Soiling of materials and visibility reduction are the two most important welfare effects of particulate matter. Though the relationship between fine particles and visibility reduction is well documented, the evidence for the relationship of ambient particles to soiling is scant and poorly understood. Considerable uncertainty exists concerning the amount of surface accumulated particulate matter that leads to a decision to clean, since not only are socioeconomic factors involved in such a decision, but accumulation of the particles must first be perceived as soiling. Information regarding relevant physical and chemical properties of airborne particles are reviewed (e.g. factors affecting deposition velocity and color). Considerations to assure congruency between physical measurement techniques and human observations are discussed. Factors are recommended for inclusion in development of psychophysical models to relate psychological and physical scales of soiling.