further subdivided by the driving force for the resuspension: wind or mechanical disturbance. Local resuspension occurs as a result of a localized disturbance of the soil causing material to become airborne. The exposure is to only one or a few individuals in the immediate vicinity of the disturbance. In general, meteorological conditions play a minor part. An illustration of local resuspension would be an in- dividual digging in the area and producing a cloud of dust in his breathing zone. An interesting variation, which should be given a different name such as "transfer resuspension," is the movement of contamination from the soil to another object from which an increased probability of inhalation may occur, usually at a later time. An example of this is the individual digging in the ground, contaminating his clothing, with the contamination released by the movement of the fibers in a region close to the nostrils. shirt over one's head. An extreme example would be pulling a contaminated In the application of resuspension data to various problems, three pri- mary models have been used: the resuspension factor, the mass-loading of the air, and the resuspension rate. These models are discussed briefly before noting the appropriate use. The resuspension factor is the ratio of the concentration of contaminant in the air to the quantity of contaminant per unit area at the location where the air is sampled. Thus, the value is in length-', usually m-'. It is the oldest of the descriptive models. Although frequently attributed to Langham, it was used in the late 40's with numerical values obtained from data on dust in the air during demolition operations in the rebuilding of London. Its advantages are its simplicity and the fact that nearly all of the information that has been gathered up to a few years ago has been expressed in this form. Thus, values are available for use. Its disadvantages are 1) the Jack of a built-in classification as to the type of terrain and cause of the resuspension, and 2) the denominator expresses only the very local condition while the numerator, for general resuspension, includes the integrated effect from upwind. .Thus, if the area upwind contains a "hot spot" the resuspension factor will be high. The lack of classification is wel] illustrated by Mishima's tabulation (Mishima, 1964) where the values vary by 13 orders of magnitude. However, in addition to the apparent change in resuspension with aging, some of the results represent wind resuspension, others mechanical resuspension upwind and, still others, local resuspension from automotive traffic. For more closely defined activities one should be able to reduce the range of values for these activities by a wide factor. The mass-Joading approach considers the contaminant in the air to be directly associated with the soils in the immediate region. The air concentration is estimated by multiplying the concentration contaminant in the soil by the concentration of dust in the air. Its advantage is, again, the simplicity of application. However, for general resuspension, it suffers from much the same difficulties as the resuspension factor in describing the effect of a source upwind. One problem, which is not well defined, is the potential partitioning of the plutonium in the soi] particles. Not all particles have equal probability of resuspension 212 and the particle size distribution in the air is distorted by preferential deposition. These can lead to distortion even if particle size analysis is done. In spite of these difficulties, however, Anspaugh et al. (1975) have shown some interesting correlations with total soil content. The first documented use of the resuspension rate was in a paper in the second Geneva Conference (Healy and Fuquay, 1958). The resuspension rate describes the fraction of the material on the ground that becomes airborne under the influence of a given action. This provides a “source term" or the quantity entering the air per unit time, usually per second. The dispersion downwind and subsequent deposition can be obtained by use of the meteorological correlations which are used in dispersion calculations. The major advantage of the resuspension rate is that ft permits realistic estimates of the contribution from areas upwind. Such estimates are not simple, however, since they involve numerical integration over a predetermined deposition pattern. Disadvantages, aside from complexity, are primarily the lack of accepted values for the resuspension rate in different terrains and the variation of this rate with such variables as wind-speed or soil conditions. Data are being obtained, however, primarily through the work of Anspaugh (Anspaugh and Phelps, 1974) at the Nevada Test Site, Sehmel (Sehmel and Lloyd, 1976) with tracers at Hanford and plutonium in the Rocky Flats area, and Gillette (Gillette and Blifford, 1974; Gillette et al., 1972) in studies of the resuspension of soil in agricul- tural fields. Tt Ts believed that future extension of this work will permit choosing realistic values for application in other areas. It should be noted, however, that this is a far more sophisticated approach than the resuspension factor in that it requires knowledge of the contamination contours, the characteristics of the soils in the area, and the detailed meteorological patterns. Thus, it is doubted that a “handy-dandy" rule of thumb to be applied to only a few measurements will ever be derfved from this concept. Earlier we mentioned the types of resuspension. application of the basic models to these types. We will now discuss the Before that, however, I would like to recall a long series of discussions with Wright Langham concerning the relative merits of the resuspension factor and the resuspension rate. I had always felt the resuspension factor to be too simplistic in its concept and provided unreal answers. Finally, Wright, in his pragmatic manner, replied that he didn't see what the fuss was all about. If the resuspension factor is measured under the worst conditions of soil disturbance, it will give a safe answer. correct. I could not really answer him since, for use in this manner, he was For general resuspension, which implies the estimate of ambient concentra- tion from material arising from an upwind area, the resuspension rate should be the method of choice. This technique permits the concentration at different positions in and around the contaminated area with nonuniform deposition to be estimated and accounts for different meteorological and soil conditions. It is of interest that a large portion of the values for the resuspension rates now available have been measured for short time periods so that the variations in meteorological conditions