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Chlorination, Clean Water and the Public Health Progress that Changed America

20th Century Lessons-Learned for 21st Century Developing Nations

At the start of the twentieth century, high mortality rates and substandard living conditions were commonplace in urban America. Mortality rates in major cities were substantially higher than in rural areas, a phenomenon known as the "urban penalty". Yet by 1940, mortality rates had declined by a full 40 percent, mostly due to a reduction in infectious diseases. In addition, the urban penalty had largely disappeared, with life expectancy rising from 47 to 63 years.

What fostered this rapid and revolutionary change, the most dramatic reduction in mortality ever recorded in the United States? There has been much debate on this question, but to-date relatively little empirical data to provide a verifiable answer. However, a new study by two Harvard University researchers, David Cutler of the Department of Economics and Grant Miller of the Health Policy Program, finds the most compelling root cause for the drop in U.S. mortality rates in the twentieth century was the adoption of treatment technologies for public drinking water supplies.

In their recent article, "The Role of Public Health Improvements in Health Advances: The Twentieth Century United States,"1 Cutler and Miller conclude that clean water technologies, filtration and chlorination, were responsible for nearly half of the total mortality reduction in major cities between 1900 and1936, with even greater impact on infant and child mortality rates during that same time period. Significantly, these technologies led to the near-eradication of typhoid fever, the waterborne disease that was one of the major scourges of that era.

Considering the cost of water infrastructure and the value of reduced mortality, Cutler and Miller's analysis also concludes that clean water technologies were not only a boon to public health, but were tremendously cost effective in doing so. Their data demonstrate that these technologies, working together, yielded an estimated $23.00 in benefits for every $1.00 invested. Applying these findings to today's world, the authors suggest a potential for tremendous public health and individual wellness benefits from adopting inexpensive water disinfection technologies in developing nations.

The Adoption of Clean Water Technologies

In the later half of 19th century, many large U.S. cities had municipal water and sewer systems. As Cutler and Miller note, however, these early systems did not include treatment and did not prevent significant outbreaks of waterborne diseases. In fact, drinking water quality deteriorated, due to the dumping of untreated sewage into rivers and lakes used as source water. Often, primary sewer outfalls emptied upstream or in close proximity to water intakes. Even the few cities that addressed this problem early suffered from the dumping of untreated sewage by upstream communities.

While modern sewage treatment was not widely adopted until the 1930's and 1940's, many major U.S. cities adopted clean drinking water technologies by 1920. Two major methods of filtration, slow sand and rapid (or mechanical), emerged. Originally designed to reduce turbidity, discoloration, and bad taste, these methods were also found to improve the microbial quality of water. However, it was clear that filtration did not remove all the bacteria from public-use water. Complimentary disinfection processes were evaluated to expand the effect of clean water on urban populations. Found to be the least costly option, chlorination was rapidly and widely adopted. The first significant adoption of water chlorination took place at the Boonton Reservoir of the Jersey City, New Jersey waterworks in 1908, with most major cities following suit in the ensuing decade.

Impact of Clean Water on Public Health

To measure how clean water affected mortality, Cutler and Miller matched municipal-level mortality statistics to knowledge of where and when filtration and chlorination were adopted. By examining changes in mortality just around the time that filtration and chlorination were introduced in each city, they were able to distinguish these impacts from other changes occurring during the same time period (such as knowledge of appropriate personal health practices). The final study sample included thirteen cities where sufficiently complete and reliable data were available: Baltimore, Chicago, Cincinnati, Cleveland, Detroit, Jersey City, Louisville, Memphis, Milwaukee, New Orleans, Philadelphia, Pittsburgh, and St. Louis.

