Hypersensitivity pneumonitis is what happens when lungs react to something you keep breathing in: moldy hay, bird droppings, a contaminated humidifier, metalworking mist. Doctors used to call it extrinsic allergic alveolitis, and the old names still help. Bird fancier's lung. Farmer's lung. The pattern is delayed, which is exactly why it gets missed. Symptoms typically appear 4 to 8 hours after exposure, so someone who cleans a pigeon coop at noon feels fine and is coughing and feverish by dinner.
This is not asthma, and it is not an allergy in the sneezing sense. It is an immune reaction inside the air sacs, and it can scar lung tissue when the trigger stays in the air. Registry series report 5-year mortality near 20 percent once fibrosis is visible on CT, which is why decisions made in the first year matter more than any supplement on this page. Here is what drives it, what it feels like, and the mistakes that keep lungs inflamed long after the source is gone.
Causes
Roughly 200 organic antigens can set off hypersensitivity pneumonitis, and they share one feature: they are small enough to reach the air sacs. Mold spores from hay and compost, proteins in bird droppings and feather dust, contaminated humidifiers and air conditioners, metalworking fluids, and Mycobacterium avium colonizing a warm hot tub. The reaction is not immediate. Immune complexes and T cells build over hours, so cough and fever arrive 4 to 8 hours after the dust settles. That delay is the single biggest reason the diagnosis gets missed for years.
Dose and duration shape the disease
Screening studies in pigeon keepers have found HP in roughly 1 to 5 percent of people with regular exposure, which tells you ventilation and genetics matter as much as the birds themselves. High-dose bursts, like mucking out moldy hay, produce the acute, flu-like version. Low-dose daily exposure, like a cockatiel in a small bedroom, produces the slow version: creeping breathlessness, a nagging dry cough, falling exercise tolerance. Both end in the same place if the antigen stays airborne.
Two genetic signals are worth knowing. Certain HLA variants, HLA-DR7 among them, raise susceptibility, and a MUC5B promoter variant is linked to a higher risk of the fibrotic pattern on CT. A family history of interstitial lung disease is a real risk factor here, not background noise.
What does not cause it
Smoking is the strange one. Most case series report fewer current smokers among HP patients than expected, a protective association that flips once fibrosis appears and outcomes worsen. Pet hair on its own does not trigger it; the protein has to be airborne and inhalable. Household dust is rarely the culprit unless a moisture problem is feeding mold. If nobody has asked you about birds, hot tubs, humidifiers, down bedding, or your workplace, that conversation is the missing piece.
Symptoms
Acute HP feels like flu that arrives on schedule. Four to eight hours after exposure: chills, a temperature that can reach 38.5 C, aching muscles, dry cough, tight chest. Everything fades over 24 to 48 hours away from the trigger, which is why people blame a recurring virus. If you can draw a line from coop cleaning on Saturday to a fever on Saturday night, that pattern is the diagnosis talking.
Chronic HP is quieter and more dangerous. Breathlessness that used to show up on hills shows up on stairs, then on the flat. A dry cough that never quite clears. Fatigue, appetite loss, and 5 to 10 percent of body weight gone without trying. Fine crackles at both lung bases and clubbing of the fingertips turn up in a substantial share of chronic cases, best heard after a few deep breaths.
- Reach for urgent care if lips or fingertips turn blue, if you are breathless at rest, or if crossing a room leaves you unable to finish a sentence.
- Two tests do most of the work: spirometry and a 6-minute walk test. A restrictive pattern, meaning low FVC with a normal or high FEV1/FVC ratio, plus oxygen desaturation while walking, is the usual signature.
- Asthma behaves differently. HP rarely wheezes and does not loosen within minutes of a bronchodilator, and it does not run on the same IgE pathway as hay fever. A drip down the back of the throat that keeps you coughing points somewhere else entirely, which is why persistent post-nasal drip is worth ruling out before you accept a lung diagnosis.
- Keep a two-week diary that timestamps symptoms against exposures. Dated notes beat a vague memory of sometimes coughing when you are sitting in a pulmonologist's office.
Natural Remedies
No herb undoes ongoing antigen exposure. Removal comes first, and it is not a soft recommendation. If the source is a bird, rehoming stops progression in most people who have not yet developed fibrosis; if it is a humidifier, replacing it costs less than a year of steroids.
Track the antigen down in week one
Walk through the house and the workplace with a notebook. Birds, rodents, down pillows and comforters, damp drywall, drip trays, dehumidifier reservoirs, indoor compost, hot tub covers, and metalworking fluid mist are the usual suspects. Ask your employer for safety data sheets on any mist or fluid you breathe. If nothing indoors fits, a pulmonology referral for antigen-specific IgG testing and possibly a controlled exposure challenge is the next step. The 2020 American Thoracic Society guideline with JRS and ALAT partners laid out how that workup should run, including when CT patterns point to the fibrotic form.
