No single measurement can adequately describe the soiling ability of airborne particulates or the soiled appearance of surfaces on which these particulates are deposited. While concentration and size distribution give valuable information, they tell nothing of refraction and absorption indices, shape, and orientation, all of which determine the optical properties and thus the soiling ability of the particles. Since a complete analysis of individual particles is impractical, it becomes necessary to measure convenient overall properties of the particulates, e.g. filter paper staining ability, which, it is hoped, may be related to surface soiling. Such tests are relatively easy to conduct, but it is important to understand what a given test measures and how it evaluates the particulates.

This paper compares four types of particulate measurements with respect to particle size: filter paper staining, forward light scattering by airborne particles, electrical charge carrying ability, and light scattering by particles deposited on a front surface mirror.