USB standards would be 5V or 5 Volts. Since the usual power bank has the usual USB socket and it's the same as some USB port (voltage) then I'd say it's OK.
HOWEVER for those that worry the ONLY person that can give more assurance is the maker of the nebulizer.
I am replacing a portable nebulizer that uses AA or USB cable. USB is ok around the house, but AA last 10-13 minutes and rechargeable last 13-15 minutes. It uses 3v. Can I use a power bank via the USB cable to run it without damage.? Or would they have too high of volage?