On Tue, 04 May 2021 12:45:11 -0400, micky posted for all of us to digest...


One of my n'bors is getting her house's air ducts cleaned now.

It seems to be that the dirt and dust that is loose gets blown into the
house every day with no need for a cleaning service (and before the
cleaning service can arrive), and the dirt and dust that isn't loose,
that requires a cleaning service, just sits there in the duct anyhow and
doesn't bother anyone.

When did cleaning air ducts become a desirable thing?

When it was noted that money could be made from it.

House dust is composed of 50% skin cells.

Further research is left to the OP.

--
Tekkie