enters through a break in the skin, typically on the hands, forearms, knees, or face.
The lesions can look quite different from case to case, which makes diagnosis tricky. They may appear as painless nodules under the skin, crusty ulcers, warty-looking growths, or fluid-filled cysts. In severe cases, sinus tracts (channels draining from deep tissue to the skin surface) can develop. Eye, sinus, and nail infections are possible but uncommon. Because the lesions aren’t distinctive enough to identify on sight, diagnosis typically requires a tissue sample and lab culture to confirm the fungus is present.
Reducing Your Exposure
Controlling indoor humidity is the single most effective step. Keeping relative humidity below 60%, and ideally below 50%, makes it difficult for A. alternata to establish colonies. Proper ventilation in bathrooms, kitchens, and basements matters, as does fixing leaks promptly. Regularly cleaning carpets and bedding removes spores that have settled from the air.
Outdoors, spore counts peak in late summer and early fall. They’re highest on warm, windy days and during thunderstorms. If you’re sensitized, keeping windows closed during these conditions and using HEPA filtration can reduce the amount of Alternaria spores in your indoor air. For food safety, inspecting fruits and vegetables for dark spots or soft rot before eating them is a practical way to avoid the most heavily contaminated items, since visible mold growth correlates with higher mycotoxin levels in the surrounding tissue.