ABSTRACT
Various boundaries are in current use for both the upper and lower limits for the diameter of ultrafine particles. Ultrafine particles include those in the atmospheric nuclei mode, plus some of the smaller particles classified as “accumulation” mode particles. The lower limit is generally regarded as about 1 nm, but some reports refer to those up to 10 nm as “nanometer” particles, reserving ultrafine for particles with diameters larger than 10 nm. A convenient upper limit of 100 nm is frequently used. This is because a 100 nm diameter particle is an acceptable lower size limit to the accumulation mode, and an upper limit to the nucleation mode (modes are defined in Chapter 2). In this size range, particle aerodynamic behavior is dominated by Brownian diffusion, and particle size is adequately described by a thermodynamic diameter. The thermodynamic diameter is the
diameter of a sphere of unit density that would have the same diffusion coefficient in air as the particle of interest. An upper limit value of 150 nm is chosen in this chapter, because it represents the particle size at which gravitational and inertial effects are of little importance when particles are inhaled. Thus, ultrafine particle behavior in human airways is dominated by diffusion.
