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274 September, 1970 IR3 day after day, without adverse effect. The values listed refer to time-weighted average concentrations for a normal work day. AIR POLLUTION Silver-Iodide Generator.-Of the materials present in the effluent of a silver-iodide generator, iodine vapor might present a hazard to an individual inthe immediate vicinity if the concentration is excessive (3). The concentra- tion of iodine vapor can be calculated. The hazard can also be determined since theACGIH accepted threshold level for iodine vapor has been establi@~1 mg per cu m of air. Consider a propane-acetone slIver-iodide generator. These are in wide use by cloud seeding companies. Such generators operate at a normal output rate of 6 g of AgI per hr, although higher rates of up to 24 g of AgI per hr are sometimes used. Considering the normal output rate, in an ambient air temperature of 700 F, the concentration of iodine vapor in the ef- fluent at a point where the effluent temperature is 150" F, is estimated to be 0.47 mg per cu m and is probably even much less (Appendix I). This is less than one- half of the ACGIH threshold level. If an ambient air temperature of 0" F is assumed, the concentration of iodine vapor is 0.87 mg per cu m at the pointwhere the effluent temperature is 150" F. On this basis, the hazard from iodine vapor to an individual in the immediate vicinity of a silver-iodide gen- erator is nonexistent, and the hazard to the general population from air pollu- tion by iodine vapor emitted by a silver-iodide generator is also nonexistent. The other materials pre sent which may be of concern are silver iodide and its complexes, and acetone. With the exception of external irritants, toxic substances must be absorbed into the body and distributed by means of the bloodstream in order for poisoning to occur. Poisoning ordinarily does not occur without absorption. An important factor in absorption is the solubility of the material in human tissue fluids. The Merck Index states that silver iodide is the most water insoluble salt known (3). Its solubili.ty is 0.03 mg per I of water [30 ppb (parts per billion)). There are no data on threshold levels for silver iodide; however, in all re- corded cases which involved the absorption of soluble silver compounds, there' were no constitutional symptoms and no physical disability (10). In view of the insolubility of silver iodide and its complexes in water and the absence of . symptoms and disability in cases where soluble silver compounds were ab- sorbed, the hazard to an individual from the silver iodide and its complexes emitted from a silver-iodide generator is nonexistent. The hazard to the general population from air pollution by these materials as emitted from a silver-iodide generator is also nonexistent. In the generator, the acetone solution is sprayed into the propane flame. It is conceivable that some of the acetone is not completely oxidized despite the high flame temperature and the ready availability of oxygen. Incomplete oxi- dation of the acetone would result in the formation of carbon monoxide as well as carbon dioxide and water. The Atmospheric Simulation Laboratory of Colo- rado State University has examined the effluent of a silver-iodide generator operating at the rate stated earlier. The concentration of carbon monoxide was found to average 0.4 %. In an ambient air temperature of 700, the con- centration of CO, at a point where the effluent temperature is 150" F, is 150 ppm. In an ambient air temperature of 00 F, the concentration at 150" F is 280 ppm.