EPIDEMIOLOGICAL AND BIOLOGICAL PROBLEMS 27 



results obtained with succinchlorimid were oftentimes irregular, due appar- 

 ently to its unstability in solution. 



The dichloramines are more cysticidal than the monochloramines and 

 among the dichloramines, ammonia-dichloramine seemed to be the most 

 cysticidal. About 6 ppm of residual chlorine as ammonia-dichloramine 

 destroyed all cysts in 10 minutes at 23° C while the commonly used organic 

 dichloramine, Halazone, reached the same efficiency at a residual chlorine 

 concentration of 12 ppm (Fair, Chang and Morris 1945; 1946). 



Chlorine dioxide as tested in our laboratory, seems to be less cysticidal 

 than HOCl on a basis of ppm as titrable chlorine, and we were uncertain 

 as to the method for determining the amount of active chlorine in this 

 compound. 



Bromine, in its elemental form, has also been tested in our laboratory 

 for its cysticidal efficiency. In a small number of experiments, it has 

 been found that this halogen is slightly less cysticidal than HOCl on a 

 weight basis and slightly more cysticidal molecule for molecule (Fair, 

 Chang and Morris 1945). This is believed to be due to the fact that bromine, 

 having a smaller hydrolysis constant, exists to a greater extent in elemental 

 form than chlorine, the elemental forms of the halogens being thought to 

 penetrate cysts more easily than their compounds. However, bromine 

 showed a considerably greater loss to the halogen demand of the water 

 than HOCl. 



Iodine has been extensively studied in our laboratory. Its cysticidal 

 efficiency, as reported by us (Fair, Chang and Morris 1945; 1946) is less 

 than that of HOCl on a weight basis but slightly more on a molar basis. 

 This is again believed to indicate that elemental iodine penetrates cysts 

 more easily than HOCl. The advantage of elemental iodine as a cysticidal 

 agent over HOCl lies in the fact that it is less affected by changes in 

 temperature, does not form iodomines with ammonia or amino compounds, 

 and does not impart as much objectionable odor and taste to the treated 

 water as does HOCl at the cysticidal level. The cysticidal efficiency of 

 elemental iodine has also been expressed in concentration-time relationship 

 as follows: 



t = 171.5C-1-* at 3° C 

 t = 118.7C-1-' at 10° C 

 t = 57 AC-'-' at 23° C 



The effect of temperature on the cysticidal efficiency of iodine has also 

 been calculated to give a value of Q^o of 1.6 (Fair, Chang and Morris 1946) . 



Ozone was reported by Kessel and his associates (1944) to be more 

 cysticidal than active chlorine. These investigators found that a residual 

 of 0.1 to 0.3 ppm of ozone (using orthotolidine test) destroyed all the cysts 

 in water in a few minutes. 



The cysticidal efficiency of synthetic detergents has been studied by 

 Kessel and his associates (1946) and also in our laboratory. In the report 

 of Kessel et al. (1946), it is noticed that some of the cationic detergents 

 were cysticidal at a concentration of less than 12.5 ppm. In our reports 



