Hypersensitivity Pneumonitis: Cough and Breathlessness from Environmental Triggers, Not Medications

When you start coughing and feel short of breath, it’s easy to blame a cold, allergies, or even a new medication. But if these symptoms keep coming back-especially after being around birds, moldy hay, or humidifiers-you might be dealing with something more specific: hypersensitivity pneumonitis. And here’s the catch: it’s not caused by pills you swallow. It’s caused by what you breathe.

What Hypersensitivity Pneumonitis Really Is

Hypersensitivity pneumonitis (HP), sometimes called extrinsic allergic alveolitis, isn’t an infection or an allergy like asthma. It’s an immune reaction in the tiny air sacs of your lungs-the alveoli-triggered by repeated breathing in of certain airborne particles. These aren’t chemicals you take orally. They’re things you inhale: bird droppings, mold from damp hay, fungi in humidifiers, or dust from hot tubs.

This condition doesn’t happen after one exposure. It takes repeated contact over weeks or months. Your immune system gets sensitized. Then, every time you breathe in the same dust or particles, your lungs swell up. That’s when coughing and breathlessness start.

The Merck Manual and other top medical sources make this clear: HP is caused by environmental antigens, not medications. If you’re thinking your new blood pressure pill or antibiotic caused your lung symptoms, you’re likely looking at a different problem-like drug-induced interstitial lung disease. That’s not HP. It’s a different disease with different causes and different lung damage.

How Cough and Breathlessness Show Up

The symptoms of HP don’t come on like a flu. They creep in, or they hit hard and fast-depending on how often you’re exposed.

In the acute form, you might feel fine in the morning, then by late afternoon, you’re sweating, shivering, coughing, and struggling to catch your breath. It usually starts 4 to 8 hours after breathing in a big dose of the trigger-like cleaning out a bird cage or walking through a moldy barn. If you get away from the source, you often feel better within a day or two.

The subacute form is sneakier. You don’t have a fever or chills. Instead, you just feel tired. Your cough sticks around. You get winded climbing stairs, even if you used to be fine. This goes on for weeks or months. People often think they’re just getting out of shape-or have a lingering cold.

Then there’s chronic HP. This is the dangerous one. You’ve been exposed to the trigger for months or years. Your lungs start to scar. The cough doesn’t go away. You lose weight without trying. Your fingers might even club-curving at the tips. At this stage, the damage can be permanent. Pulmonary fibrosis sets in. Your lungs stiffen. Breathing becomes a constant effort.

What Triggers It (And What Doesn’t)

Here’s where people get confused. Medications like amiodarone, nitrofurantoin, or chemotherapy drugs can cause lung inflammation. But that’s not hypersensitivity pneumonitis. That’s drug-induced lung injury. The immune response is different. The pattern of scarring is different. And crucially, you don’t need to inhale those drugs-they’re taken by mouth or IV.

True HP comes from things you breathe in:

• Bird proteins-from feathers, droppings, or dust around parrots, pigeons, chickens, or even pet birds. This is called bird fancier’s lung.
• Moldy hay or grain-farmers, barn workers, and people who handle stored grain get farmer’s lung.
• Fungi in humidifiers or air conditioners-especially if the water isn’t changed regularly. The bacteria in the water get aerosolized and inhaled.
• Mushroom spores-workers in mushroom farms are at risk.
• Hot tub biofilm-mold and bacteria in the warm, wet environment can become airborne when the tub is used.

These are all airborne. You have to be breathing them in. That’s why HP is called an inhalational disease.

How Doctors Diagnose It

There’s no single blood test for HP. Diagnosis is like solving a puzzle. Doctors look at five things:

1. Your history-Do your symptoms get worse at work? After cleaning the birdcage? After using the humidifier? Do they improve when you’re away from home or work?
2. Imaging-A high-resolution CT scan of your chest shows telltale signs: patchy ground-glass shadows, mosaic patterns, and air trapping (especially on exhale scans).
3. Blood tests-They check for antibodies to specific antigens. If you’ve been around birds, they’ll test for pigeon serum proteins. If you’re a farmer, they’ll test for mold from hay. These antibodies are found in 60-90% of cases.
4. Bronchoalveolar lavage-A small tube is passed into your lungs, and fluid is washed out. If more than 40% of the cells are lymphocytes (a type of white blood cell), it’s a strong clue for HP.
5. Lung biopsy-In unclear cases, a small sample of lung tissue shows poorly formed granulomas and lymphocyte buildup around the small airways. That’s the fingerprint of HP.

The key clue? Symptoms improve when you avoid the trigger. If you stop cleaning the birdcage and your cough fades, that’s powerful evidence.

Treatment: Stop the Exposure

The most important treatment isn’t a drug. It’s avoidance.

If you catch HP early-in the acute or subacute stage-and you stop breathing in the trigger, your lungs can heal completely. Studies show 70-80% of people recover fully with just removal of the antigen.

For more severe cases, doctors may prescribe short-term steroids like prednisone to calm the inflammation. But steroids won’t fix the problem if you keep breathing in the dust. You’ll just be masking it-and risking permanent damage.

In chronic HP, where scarring has already started, treatment gets harder. Steroids may help slow inflammation, but antifibrotic drugs like nintedanib (used in idiopathic pulmonary fibrosis) are now being tested in HP. One trial showed a 56% reduction in lung function decline over a year.

Pulmonary rehab-breathing exercises, walking programs, oxygen therapy-helps manage symptoms. If your oxygen levels drop below 88%, you’ll need supplemental oxygen at home.

What Happens If You Don’t Act

Ignoring HP is dangerous. The longer you’re exposed, the more your lungs scar. Once fibrosis sets in, it’s irreversible. The 5-year survival rate for chronic HP ranges from 50% to 80%, depending on how much scarring is already there.

If you’re a bird owner and your cough won’t go away, don’t assume it’s just dust. If you work in agriculture and get winded faster than your coworkers, don’t brush it off as aging. If you use a humidifier and feel worse every morning, clean it-or stop using it.

Cigarette smoking, oddly enough, seems to lower your risk of developing HP. That’s not a recommendation-it’s just a curious observation from studies. Smoking damages your lungs in other ways, so don’t use this as an excuse to smoke.

When to See a Doctor

See a pulmonologist if:

• Your cough lasts more than 2 weeks without a clear cause
• You get short of breath doing things you used to handle easily
• Your symptoms improve when you’re away from home or work
• You own birds, work with grain, or use a humidifier regularly

Don’t wait until you’re struggling to walk to the bathroom. Early diagnosis means your lungs have a real chance to recover.

Final Thought: It’s Not Your Pill-It’s Your Air

We live in a world where every symptom gets blamed on a medication. But sometimes, the culprit isn’t in your medicine cabinet. It’s in your garage, your basement, your birdcage, or your humidifier.

Hypersensitivity pneumonitis teaches us that lungs don’t just react to what you swallow. They react to what you breathe. And if you’re coughing and short of breath, it’s worth asking: what’s in the air around you?