Cellulose ceiling tiles (asbestos?)
Do you know anything about the building you have them in?  Age of the building, dates of renovations?  If you can pinpoint that it might rule out the asbestos for you.  Asbestos building materials were made illegal in North America and if you can find someone (an older person with some years in) at that place they might be able to help give you some information.  If you haven’t been working with those tiles your exposure would be minimal, for asbestos to be dangerous it must be air born and the amount of exposure determines your health risk.  So if those tiles were broken a long time ago and you didn’t disturb them in your work your exposure would be minimal.  If you have been working with them it does increase your risk unfortunately.