On Tuesday, May 4, 2021 at 12:45:20 PM UTC-4, micky wrote:
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 we started panicking about mold.

I don't often agree with you but I'm with you on this one. If it's loose it will get blown out, if it's not it won't hurt anything.

But we do it all the time where I work.

People think the dirt comes from outside and cleaning the ducts will make the house cleaner. Actually residential systems don't have ventilation air. All the dirt in your vents came from your house, sucked up into the return duct. If you want cleaner ducts, clean your house more often.