Health hazards fi om the deposition of inhaled medium-density fiberboard (MDF) particles containing urea-formaldehyde (U-F) resins, caused by the release of formaldehyde vapor from the MDF particles within the respiratory tract, have been studied by a number of researchers [1-3]. Some of the epidemiology studies indicate a link between the inhalation of formaldehyde impregnated particles and increased cancer rate [4,5]. The objective of this report is to provide a brief description of: (1) the aerodynamic properties of MDF particles, (2) the microstructural features of MDF particles containing U-F resin formulation, and (3) the respiratory tract deposition characteristics of inhaled MDF particles and the associated health hazards. Scanning Electron and Epi-Fluorescence-Optical microscope analyses of MDF particles show the particles are of widely ranging diameters (10 to 50 mu m) and lengths (10 to 1000 mu m) of irregular shape with sharp edges. A major fraction of MDF particles released during the manufacturing process have equivalent aerodynamic diameters less than 50 mu m Therefore, these particles are inhalable. The aerodynamic properties of fibers are determined primarily by their diameters, hence, they can remain airborne for a long time period and can negotiate their flow through respiratory airways. Therefore, fibers can carry relatively greater mass of chemicals per particles compared to the nonfibrous particles of equivalent aerodynamic diameter. A small fraction of MDF particles are respirable, with aerodynamic diameters smaller than 10 mu m. Sander dust samples of MDF showed presence of PM10 particulates with a count median aerodynamic diameter of 6.5 mu m. Therefore, while inhaled formaldehyde vapor does not penetrate to the deeper lung, a small fraction of the total inhaled MDF particles can penetrate to the lower respiratory tract and release formaldehyde. It is therefore necessary to establish a safe permissible exposure limit for MDF particles.