Objectives: Dental personnel are more at risk to develop asthmatic disease, but the exact reason is so far unknown. During abrasive procedures, dental personnel are exposed to nano-sized dust particles released from dental composite. The aim of this study was to investigate whether respirable composite dust may also release monomers. Methods: Respirable (<5 mu m) composite dust was collected and the release of methacrylate monomers and Bisphenol A (BPA) in water and ethanol was evaluated by liquid chromatography/mass spectroscopy (LC-MS/MS). The dust was ultra-morphologically and chemically analyzed by transmission electron microscopy and energy-dispersive X-ray spectroscopy (TEM-EDS). Results: LC-MS/MS analysis revealed that, irrespective of the type of composite, the respirable fraction of composite dust may release relatively high concentrations of unpolymerized methacrylate monomers, both in water and ethanol. Higher release was observed in ethanol. The endocrine disruptor BPA also emanated from the composite dust particles. TEM showed that most particles were nano-sized, although particle size ranged between 6 nm and 5 mm with a mode value between 12 and 39 nm. Most particles consisted of several filler particles in resin matrix, although single nano-filler particles could also be observed. Elemental analysis by TEM-EDS proved that the particles collected on the filters originated from the dental composites. Conclusion: Theoretically, composite dust may function as a vehicle to transport monomers deeply into the respiratory system. The results of this study may shed another light on the increasing incidence of respiratory disease among dental personnel, and more care should be taken to prevent inhalation of composite dust. Clinical significance: Special care should be taken to prevent inhalation of composite dust, as the dust particles may release methacrylate monomers. (C) 2017 Elsevier Ltd. All rights reserved.