Mold gets into your lungs the same way air does: you breathe it in. Mold reproduces by releasing microscopic spores into the air, and these spores are small enough to travel deep into your airways. Most healthy people inhale small amounts of mold spores every day without any problems. The immune system traps and destroys them before they can take hold. But when spore concentrations are high, when exposure is prolonged, or when your immune system is compromised, mold can colonize lung tissue and cause infections ranging from mild allergic reactions to life-threatening invasive disease.
Why Most People Don’t Get Sick
Mold spores are everywhere, indoors and outdoors. Your respiratory system has multiple layers of defense against them. Mucus lining the airways traps inhaled particles, tiny hair-like structures called cilia sweep them back toward the throat, and immune cells in the lungs destroy whatever gets through. For the vast majority of people, this system handles mold spores effortlessly.
The CDC does not even set a threshold for “safe” versus “unsafe” levels of indoor mold, because health effects vary so widely between individuals. What matters more than the exact spore count is the combination of how much mold you’re exposed to, how long the exposure lasts, and how well your body can fight it off.
Who Is Most Vulnerable
The people who develop serious mold infections in the lungs almost always have a weakened immune system or an underlying lung condition. Specific risk factors include:
- Organ transplant recipients, who take medications that suppress immune function to prevent organ rejection
- Cancer patients, particularly those with blood cancers like acute myeloid leukemia receiving chemotherapy or targeted therapies
- People on long-term corticosteroids or biological immune-suppressing drugs, which reduce the body’s ability to fight fungal invaders
- People with severe viral respiratory infections, including influenza and COVID-19, especially those who develop acute respiratory distress syndrome
- People with inherited immune deficiencies or reduced counts of certain white blood cells
- People with chronic lung diseases like asthma, cystic fibrosis, COPD, or a history of tuberculosis, where damaged airways give spores a place to settle
One large study found that among people who developed invasive aspergillosis (the most common serious mold lung infection), only 59% of solid organ transplant recipients survived one year, and just 25% of stem cell transplant recipients did. These numbers reflect how dangerous mold becomes when the immune system can’t mount a proper defense.
Jobs That Increase Your Exposure
Certain occupations put you in contact with unusually high concentrations of fungal spores. The CDC has documented outbreaks among workers whose jobs disturb soil, decaying organic material, or contaminated structures.
Construction and demolition workers face elevated risk because tearing into walls, foundations, and old buildings releases trapped spores in large bursts. Agricultural and forestry workers encounter fungi that thrive in soil and plant matter. Landscapers and tree removal crews disturb environments where fungi grow. Mining, oil, and gas extraction workers are exposed to underground fungal reservoirs. Even wildland firefighters have experienced outbreaks of valley fever after working in areas where soil-dwelling fungi were present.
Histoplasmosis, caused by a fungus that grows in soil contaminated with bird or bat droppings, has caused outbreaks among bridge workers, prison workers, and people who work in caves. A 2023 blastomycosis outbreak hit workers at a paper mill in Michigan. These aren’t exotic scenarios. They’re ordinary workplaces where spore concentrations can spike without warning.
Types of Mold-Related Lung Disease
Not all mold lung problems look the same. The type of disease depends on your immune status and the specific fungus involved.
Allergic Reactions
The mildest form is an allergic response. Allergic bronchopulmonary aspergillosis (ABPA) occurs when the immune system overreacts to Aspergillus spores in the airways, causing wheezing, coughing, and worsening asthma symptoms. An estimated 2.5% of adults with asthma have ABPA, which translates to roughly 4.8 million people worldwide. It’s also common in cystic fibrosis, affecting between 1% and 15% of patients.
Chronic Pulmonary Aspergillosis
When mold gradually colonizes damaged areas of the lungs, it can form a fungal mass called an aspergilloma, or “fungus ball.” This tends to happen in people who already have cavities in their lungs from previous tuberculosis, sarcoidosis, or other conditions. Roughly 1.2 million people worldwide are estimated to have chronic pulmonary aspergillosis following tuberculosis, and over 70,000 have it as a complication of sarcoidosis. Symptoms develop slowly over weeks or months: persistent cough, fatigue, weight loss, and sometimes coughing up blood.
Invasive Aspergillosis
This is the most dangerous form. The fungus penetrates lung tissue and can spread through the bloodstream to other organs. It occurs almost exclusively in severely immunocompromised people. Hospitalizations for invasive aspergillosis in the United States increased an average of 3% per year between 2000 and 2013, reaching nearly 15,000 aspergillosis-related hospitalizations in 2014 at an estimated cost of $1.2 billion. Symptoms come on fast: fever that doesn’t respond to antibiotics, chest pain, cough, and difficulty breathing.
Mucormycosis
A rarer but aggressive infection caused by a different group of molds. It tends to invade blood vessels, cutting off blood supply to tissue. On CT scans, doctors can often distinguish it from aspergillosis by a characteristic ring-shaped pattern visible in about 54% of mucormycosis cases but only 6% of aspergillosis cases. It requires urgent treatment.
How Doctors Find It
CT scans are the primary tool for evaluating suspected mold infections in the lungs. No single pattern on a scan is unique to fungal disease, but certain findings point strongly in that direction. A “halo sign,” a fuzzy ring of inflammation surrounding a dense area, suggests invasive aspergillosis. Later in the disease, an “air crescent sign” can appear as the immune system begins to recover and tissue around the fungal mass breaks down. New cavities forming in the lungs are another red flag, particularly in moderately immunocompromised patients.
Because aspergillosis isn’t a reportable disease in the United States, exact case numbers are hard to pin down. Population-based estimates from the San Francisco Bay Area suggested a rate of 1 to 2 cases of invasive aspergillosis per 100,000 people per year, but the real number is likely higher given the rise in immunosuppressive therapies over the past three decades.
What Treatment Looks Like
Treatment depends on the type and severity of the infection. Allergic forms are typically managed with corticosteroids to calm the immune overreaction, sometimes combined with antifungal medication. For invasive aspergillosis, antifungal drugs are the standard treatment, and therapy often lasts weeks to months depending on how well the infection responds and whether the patient’s immune system recovers.
Chronic forms, especially fungus balls, sometimes require surgical removal if they cause significant bleeding or don’t respond to medication. Recovery timelines vary enormously. Someone with ABPA might manage their condition long-term with periodic flare-ups, while a patient with invasive disease may need months of intensive treatment and close monitoring.
Reducing Your Risk at Home
You can’t eliminate mold spores from the air you breathe, but you can keep indoor concentrations low. The most effective single step is controlling moisture: keep indoor humidity below 50% using a dehumidifier or exhaust fans. Fix leaks in bathrooms, kitchens, and basements as soon as they appear, since mold can begin growing on damp surfaces within 24 to 48 hours.
HEPA air purifiers capture at least 99.97% of particles 0.3 microns and larger, which includes mold spores. They’re worth considering in damp areas or rooms where mold has been a recurring problem. Standard HVAC filters rated MERV 5 capture only about 20% of mold spores, so upgrading to MERV 11 through 13 filters significantly improves capture rates. Ventilating your home by opening windows during dry weather helps dilute indoor spore concentrations, and cleaning visible mold promptly with vinegar or hydrogen peroxide prevents colonies from spreading and releasing more spores into the air.
For people working in high-risk occupations, properly fitted N95 respirators provide meaningful protection against inhaling concentrated spore clouds during demolition, excavation, or remediation work.