Hypersensitivity Pneumonitis: When Cough and Breathlessness Come from Airborne Triggers, Not Medications

Many people assume that if they develop a persistent cough and trouble breathing, it must be from a medication they’re taking. But when it comes to hypersensitivity pneumonitis, the real culprit is almost never a pill or injection. It’s the air you breathe.

What Exactly Is Hypersensitivity Pneumonitis?

Hypersensitivity pneumonitis (HP) is a lung condition caused by repeated breathing in of tiny particles from the environment-like mold, bird droppings, or dust from hay. Your immune system overreacts to these harmless substances, triggering inflammation deep in your lungs. This isn’t an allergy like hay fever. It’s a more serious, delayed immune response that damages the tiny air sacs (alveoli) where oxygen enters your blood.

The name sounds complicated, but the symptoms are simple: a dry cough that won’t go away, and breathlessness that gets worse over time-especially when you’re doing things you used to handle easily, like climbing stairs or walking the dog. These symptoms don’t show up right away. They build up slowly, often over weeks or months, which is why many people ignore them until it’s too late.

Why Medications Don’t Cause True Hypersensitivity Pneumonitis

You might have heard of drug reactions that affect the lungs-like amiodarone or nitrofurantoin causing lung scarring. But these aren’t the same as hypersensitivity pneumonitis. Those drug reactions are called drug-induced interstitial lung disease (DILD). They happen because the medicine directly poisons lung cells or triggers a different kind of immune response. They don’t form the telltale poorly formed granulomas or the specific pattern of lymphocyte buildup around small airways that define true HP.

Major medical references like the Merck Manual, the Pulmonary Fibrosis Foundation, and UCSF Health all agree: HP is caused by inhalation of antigens. Medications are swallowed or injected. They don’t get inhaled in the way mold spores or bird proteins do. That’s why no authoritative source lists medications as a cause of true hypersensitivity pneumonitis. If you’re coughing and short of breath after starting a new drug, it’s likely DILD-not HP.

Common Triggers You Might Not Realize Are Dangerous

Most people think of HP as something that affects farmers or bird owners. But the triggers are more common than you think:

• Bird fancier’s lung: Caused by proteins in bird feathers, droppings, or serum. Even pet parakeets or pigeons can be the source if you clean cages regularly.
• Farmer’s lung: From moldy hay, grain, or straw. This is the classic form, first described in the 1950s.
• Mushroom picker’s disease: From spores in mushroom-growing environments.
• Humidifier lung: From bacteria or mold growing in poorly cleaned humidifiers, air conditioners, or hot tubs.
• Hot tub lung: Caused by inhaling aerosolized mycobacteria from contaminated water.

These aren’t rare. In fact, in some farming communities, up to 1 in 5 people exposed to moldy hay show signs of HP. And if you’re a bird enthusiast, a home hobbyist who uses a humidifier daily, or work in a building with old HVAC systems, you’re at risk too.

How Do You Know It’s HP and Not Asthma or COPD?

HP can look a lot like asthma or chronic bronchitis. But here’s how to tell the difference:

• Asthma: Wheezing, tight chest, symptoms improve with inhalers. Affects airways, not air sacs.
• COPD: Long-term smoker, chronic mucus production, slow progression over years.
• HP: No wheezing, cough is dry, breathlessness gets worse when you’re near the trigger and improves when you leave.

The biggest clue? Symptoms get better when you’re away from home or work. If your cough and shortness of breath vanish on weekends or during vacation, but come back when you return, that’s a red flag for HP. Doctors call this the “away-from-exposure” improvement test. It’s one of the most reliable diagnostic signs.

What Tests Confirm HP?