Clean the air while the lungs settle
Keep indoor relative humidity between 30 and 50 percent. Above 50 percent, mold has what it needs. Run a HEPA H13 filter in the bedroom, sized for the room, and vacuum or change the prefilter every 2 to 4 weeks. Skip ozone generators entirely; they irritate airways and do nothing for antigen load. In a basement that smells musty, a dehumidifier is not optional.
Rebuild breathing capacity
Steam inhalation eases the cough: 10 minutes over a bowl of water at about 40 C, towel over the head, twice daily during flare-ups. Do not use boiling water, and never add essential oils for a child under 5. Then train the muscles. Inspiratory muscle training at 30 to 40 percent of maximal inspiratory pressure, 30 breaths twice daily for 6 weeks, improves dyspnea scores in small trials of restrictive lung disease, and slow breathing work overlaps with what helps in chronic hyperventilation syndrome. Pursed-lip breathing, 4 seconds in and 8 seconds out, keeps small airways open when you are short of breath. Walk daily; the 6-minute distance tracks recovery better than how you feel. If weight loss has cost you muscle, aim for 1.2 g of protein per kilogram of body weight, and ask about vitamin D, since 1000 to 2000 IU daily is cheap and deficiency is common in chronic lung disease.
Herbal Treatments
Herbs will not remove antigen, and none reverse fibrosis. Several can calm inflammatory tone in the airways, loosen secretions, and make coughing less exhausting while the immune system settles. The evidence ranges from thin to moderate, and the NIH's complementary health center, NCCIH, is blunt that most turmeric trials are small and short, which is a fair description of this whole category.
- Turmeric (curcumin). The best-studied anti-inflammatory here in general terms, though trials in HP specifically are essentially absent. Lung and joint studies usually land at 500 mg of curcuminoids twice daily, taken with black pepper or a lipid carrier. A 1998 study in Planta Medica found piperine raised curcumin absorption roughly 20-fold, which is why turmeric powder stirred into food does very little. Give it 6 to 8 weeks before judging. Avoid it alongside warfarin, apixaban, rivaroxaban, or clopidogrel, and stop 2 weeks before surgery. Skip it if you have gallstones.
- Thyme is the traditional expectorant for a reason: thymol and carvacrol thin mucus and relax bronchial smooth muscle in laboratory work. Steep 1 to 2 g of dried leaf in 250 ml of just-boiled water, covered, for 10 minutes, and drink up to 3 cups a day. Thyme-ivy combinations carry the most clinical data, mostly in acute bronchitis rather than HP, so treat this as secretion management and comfort. Large medicinal doses are not for pregnancy.
- For a raw, tickly cough that keeps you awake, marshmallow root coats the throat. A cold infusion works better than tea: 2 to 3 g of dried root in 250 ml of cold water, left 4 to 8 hours, then strained. Take it 1 hour away from any oral medication, because mucilage can slow the absorption of everything else you swallow. The trial evidence here is thin; the tradition is older than the data.
- Elecampane (Inula helenium) has been used for centuries for deep chest congestion, and its alantolactone content shows antimicrobial and anti-inflammatory activity in vitro. No good human trial exists. A typical preparation is 1 to 2 g of dried root simmered 15 minutes, or 1 to 2 ml of a 1:5 tincture three times daily, for no more than 2 to 3 weeks at a stretch. Avoid in pregnancy. Skip it if you react to ragweed, chamomile, or echinacea, since it sits in the same plant family, and expect skin irritation if you handle the fresh root.
Prevention
Ranked honestly, what changes the odds is getting the antigen out of your air, keeping humidity down, and watching lung function over years rather than months. Everything else is support.
If birds stay, that is a decision to make with clear eyes. Moving a cage to the garage does not fix a shared air supply, because avian protein travels. Keepers who reduce exposure with ventilation and masks often stabilize, but their fibrosis risk runs higher than that of people who rehome. For farmers, drying hay below 20 percent moisture before baling prevents the thermophilic actinomycete growth behind farmer's lung, and an N95 during barn work is cheap insurance.
Home measures that genuinely work: fix leaks within 48 hours, run exhaust fans in bathrooms and kitchens, clean humidifier and dehumidifier reservoirs weekly with diluted vinegar, and swap furnace filters every 2 to 3 months during heavy use. If you have hot tub lung, the fix is the tub's maintenance or the tub itself, not a supplement. Influenza and pneumococcal vaccines matter too, since respiratory infections push scarred lungs into trouble faster than anything else on this list.
Finally, keep a baseline. Spirometry and a 6-minute walk test once or twice a year, plus a CT when your doctor calls for it, show whether the disease is quiet or creeping. Symptoms lag behind scarring by a wide margin.
