What is this white chalky powder under the carpet and underlay

[cherryblossom123]

Hi, I'm replacing my carpet. Just removed the old carpet and underlay but there is a chalky white substance underneath the underlay. Wondering if this is anything to be conserned about.

It is only present on the upstairs landing, wasn't on the stairs or other rooms I've decorated previously. It's quite chalky and powdery, when I stood on it a bit transferred to me shoes and a bit dusty.

 

[Fred Blogs]

It's from the foam carpet backing. It is breaking down and gone chalky. I had that when we bought the house we are in now. Al the carpets were quite old and that chalky substance was under most of them.

 

[Wayners]

Maybe a vacuum bag split and the dust went through carpet.
It's possible the dust was left over from renovation and carpet layed over.
You don't think they carpet backing has crumbled?