There’s no single blood test for HP. Diagnosis needs a puzzle of evidence:

• High-resolution CT scan: Shows patchy ground-glass opacities and air trapping-especially on expiratory images.
• Pulmonary function tests: Usually show reduced lung volume and lower diffusion capacity (DLCO), meaning your lungs can’t transfer oxygen well.
• Bronchoalveolar lavage: A procedure where fluid is washed into the lungs and collected. In HP, over 40% of the cells are lymphocytes-a key marker.
• Blood tests for antibodies: Can detect antibodies to specific antigens (like bird proteins or mold), though these aren’t always present.
• Lung biopsy: Only needed in unclear cases. Shows the hallmark: small clusters of immune cells (granulomas) and inflammation around the smallest airways.

Doctors don’t rely on one test. They look at your history, your symptoms, your imaging, and your lab results together. If you’ve been exposed to a known trigger and your symptoms improve when you avoid it, that’s often enough to make the diagnosis.

Can It Be Treated? What Happens If You Ignore It?

Yes-early HP can be reversed. The most important treatment isn’t medicine. It’s stopping the exposure.

If you catch it in the acute or early subacute stage and remove yourself from the trigger, your lungs can heal completely. Many patients recover fully within weeks. Corticosteroids like prednisone may be used for a few weeks to calm inflammation, but they’re not a long-term fix.

But if you keep breathing in the trigger? That’s when things turn serious. Over months or years, inflammation turns to scar tissue-fibrosis. Once that happens, it’s permanent. Your lungs stiffen. Your oxygen levels drop. You may need supplemental oxygen, pulmonary rehab, or even a lung transplant.

Studies show that 30-50% of people with chronic HP develop irreversible lung scarring. The survival rate drops to about 50-80% over five years if fibrosis is present. That’s why early diagnosis isn’t just helpful-it’s life-saving.

Who’s Most at Risk?

You’re at higher risk if you:

• Own birds (parrots, pigeons, chickens, even budgies)
• Work in agriculture, especially around grain, hay, or compost
• Use a humidifier, air conditioner, or hot tub regularly without cleaning it
• Work in mushroom farms, veterinary clinics, or HVAC maintenance
• Live in a damp, moldy home with poor ventilation

And here’s something surprising: smokers have a lower risk of developing HP. It’s not because smoking is healthy-it’s because smoking damages the immune cells in the lungs that normally respond to these airborne triggers. But that doesn’t mean smoking protects you. It just means your body might not react the same way, which can delay diagnosis.

How to Protect Yourself

Prevention is simple, but it takes action:

• If you have birds: Wear a mask when cleaning cages. Wash hands after handling them. Keep cages in well-ventilated areas, not bedrooms.
• If you use a humidifier: Clean it daily with vinegar or hydrogen peroxide. Use distilled water. Never let water sit for more than 24 hours.
• If you’re a farmer or work with grain: Use respirators rated for organic dusts. Keep storage areas dry. Avoid working in dusty environments without protection.
• If you have a hot tub: Maintain chlorine or bromine levels. Clean filters weekly. Shower after use.
• If you have unexplained cough and breathlessness: Ask yourself-does it get worse at home? Worse at work? Does it improve on vacation? If yes, talk to a pulmonologist about HP.

Don’t wait for a diagnosis. If you’re exposed to known triggers and have symptoms, get tested. Early action can mean the difference between full recovery and permanent lung damage.

Frequently Asked Questions

Next Steps If You Suspect HP

If you have a persistent cough and breathlessness-and you’re exposed to birds, mold, or humidifiers-take these steps:

1. Stop using humidifiers or hot tubs for two weeks. See if symptoms improve.
2. Change your routine: Avoid cleaning bird cages, handling hay, or working in damp areas.
3. Write down when and where symptoms happen. Does your cough worsen at home? At work?
4. Ask your doctor for a high-resolution CT scan and pulmonary function tests.
5. If HP is suspected, ask for referral to a pulmonologist who specializes in interstitial lung disease.

Don’t assume it’s just a cold or allergies. If your lungs are reacting to something in the air you breathe, ignoring it could cost you your breathing for life. Early action saves lungs-and lives.