As with all airborne particulates, wood dust particle sizes are classified with regard to effect on the human respiratory system.  For this classification, the unit for measurement of particle sizes is the micrometre or micron (μm), where 1 micrometre = 1 micron. Particles below 50 μm are not normally visible to the naked human eye. Particles of concern for human respiratory health are those <100 μm (where the symbol < means ‘less than’).
Zhang (2004) has defined the size of indoor particulates according to respiratory fraction:
Particles which precipitate in the vicinity of the mouth and eyes, and get into the organism, are defined as the inhalable fraction, that is total dust. Smaller fractions, penetrating into the non-cartilage respiratory tract, are defined as respirable dust. Dust emitted in the wood industry is characterized by the dimensional disintegration of particles up to 5 μm, and that
is why they precipitate mostly in the nasal cavity, increasing the risk of cancer of the upper respiratory tract.