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<br />material. make it unlikely that other agents. \....ith the exception of dry ice, will be used on a large <br />scale. unless there are improvements in delivery systems and major changes in the economics of <br />silver availability:' In the same book a summ.u)' of potcntial impacts on humans is as follows: <br />"The effects on humans of ingestion or topical contact with silver iodide used in cloud seeding <br />can be considercd negligible. Decade-long observations of cases (unrelated to cloud seeding) of <br />ingestion of large silver doses revealed no physiological concern. In addition. surveys of secding <br />generator operators who have had long-term intensive contact with silver iodide reveal that they <br />have not expcrienced medical ditTicuhies." <br /> <br />A report prepared by Tom Ryan (Ryan. 2005) of the Metropolitan Water District of <br />Southern California contains the following summary on the topic of possible toxicity of silver <br />iodide: <br />There has been a concern about the toxicil)' of the mosl ('ommon cloud seeding <br />malerial. silver iodide (Agl) on Ihe environment. l11e IJ11ical concenlration (~fsilver in <br />rainwater or snowfrom a s!'eded cloud is less than 0, I micrograms perliteI'. The <br />Em:ironmental Protection Agency recOImnend\. that the concentration afsilver in <br />drinking water not exceed o. 10 milligrams per liter of water, Many regions have milch <br />higher concentrations ofsilrer in the soil than are fiJUnd in .\'eeded clouds. Indllstryemits <br />100 times as milch sil1'er into the atmosphere in many parts oj the country, and silver <br />from seeding isfar exceeded by indil-'idual exposure from tooth filling~. The <br />('ollcentratioll of ioc/ine in iodized sallllsed onfood is fill' ahove the ('ollcentrationfinmd <br />in rainwater from a seeded storm. /11'0 signijicllfll environmeflllll effects JIll\'e been noted <br />around o{X'rafional projects, mWl)' of which have been in operation/hI' 30 to./O years <br />IWAtA. 1996). <br /> <br />The conccntration of silver in rain water or sno\v from a seeded cloud using the above <br />information is on the order of 1000 times less than the EPA Standard. <br /> <br />Sccdin~ Susllcnsions <br /> <br />Almost all oftoday's cloud seeding program designs iuentify situations in which seeding <br />activitics should be suspendcd. Example:s of reasons for suspcnsions may include avalanche <br />warnings. l1ash 1100d warnings. and excess snowpack accumulation. The last type of suspensions <br />insures that cloud seeding docs not result in sno\....packs that excccd long-lcrm historical <br />maximum values. This factor was considcrcd in lhe consideration of potential environmental <br />impact studics such as that referenced in the Steinhoff study (1976). <br /> <br />.7.0 Potential Legal Issues <br /> <br />There are ccrtain licensing and pennit rcquiremcnts in each of the four states of Arizona. <br />Colorado. Utah and Wyoming related pertaining to the conduct of weather modificalion (cloud <br />secding) programs. Somc special consideration may need to be given to be needed for pOlenlial <br />target areas that straddle state lines (e.g. the Sierra MadreiPark Range eomplcx in northern <br />Colorado and southern Wyoming). In thcse cases would pennits be reqllircd from both states or <br />could ajoint permitting system be deHlopcd? Additional water gcnerated from cloud seeding <br />activities is typically trcated as natural water that is approprialed according to lhe existing waler <br />