INDUSTRIAL FUMIGATION AGAINST INSECTS 23 



mated that a man protected by a good gas mask can remain in air 

 containing 2 ounces of hydrocyanic acid gas per 1,000 cubic feet for 

 half an hour without being affected ; in a concentration of 4 ounces 

 per 1,000 cubic feet this time should be reduced to 15 minutes, and in 

 one of 8 ounces to 5 minutes. The same author calculates that, in 

 distributing discoid s at the rate of 8 ounces of hydrocyanic acid per 

 1,000 cubic feet, a fumigator will be actually exposed for the greater 

 part of the time to from 1 to 2 ounces per 1,000 cubic feet. Allow- 

 ance is made for the fact that the fumigator is constantly moving 

 away from the discoids and that the full quantity of the gas is not 

 given off immediately. If the discoids are chilled before being used, 

 the concentration to which the fumigator is exposed will be less. 



From the foregoing data it is evident that, where large quantities 

 of discoids are used, the fumigating crew must be large enough so 

 that the distribution of the fumigant will be speedy and no one man 

 is exposed to the gas for a dangerously long period. The use of this 

 material is not recommended for any but professional fumigators. 



Disposing of Spent Discoids 



When the fumigation is finished and the building has been well 

 ventilated, the spent discoids and the empty cans can be gathered up 

 and thrown away. At the end of a 24-hour fumigation the discoids 

 will retain not more than a trace of hydrocyanic acid. 



THE POWDER METHOD 



Calcium cyanide in dust form is used in much the same way as the 

 discoids. On exposure to the air the powder absorbs moisture and a 

 chemical reaction takes place by which hydrocyanic acid gas is given 

 off. 



Applying the Powder 



The required number of cans of calcium cyanide are distributed 

 throughout the building. They are then opened and the contents 

 scattered over the floor in a layer not more than half an inch thick. 

 To facilitate removal of the dust after the fumigation, it may be 

 scattered on strips of paper previously laid on the floor, although it is 

 sometimes placed directly upon the floor (fig. 24). Each can of 

 fumigant is equipped with a special perforated top, which the fumi- 

 gator puts in place of the friction top when he is ready to use it. 



Inasmuch as the gas is given off very rapidly after the dust is 

 exposed to the air, the fumigator should wear a gas mask while dis- 

 tributing it. As in the case of the discoids, he should begin distribut- 

 ing the dust at the point farthest from the exit, so that he will be 

 working away from the gas that is being given off. 



After the fumigation, the paper on which the dust is spread can 

 be rolled up and thrown away, or the dust can be swept from the 

 floor and placed in containers, to be disposed of immediately. The 

 residue, which is mostly calcium hydroxide, is likely to absorb some 

 of the hydrocyanic acid from the air ; hence it is advisable to dispose 

 of it outside the building, where small quantities of escaping hydro- 

 cyanic acid gas will harm no one. After all the hydrocyanic acid 

 has been given off, the residue is nonpoisonous. 



