I have been told that some disposable dust masks are reusable, is that true?

Surprisingly, this is true. You will find “NR” - not reusable or “R” - reusable marked on the disposable dust mask. The reusable masks must be carefully stored to ensure that the contamination from the outside of the mask does not cross-contaminate into the inside the mask as the inner face seal should be kept clean. In practice, it can be difficult to keep a disposable dust mask in a way in which it can be reused. 

All FFP (filtering facepieces) dust masks are disposable and aligned to an 8-hour working life expectancy although there will be times when you will decide otherwise on inspection/compromise.