Are you asking if it is mold dust or if breathing it in general is a good idea?

Either way you should protect yourself with a dust mask at minimum. The dust can have a wide range of things in it from harmles ash to mold or spores from rat feces.

Its always best to protect yourself and your lungs. A house that old would have been heated with firewood and the soot from that gets everywhere. If in doubt have an inspection done.

Certainly a concern!

Imagine what has occurred in the last 180 years!The dust should be contained,the ceiling could have (like califdadof3 said) anything on top of it from feces to dust from fireplaces,oil furnaces,bird droppings,raccoon,anything that might have nested in there including bats.

The removal and exposure should be kept to a minimum,wet it down with a garden sprayer to keep it limited,wear masks and gloves,clean any cuts as soon as you can,etc.

This can be bad,or it can be nothing... it's up to you the risks.