When a 43-year-old patient reported in with severe lung inflammation, doctors weren’t really sure what to do. The man (whose identity was not disclosed) was not a smoker, but he reported intense fatigue and breathlessness, even when carrying out the simplest of tasks.

His state worsened up to the point where his symptoms would become unbearable. He could only stand for a few minutes without feeling as if he was about to pass out. He did little other than sleep all day and was not capable of work.

Doctors quizzed him about potential causes for these symptoms. He had a bit of mold in the bathroom, but that was very unlikely to be the cause. He also owned a cat and a dog, but no birds — so again, these were unlikely to be responsible. Doctors particularly asked about birds (as they should in this scenario) because the inhalation of dust from duck feathers can cause similar symptoms — it is not uncommon for bird owners to come up with similar symptoms. But if the man was not in contact with any birds, this avenue was not pursued.

Nevertheless, the blood tests came out normal, aside for a test for avian precipitins — bird antigens.

“Based on our patient’s initial history of fatigue, malaise and breathlessness together with his CXR, we considered interstitial lung disease as the most likely diagnosis,” write the doctors in a study published in BMJ Case Reports.

But where could the contact with birds come from? The eureka moment came when the man told doctors he had purchased a feather duvet. During the last half century, feathers stuffed linens have increased massively in popularity. In the UK alone, over 10 millions are sold every year.

The man was suffering from what can be described as feather duvet lung — a type of hypersensitivity pneumonitis — a condition in which the air sacs and airways in the lungs become severely inflamed as a result of the body’s exaggerated immune response to a particular trigger. “Feather duvet lung” essentially is an inflammatory reaction to goose or duck feathers in linens (most commonly pillows or duvets)

When they realized what the problem was, doctors told him to switch to a synthetic duvet. They prescribed steroids which had an almost immediate positive effect. After a week, the patient was feeling much better, and within 6 months, his symptoms were entirely gone.

This is just one case, but the researchers caution that since so many feather duvets are sold, there’s a good likelihood that many cases go entirely unnoticed. They recommend that doctors diagnosing patients with similar symptoms should consider that feather duvets can also be a source of exposure. If untreated, this can lead to progressive and irreversible lung fibrosis.

This is something you should pay attention to if you are using feather linens — or better yet, avoid using them at all.

The study has been published in BMJ Case Reports.