MYTH: The library’s fire-extinguishing system removes the air from the book stacks in the event of a conflagration, dooming any librarians inside to a slow death by asphyxiation.
MOSTLY FALSE: According to Jones, this legend has a kernel of truth:
Instead of water sprinklers that would harm the rare books
collections, he said, a combination of halon and Inergen gases would
be pumped into the stacks to stop the combustion process, and thus the
spread of fire.
“They do lower the percentage of oxygen, but not enough to kill any
librarians,” Jones said.
For anyone who was interested but couldn't be arsed to look it up, normal* air is 21% oxygen, British Library is 15%, and it sounds like it's done by adding extra nitrogen.
** Not sure whether Euston Road counts as normal air
Done the google, though quickly, and can confirm reduced oxygen levels at long term storage facilities
https://www.ifla.org/wp-content/uploads/2019/05/assets/pac/British-Library-low-oxygen-case-study.pdf
From a post about another library
https://yaledailynews.com/blog/2010/02/04/myths-abound-about-beinecke/
So true for the long term storage areas, but not for the open to public areas afaik. Carry on.