What is this?

Inspecting an attic with loose fiberglass blown it. This was sitting on top of 4" of wood chips. I always check to see if there is a vapor barrier present. When I moved the top 4" of insulation I found this white substance underneath. Can someone tell me what this might be? Limited soffit vents (4) with attic baffles. 2 gable vents one vent blocked with debris facing street. Could this be from years of street cleaning? This is a corner house adjacent to a busy street. The insulation was very dirty at the gable vent. Built 1963.

Looks like degraded UFFI to me

It was on top of the wood chips, but some places did not have the substance. When disturbed it was a fine dust. They put up uffi in walls and wood products like press board. Even if it was uffi would it still have any negative effects to the occupants? 50 years after it was put in?

Minimal if any heath effects after this many years…

I read the article and found it very informative. It answered all my questions and concerns.
Thank you.