What are the dangers of cutting PCB with Dremel?
I found a MSDS datasheet for FR-4 material, which is commonly used in PCBs.

Highlights:


  Machining, grinding or sawing this material may generate harmful dusts. Continuous filament glass fiber is not
  considered flbrogenic; however, it Is woven from E-Glass fibers which are listed by IARC as "special purpose glass
  fibers" and designated as "possibility of carcinogenic in human.” Inhalation of copper fumes, while not expected to
  occur under typical conditions of use, may cause metal fume fever. See Section 8 for exposure controls.


The cited section 8 says:


  ENGINEERING CONTROLS:
  Use local exhaust when machining, grinding or sanding to minimize exposures and maintain airborne dust and fiber
  concentrations below occupational exposure limits.
  
  PERSONAL PROTECTIVE EQUIPMENT:
  RESPIRATORY PROTECTION: In the absence of adequate general or local ventilation, use a NIOSH-or local
  authority-approved respirator with at least an N95 filter if exposure limits are exceeded. 
  
  SKIN PROTECTION: Wear
  gloves during prolonged contact to avoid skin Irritation from dust.
  
  EYE PROTECTION: Use safety glasses or a face
  shield when machining, grinding or sawing this material.


So, by using a mask and fan, you've likely kept yourself reasonably safe. You might want to consider getting an electronics-grade vacuum cleaner that meets N95 to collect and contain dust that otherwise may escape into the air.