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~ I I~ I IR3 SILVER IODIDE GENERATOR 275 The ACGIH threshold level for carbon monoxide is 100 ppm in air. This is the concentration which can be tolerated day after day without adverse effect. Higher concentrations can be accepted for limited periods. Concentrations of 400 ppm to 500 ppm in air can be inhaled for 1 hr without appreciable effect. Exposure to 600 ppm to 700 ppm for 1 hr wil.l produce barely appreciable effects. The same exposure to 1,000 ppm to 1,200 ppm is dangerous. Exposure to 4,000 ppm or greater is fatal in less than an hour (10). It is highly unlikely that an individual would expose himself to an effluent temperature of 150" F for more than a few minutes. Cooling and further dilu- tion to a tolerable temperature will reduce the carbon monoxide concentration well below the ACGIH threshold level. Therefore the hazard to an individual from the' carbon monoxide emitted from a silver iodide generator is non- existent. ,The hazard to the general population by this material is also nonexistent. Propane-acetone silver-iodide generators have been in common use for over 15 y,r. During this arne, only one instance of reaction to the effluent has been reported. In this case, an individual was servicing a silver-iodide gener- ator mounted on a tower and remained within a few feet of it, directly in the effluent, fbr a period of 6 hr. He suffered a skin rash which, while inconvenient, was not $erious enough to cause loss of work. No other substantiated case of injury (or reaction) to any individual has been reported as arising from the chemical icontent of the effluent of a silver iodide generator. To further emphasize the inoffensive nature of a silver-iodide generator, the materials it releases can be contrasted with other materials being re- leased to,the atmosphere on a routine basis. Industrial plants, home heating furnaces and incinerators, aircraft, automobiles, and many other products of our advanced civilization are releasing tons of materials to the atmosphere each day. Public concern with the problem of air pollution is steadily increas- ing and sqme legislative action has been taken to lessen this problem. A silver-iodide generator and an automobile engine are similar in size and fuel consumption rate. Both release unfiltered combustion products to the free air close to the ground. It is therefore reasonable to contrast their effluents. . The n~mber of silver-iodide generators which would be employed for a target area will vary somewhat depending upon the operator. Fifteen to 30 generators might be installed for a rectangular target area of 2,000 sq miles. During seeding operations no more than about half of them would be used at anyone time. In general, one generator is used for each 100 sq miles of target area. On the other hand, automobiles are in common use throughout the nation; their density dependent upon the population. AUTOMOBILE EXHAUSTS It is w~ll known there are many materials found in automobile exhaust which contribute substantially to air pollution (8). Among these are nitrogen oxides, unburned hydrocarbons, metallic salts, and carbon oxides. Two materials found in automobile exhaust were selected for comparison with the c0mponents of the effluent of a silver-iodide generator. These two materials in automobile exhaust present potential and actual toxic hazards. Carbon monoxide, highly toxic, is present in automobile exhaust in concen- trations and temperatures as follows (2): i .