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

When you start coughing and feel like you can’t catch your breath, it’s easy to blame a cold, the flu, or even a new medication. But if your symptoms keep coming back after you’re away from work or home for a few days - and then flare up again when you return - something else might be going on. It’s not asthma. It’s not pneumonia. It could be hypersensitivity pneumonitis.

What Is Hypersensitivity Pneumonitis?

Hypersensitivity pneumonitis (HP) is a lung disease caused by breathing in tiny particles from the environment - not from swallowing pills or getting injections. These particles trigger your immune system to overreact, leading to inflammation deep in your lungs, specifically in the air sacs (alveoli). Over time, this inflammation can turn into scarring, or fibrosis, which makes it harder for oxygen to get into your bloodstream.

This isn’t a reaction to drugs. Despite what some people assume, medications like antibiotics, blood pressure pills, or chemotherapy rarely cause true hypersensitivity pneumonitis. What they can cause is a different kind of lung injury called drug-induced interstitial lung disease (DILD). That’s not the same thing. HP requires you to inhale something specific - and it has to happen repeatedly.

Common Triggers You Might Not Realize Are Dangerous

The real culprits behind HP are everyday things you might not think of as harmful:

• Bird proteins - from feathers, droppings, or even dust from pet birds. This is called bird fancier’s lung. People who keep parrots, pigeons, or even chickens in their garages are at risk.
• Moldy hay or grain - farmers, grain workers, and even people who store damp hay in barns can develop farmer’s lung.
• Mushroom spores - mushroom growers working in humid greenhouses often get exposed.
• Humidifiers and air conditioners - if not cleaned regularly, they can grow bacteria and fungi that get blown into the air.
• Hot tubs - the mist from warm water can carry bacteria that trigger HP, especially if the tub isn’t properly sanitized.

These aren’t rare causes. In fact, they’re the only well-documented triggers of true hypersensitivity pneumonitis. The Merck Manual, one of the most trusted medical references, lists these as the primary causes - and makes no mention of medications as a cause.

Why Cough and Breathlessness Are the Hallmarks

If you have HP, you’ll likely notice two main symptoms: a dry cough and shortness of breath. But here’s what makes it different from other lung problems:

• Timing matters - Symptoms often start 4 to 8 hours after exposure. You might feel fine at work, but by evening, you’re coughing and winded. By the next morning, you feel better - until you go back the next day.
• It’s not constant - Unlike chronic bronchitis or COPD, HP symptoms come and go with exposure. That’s why many people dismiss it as “just a bad cold” or “allergies.”
• It gets worse over time - In the early stage (acute), you might have fever, chills, and muscle aches along with cough. In the subacute phase, symptoms build slowly over weeks. In chronic HP, you might just feel tired, lose weight, and notice you can’t walk as far as you used to.

Advanced cases can lead to clubbing - where your fingertips become rounded and enlarged - and irreversible lung scarring. Once fibrosis sets in, the damage can’t be undone.

How Doctors Diagnose It (And Why It’s Tricky)

There’s no single blood test or X-ray that confirms HP. Diagnosis is like solving a puzzle:

1. History - Your doctor will ask about your job, hobbies, pets, and home environment. Did you start keeping birds last year? Are you renovating a damp basement? Have you noticed symptoms improve on vacation?
2. Imaging - A high-resolution CT scan of your chest shows patterns like ground-glass opacities, mosaic breathing, or air trapping - signs of inflammation and early scarring.
3. Blood tests - Some people develop antibodies to the triggering antigen. For example, 70% of bird fancier’s lung cases test positive for antibodies to bird proteins.
4. Bronchoalveolar lavage - A tiny tube is inserted into the lungs, and fluid is washed out. If more than 40% of the cells are lymphocytes, it points to HP.
5. Lung biopsy - In unclear cases, a small piece of lung tissue is examined. HP shows poorly formed granulomas and lymphocytes clustering around small airways - a signature pattern.

What makes this hard is that 20-30% of cases have no identifiable trigger. But even then, the pattern of symptoms and imaging often points to HP.

What You Can Do - And What You Shouldn’t

The single most important treatment? Stop breathing in the trigger.

• If it’s birds - rehome them. Don’t just clean the cage. The dust lingers in carpets, curtains, and ventilation systems.
• If it’s moldy hay - change your job or use proper respirators. A simple dust mask won’t cut it.
• If it’s a humidifier - clean it weekly with vinegar and water. Don’t use tap water; use distilled.
• If it’s a hot tub - keep chlorine levels high and drain it monthly.

Medications? Only in severe cases. Corticosteroids like prednisone may be used short-term to calm inflammation, but they don’t fix the root cause. Long-term use has side effects - weight gain, bone loss, diabetes - and won’t help if you keep breathing in the trigger.

And here’s something surprising: cigarette smokers have a lower risk of developing HP. Not because smoking is good - it’s not - but because it suppresses the immune response in the lungs. That’s why doctors don’t recommend smoking as a treatment. It’s just a grim statistical quirk.

What Happens If You Ignore It?

If caught early - especially in the acute or subacute phase - most people recover fully after avoiding the trigger. Up to 80% of acute cases bounce back with no lasting damage.

But if you keep exposing yourself? The inflammation turns to scarring. About 30-50% of chronic HP cases develop progressive fibrosis. Once that happens, survival rates drop. Five-year survival ranges from 50% to 80%, depending on how much scarring is present.

Advanced cases may need oxygen therapy, pulmonary rehab, or even a lung transplant. Transplant success rates for HP are about 50-60% at five years - similar to other severe lung diseases.

What Medications Actually Do to Your Lungs

You might be thinking: “But my doctor said my cough is from my medication.” That’s possible - but it’s not HP.

Drugs like amiodarone (for heart rhythm), nitrofurantoin (an antibiotic), and bleomycin (a chemo drug) can cause lung injury. But they don’t cause the same immune reaction as HP. Instead, they cause direct cell damage, chemical buildup, or allergic reactions that look different under the microscope.

Doctors call these drug-induced interstitial lung diseases (DILD). They’re serious - but they’re not hypersensitivity pneumonitis. Confusing the two can lead to wrong treatments. You don’t need to avoid your bird if your cough is from amiodarone. And you don’t need to stop your antibiotic if your cough is from moldy hay.

What to Do Next

If you’ve had a persistent cough and breathlessness - especially if it gets worse at work or home - ask yourself:

• Do I feel better on weekends or vacations?
• Do I have pets, a hot tub, or a humidifier?
• Have I recently started gardening, farming, or working in a damp space?
• Do I have a bird or visit someone who does?

If you answered yes to any of these, talk to a pulmonologist. Bring a list of your daily activities. Don’t assume it’s asthma or a cold. HP is rare - but it’s treatable if caught early.

Don’t wait for permanent scarring. Your lungs can heal - if you remove the trigger.