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96 COOPER AND JOLLY ecosystem and if they were initially nontoxic in the quantities present in precipitation. Of the agents mentioned, metaldehyde is the only one that appears to offer any problem. This compound is used as a snail and slug poison, and FDA standards permit no measurable residues on raw agricultural commodities. Neither 1, 5-dihydroxynaphthalene nor phloro- glucinol is listed in standard pesticide hand- books. Fog dispersal agents. Common salt and var- ious organic compounds have been used to dis- perse warm fog over airports. About 90 kg of NaCI is needed to clear a zone 500 meters wide, 100 meters high, and 2000 meters long. Five such applications per hour, or about 450 kg/hr, would be required to keep a fogged-in airport continuously open with normal winds [Jiusto et aZ., 1968J. This amount of salt would be highly toxic to vegetation and to life in streams and lakes reached by runoff from paved run- ways. However, salt is so corrosive to aircraft and other machinery that it almost certainly will not be used in this way. Recent fog dispersal tests have involved proprietary compounds identified only as poly- electrolytes and surfactants [Flynn and Beck- with, 1968]. As a matter of public policy, authorities should unquestionably require full disclosure of the nature of these or similar materials before authorizing their use on any but an experimental basis. The time has passed when dispersal of unidentified material into the environment can be permitted after no more than a tenuous showing that it is not directly and immediately harmful to human health. CONCLUSIONS AND RECOMMENDATIONS There is a dearth of published data that are useful for drawing conclusions about possible ecological effects of long continued dispersal of silver compounds into the environment as a part of a weather modification program. Ionic silver is known to be among the most toxic of heavy metals, but it is more rapidly immobilized in biologically inactive compounds and complexes than such metallic environmental contaminants as lead and mercury. The insolubility of its compounds, and the small quantities that will be used in cloud seeding, suggest that the im- mediate danger from silver is not sufficient to preclude its use in preliminary weather modifi- cation experiments and pilot programs. The threat is not negligible, however, and continual vigilance and reassessment is required. Microorganisms, invertebrates, and fish are more likely to be affected than terrestrial plants and animals. The literature indicates that there is relatively little likelihood that silver will con- centrate through the food chain to levels that would be harmful to. animals eating contami- nated food. There is an urgent need for additionallabora- tory research to establish the effects of various levels of silver on growth rates and metabolism of aquatic organisms. Available data relate al- most exclusively to quantities of silver required to kill a high fraction of all organisms in a short time. Studies are especially needed of the effects of low concentrations of silver on micro- organisms (particularly anaerobic bacteria im- portant in bottom muds and in sewage treat- ment) , on fish, and on aquatic invertebrates. Silver in the environment, particularly in precipitation and in lakes and streams, should be regularly monitored. It is important that an adequate set of baseline measurements be ob- tained now, before much silver has been added to the environment through cloud seeding. Public agencies and other groups supporting research in the physical science aspects of wea- ther modification have a clear obligation, which they have not yet fully accepted, to support research and monitoring programs that will enable the scientific community and the public to assess the environmental consequences of this new technology. Care should be exercised before organic com- pounds are dispersed for precipitation increase or fog dissipation. Public policy should require full disclosure of the composition of all such materials. The time has passed when widespread dispersal of unknown proprietary compounds into the atmosphere can be permitted. ,.. Ii "'~ ~... \ J Acknowledgment. This paper is based on work performed under contract 14-06-D-6576 with the Office of Atmospheric Water Resources, Bureau of Reclamation, U. S. Department of the Interior, Denver, Colorado. REFERENCES Black, W. A. P., and R. L. Mitchell, Trace ele- ments in the common brown algae and sea water, J. Marine Biol. Ass., 30, 575-584, 1952.