A Chinese couple recently discovered that their beloved home book collection has been slowly poisoning them since last year.

After developing a mysterious condition that had them constantly coughing, the unnamed pair of book-lovers from Taizhou in Jiangsu Province were informed by doctors that they were exhibiting symptoms of formaldehyde poisoning.

According to Jiangsu TV (via SCMP), even their child had developed rhinitis due to the inhalation of the toxic chemical compound.

The doctors, however, were not initially certain what was causing the illness and the couple reportedly spent around 8,000 yuan ($1,179) on treatment and medicines that did not help them.

Realizing that the air in their home could probably be to blame after some research, the woman of the house requested that local authorities to test the air quality in their home.

It was then found that there were already dangerous levels of formaldehyde in every room of their apartment. They also discovered that there were some areas in the home where the concentration of formaldehyde was alarmingly high.

The naturally occurring organic compound, which is found in many household products, is known to pose a significant danger to human health and has been “known to be a human carcinogen.“

Based on China’s national standards, a reading of over 0.08 milligrams of formaldehyde per square meter is already considered unsafe. After the air tests, they found that the level of formaldehyde concentration in the couple’s bedroom was 0.10 milligrams per square meter.

Eventually, they were able to identify the main source of formaldehyde pollution after testing the air quality near their bookshelves, which registered the highest level of 0.26 milligrams per square meter. Apparently, the formaldehyde was present in the ink used for printing books.

It was revealed in the report that the pair are actually avid book readers who bought books three to four times per month online.

Their home library currently holds tens of thousands of books, which experts are now suggesting the couple gets rid off to save themselves from further harm.

As for their doctors, they reportedly advised the couple to store the huge collection of books to one room and improve ventilation in their home.