8 CIRCULAR 3 6 4, U. S. DEPARTMENT OF AGRICULTURE 



Table 2. — Maximum and mean concentrations of hydrocyanic acid gas obtained 

 in a fumigation chamber 



[Averages of a number of tests classified as lethal and sublethal] 





Concentration and method 



Maximum 

 concentration 



Mean con- 

 centration 



Lethal concentration, single fumigation, curve a . 



Mg per liter 

 3.6 

 2.9 

 2.6 

 2.4 



Mg per liter 

 1 67 



Sublethal concentration, single fumigation, curve b 



1 56 



Lethal concentration, double fumigation, curve c 



1 65 



Sublethal concentration, double fumigation, curve d 



1.29 







Omitting the consideration of time, a comparison of curves a and 

 h shows that the higher maximum concentration is the deciding factor, 

 the lengths of exposure being only 5% minutes different and the mean 

 concentrations differing by only 0.11 mg per liter. Comparison of 

 curves c and d does not show this so clearly, the maximum concentra- 

 tions differing by only 0.2 mg per liter, while the mean concentration 

 of c is 0.36 mg per liter higher. In this case it is probably the greater 

 mean concentration that is the deciding factor. 



Taking the time of exposure into consideration, the value of the 

 higher initial concentration is shown by the fact that the curve d, 

 which represents a series of fumigations in which the concentration 

 was held above 0.4 mg per liter for a much longer period than in the 

 case of any of the other curves, was nevertheless sublethal. 



By comparing them with the average lethal and sublethal curves 

 shown in figure 3, it will be seen that the concentrations secured in 

 the houses (fig. 2, c and d) with calcium cyanide are too low to be 

 depended on to give a 100-percent kill of insects and mites. The 

 figure for the calcium cyanide dust is fairly close to the theoretical 

 lethal, but this form is much more dangerous to the operator than 

 the granular cyanide unless he is equipped with a gas mask. 



Liquid hydrocyanic acid gives a high peak of gas concentration in 

 a very short time, as compared with calcium cyanide. It is easily 

 handled by a trained operator, and as the gas is forced into the house 

 from the outside there is a minimum of danger in its use. The main 

 objections are that a pipe line must be used, and such a system, if 

 temporary, is difficult to shift while the house is hot, or, if permanent, 

 will be expensive ; and also that the cylinders are bulky and heavy. 



Sodium cyanide and acid give, in equivalent dosages, almost as high 

 a concentration as liquid hydrocyanic acid, and produce the peak con- 

 centration within 5 minutes. It involves somewhat more trouble to 

 measure out acid and weigh or count out sodium cyanide than simply 

 to weigh out calcium cyanide, but it is not so difficult or unpleasant 

 as installing a temporary pipe line for liquid hydrocyanic acid. Both 

 acid and cyanide must be handled with extreme care to avoid un- 

 pleasant results. Rubber gloves are usually worn when handling 

 either of these materials. From the standpoint of danger to the 

 operator under mushroom-house conditions, sodium cyanide is less 

 dangerous, if properly handled, than calcium cyanide, and very little 

 if any more so than liquid hydrocyanic acid. The operator starts 

 at the far end of the house and moves forward to the open door at 

 the front end. He can see where he is going at all times, minimizing 

 the possibility of stumbling over some obstacle. If very heavy or 



