Depends on the mask. A valve or filter mask? Safe for a given period of time before it needs replaced or disposed of.
Paper or cloth? Worse than useless for anything smaller or more invasive than dirt.
No.
Useful, even if they're less useful when not used properly. Not in any case "worse than useless".
https://www.sciencedirect.com/science/article/pii/S2452199X20301481
I picked that article because it reviews different types of masks, but you'll find a lot of others too.
Check and Mate
Paper or cloth? Worse than useless for anything smaller or more invasive than dirt.
Useful, even if they're less useful when not used properly. Not in any case "worse than useless".
https://www.sciencedirect.com/science/article/pii/S2452199X20301481
I picked that article because it reviews different types of masks, but you'll find a lot of others too.