ABSTRACT
Ultrafine particles are defined as particles with dimensions smaller than 0.1 µm. The smallest airborne particles are clusters of a few molecules with dimensions of about 2 nm. Smaller clusters are unstable and quickly disperse into individual molecules. Therefore, the limits of particle size for this discussion are 0.002 to 0.1 µm or 2 to 100 nm. High-efficiency particulate air (HEPA) and ultralow penetration air (ULPA) filters collect nearly all particles smaller than 10 nm in a single pass by the mechanism of Brownian diffusion. Most nuclei particles in a clean room probably come from internal sources. Ultrafine particles are only one part of nonvolatile residue. P. Lilienfeld (1986) describes the complexities of using light-scattering to detect ultrafine particles on surfaces. Several people, including Lilienfeld, have proposed condensation growth, analogous to the condensation nucleus counting method for airborne particles, to grow ultrafine particles on surfaces until they can be counted by ordinary surface scanners.