In the absence of national death records prior to 1933, Cutler and Miller utilized several research sources, including data from both an official "death registration area" comprised of 10 states and several "registration cities" outside of those states. U.S. Census Bureau monthly statistics, broken out by city, cause and age, were also referenced as the base of their analysis. While reported deaths from waterborne disease were not a fixed statistic, as many died from variations of diarrheal disease, the authors found that deaths from typhoid fever were a recognized and consistently registered cause of death during the era. Typhoid fever then serves as a marker for other waterborne and diarrheal diseases, which are estimated to account for four times as many deaths as typhoid fever alone.

In the studied cities, major infectious disease and childhood infectious disease rates plummeted in the first decades of the century, including a massive decline in tuberculosis and the near-eradication of typhoid and malaria (see Table 1).


TABLE 1 -- Percentage of Deaths, by Cause, in Major Cities
Cause of Death
Major Infectious Diseases
Diarrhea and enteritis
Typhoid Fever
Childhood Infectious Diseases
Scarlet Fever
Whooping cough
Source: U.S. Census Bureau's Mortality Statistics, 1900 and 1936.

Cutler and Miller found that filtration and chlorination together reduced mortality by an average of 13%, infant mortality by 46% and child mortality by 50% in major U.S. cities. This accounts for about 43% of the total reduction in mortality observed in these cities from 1900 to 1936. Even more striking, clean water appears to have been responsible for 74% of the reduction in infant mortality and 62% of the reduction in child mortality (see Table 2).


TABLE 2 -- Effect of Filtration and Chlorination on Mortality
Total Reduction in
Mortality Rate 1900-1936
Share of Total Due
To Clean Water
Typhoid Mortality
Total Mortality
Infant Mortality
Child Mortality
* Achieved five years after adoption of clean water technologies

Nearly all of the mortality declines are accounted for by reductions in infectious disease, which caused nearly half of all deaths in 1900 but account for only a small fraction of deaths today. Most notably, clean water appears to have reduced typhoid fever deaths by 26% initially and by another 65% after five years, leading to the near-eradication of this killer disease by 1936. Cutler and Miller suggest that clean water reduced other infectious diseases, including pneumonia, tuberculosis, and meningitis.

The Economic Impacts of Clean Water

What is the financial impact of providing clean water and reducing disease to a society? As part of their study, Cutler and Miller estimated the economic benefits of these striking reductions in disease mortality.

To do so, Cutler and Miller estimated the cost of a water system serving 100,000 persons to be $30 million per year (in 2003 dollars). Using the mortality reductions indicated by their research, they also calculated the annual number of deaths prevented by clean water technologies (1,484), the number of person-years saved (57,922), and the associated annual benefits ($679 million, in 2003 dollars).

The result? Cutler and Miller demonstrate in their study that the introduction of clean water technologies produced an eye-popping estimated rate of return of 23:1. This means that the duel clean water technologies of filtration and chlorination provide $23.00 in benefits for every $1.00 invested in clean water. Significantly, this statement of value does not tell the entire economic impact story of clean water technologies. The additional fact is that these estimates exclude the considerable ancillary benefits of clean water for public use, including overall reduced illnesses and productivity gains in the general population.


Cutler and Miller's analysis demonstrates the strikingly large and cost-beneficial role of clean water technologies. The period examined was the era of the most rapid documented decline in mortality in American history, and clean water appears to have played as large a role as any force responsible for this rapid progress.

Although findings from the early twentieth century in the United States cannot be compared directly to the current circumstances of developing countries, the results give some indication of the tremendous health and economic gains achievable through clean water technologies. Worldwide, roughly 1.1 billion people lack access to safe water and 1.7 million people die every year from diarrheal diseases. Applying results from their analysis, and assuming that only 1% of the annual deaths from diarrheal diseases could be prevented by water disinfection, Cutler and Miller estimate the corresponding social rate of return would be about $160 billion annually.

End Notes

1Cutler, D. and Miller, G. (February, 2005). The Role of Public Health Improvements in Health Advances: The Twentieth-Century United States. Demography, vol. 42, no. 1, 1-22.





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