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000123 vapor in the atmosphere. (Prior to sublimation, the dry ice will cool a portion of the cloud sufficiently to freeze water driplets which, in turn, can begin a growth process that may result in precipitation reaching the ground.) 'Carbon dioxide is a natural constituent of the atmosphere, present normally in a concentration of about 400 ppm by weight. Carbon dioxide is added to the atmosphere as a result of the decomposition of vegetable matter and the combustion of carbon-containing fuels. It is withdrawn from the atmosphere by growing vegetation as part of the process of photosynthesis of carbohydrates. Carbon dioxide is highly water-soluble and it is exchanged constantly between the atmosphere and the oceans. Within the atmos- phere, it moves freely in response to large-scale pro- cesses of diffusion.' Thus, because the sublimation process simply returns carbon dioxide vapor to the atmos- phere from whence it came in comparatively insignificant amounts, it is environmentally benign as a cloud seeding agent. "The programmatic Statement states the following concerning silver iodide release from ground-based generators: 'The American Conference of Governmental Industrial Hygienists has set a threshold limit value for silver in workroom airj under conditions of continuous exposure, at 0.01 mg/m. According to a study made for the National Institute of Environmental Health Sciences, atmospheric silver emissions have yet to produce any reported ambient air concentrations approaching this threshold limit value. This is so despite the presence in many parts of the country of industrial areas that emit more than a hundred times as much silver into the atmosphere as a cloud-seeding generator and do so almost continuously. For this reason it is considered that the ground-based silver iodide smoke generator does not constitute a health hazard provided continuous exposure at a very short distance from the generator is avoided.' [4, p. B-2] "The programmatic Environmental Statement continues: 'The pathways followed by silver iodide after it reaches the ground are uncertain. Accumulations of silver iodide from cloud seeding in soil, vegetation, and surface waters have not been large enough to measure. It has not been found in surface waters such as streams and lakes in the same concentration found in newly fallen precipitation. It is likely that some of the silver iodide is trapped in the uppermost layers of the ground and that some of it is carried by percolating water or other transport processes to deeper layers of the soil or underwater muds where it is sequestered from 26 - ~ ~ ~ - ~ ~~---~- = ~ - = = ~ -- ~ ~