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Ecological Effects 1"\. 'I storms range from 10-12 to 45 X 10-10 g ml-1 (Table 2). Typical values are 10-1. to 3 X 10-1. g ml-'. This is of the same order as the concen- tration of Ag in normal sea water, 1.5 to 3.0 X 10-1. g ml-'. Precipitation of average concentra- tion from seeded storms would deliver 0.01 to 0.03 g Ag per hectare per centimeter of rain. There do not seem to be comparable measure- ments of iodine m precipitation from seeded storms, largely because of the much greater atmospheric abundance of iodine. However, since AgI is 54% I and 46% Ag by weight, the two elements are .added in about equal amounts. \ ECOLOGICAL EFFECTS OF IODINE IN SEEDING AGENTS There is little likelihood of environmental effects from the iodine in AgI. A human con- sumer would have to drink 130 gallons of rain from a storm seeded with AgI to obtain as much iodine as there is in eggs seasoned with iodized salt [Douglas, 1968b]. Iodine is an essential trace element for most organisms. It is poisonous in excess, but instances of toxicity from naturally occurring iodine are very rare [Bowen, 1966]. Iodine is almost ubiquitous in organic and inorganic environments. Until about ~934, seaweed was commonly burned along the Atlantic seaboard of Europe to produce iodine. Iodine vapor escaped to the atmosphere in the process and was regularly detected over most of Europe, but no biological effects were ever reported [Junge, 1963]. Iodine in AgI at antic- ipated levels of use will apparently produce no adverse ecological reactions. t TABLE 1. Hourly Silver Iodide Outputs Reported for Steady State Generators 1 -\ AgI Burn Rate (g Ihr ) Reference 6 7..5 20 21 24 132 160 258 270 276 300 1589 Davis and Steele [1968J Schleusner [1968] Reinking and Grant [1968J Davis and Steele [1968J Davis and Steele [1968] Davis and Steele [1968] Bollay [196.5J Davis and Steele [1968J Davis and Steele [1968] Davis and Steele [1968J Koscielski and Dennis [1968J Butchbaker [1968J -.L~_-,. I :'~~i':. 89 TABLE 2. Silver in Precipitation from Seeded and Unseeded Storms and in Natural Fresh Waters Concentration (g X 10-" iml) Reference Seeded storms 10-1760 20-200 1-700 1Q-4500 (snow) Bollay [19661 Douglas [1968a] Warburton and Young [1968b} Warburton and Young [1968a J U nseeded storms 0-20 Douglas [1968aJ 0-20 (snow) Robertson [1968] 440 lakes in northern Maine 10-3500 (mean 94) Kleinkopj [1960] Major North American rivers 0-940 (median 90) Durum and Ha/tty [1963] CHEMICAL AND PHYSIOLOGICAL ACTIVITY OF SILVER Silver, in some respects the most toxic of the heavy metals, presents a different picture. Silver is a paradoxical substance: extensively used in industry because of its great potency as a microbial poison, it is relatively harmless to higher animals, including man. It is almost unique among metals in combining very low solubility of most of its compounds with ex- ceedingly high toxicity of the soluble fraction. Its ecologic a] behavior is primarily due to this combination of properties. For silver to be toxic in low doses, the ionic form of the metal must come into direct contact with metabolically active sites, such as the cell membranes of microorganisms or the gas ex- change surfaces of fish gills. Ag,S, which barely ionizes, is biologically almost inactive despite its relatively high solubility. AgI, much less soluble, is biologically more active because it dissociates to form free Ag+ ions. The activity of a specified amount of Ag is related to the concentration of silver ions rather than to the chemical or physical nature of their source rChambers et al., 1962]. Insoluble compounds or metallic silver can affect microorganisms by forming minute traces of ionic Ag, either through chemical reaction or through elec- trolysis due to local electrical potentials set up by mechanical deformation or by the presence of other metals [Goetz et al., 1940]. Silver's biological action apparently involves