Hygienic Results. Evidence as to the actual reduction of the number of such pathogenic germs as B. typhosus in water supplies by chlorination is most readily found in the death rates from typhoid fever in cities that have no other means of water purification. In some cases this evidence is necessarily of a circumstantial nature; in others it is definite and conclusive.
Some of the earlier results of the effect of chlorination on typhoid morbidity and mortality rates were compiled by Jennings[6] and others have been published by Longley.[7] These data have been brought up to date in [Table XXXI] and other statistics added.
TABLE XXXI.—EFFECT OF CHLORINATION ON TYPHOID RATES
| Average Typhoid Death Rate Per 100,000 Population | ||||||
| City. | Commenced Chlorination. | Before Using. | After Using. | Percentage Reduction. | ||
| Period. | Rate. | Period. | Rate. | |||
| Baltimore | June 1911 | 1900-10 | 35.2 | 1912-15 | 22.2 | 36 |
| Cleveland | Sept. 1911 | 1900-10 | 35.5 | 1912-16 | 8.2 | 77 |
| Des Moines | Dec. 1910 | 1905-10 | 22.7 | 1911-13 | 13.4 | 41 |
| Erie | Mar. 1911 | 1906-10 | 50.6 | 1912-14 | 15.0 | 70 |
| Evanston, Ill. | Dec. 1911 | 1908-11 | 29.0 | 1912-13 | 14.5 | 50 |
| Jersey City | Sept. 1908 | 1900-17 | 18.7 | 1909-16 | 8.4 | 55 |
| Kansas City, Mo. | Jan. 1911 | 1900-10 | 42.5 | 1911-16 | 14.2 | 66 |
| Omaha, Neb. | May 1910 | 1900-09 | 22.5 | 1911-16 | 10.6 | 53 |
| Trenton | Dec. 1911 | 1907-11 | 46.0 | 1911-14 | 28.7 | 35 |
| Montreal | Feb. 1910 | 1906-10 | 40.0 | 1911-16 | 25.0 | 37 |
| Toronto | Apr. 1911 | 1906-10 | 31.2 | 1912-16 | 7.8 | 75 |
| Ottawa | Sept. 1912 | 1906-10 | 34.0 | 1913-17 | 17.0 | 50 |
The figures given in this table show the effect of chlorination only; no other form of purification was used during the periods given, except at Toronto where a portion of the supply has been subjected to filtration.
It will be seen that since chlorination was adopted the typhoid death rates have been reduced by approximately 50 per cent and that the averages for the period after treatment are almost invariably less than 20 per 100,000, a figure that a few years ago was regarded as satisfactory. The average death rate for the last available year is 11 per 100,000, a result that is even more satisfactory and exceeds the anticipations of the most optimistic of sanitarians.
A portion of the reduction in the typhoid rates is no doubt due to improvements in general sanitary conditions but the reduction is much greater than can be accounted for in that manner alone and in many cases there was a sharp decline immediately following the commencement of chlorination.
In a few instances there is evidence that chlorination has reduced the typhoid rates of cities previously supplied with filtered water. [Diagram X], drawn from data supplied by Dr. West, of the Torresdale Filtration Plant, shows the effect of disinfecting the filter effluents at Philadelphia.
DIAGRAM X
TYPHOID IN PHILADELPHIA