Source: Old News 16(8), 8-10, May & June, 2005.

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Doctor John Snow Blames Water Pollution for Cholera Epidemic (cont.)

by David Vachon

The following summer, cholera broke out in London in the district where Snow was working. He suspected that it was being spread by contaminated water piped in from the Thames River. He searched through municipal records and discovered that two private companies were supplying water to the district. One firm, the Southwark and Vauxhall Water Company, was drawing water from an area along the Thames that was known to be polluted by sewage, whereas the other company, the Lambeth Water Company, had recently moved its intake facilities to a location above the sewer outlets. Snow decided to compare the mortality rates of consumers of the two sources of water.  When he learned that both companies had water pipes beneath the paving stones in most streets, he saw a chance to conduct an experiment "on the grandest scale."

Expressing his excitement, Snow wrote:

No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station, from gentle folks down to the very poor, were divided into two groups without their choice, and, in most cases, without their knowledge; one group being supplied with water containing the sewage of London, and amongst it, whatever might have come from the cholera patients-the other group having water quite free from such impurity.

Snow began by looking at two subdistricts of south London, Lambeth and Kennington, where there had been forty-four cholera deaths prior to August 12. Determining which customers were served by which water company proved difficult. Most tenants did not know, and their landlords often lived elsewhere. Snow called at each house, traced landlords to their homes, and determined that thirty-eight of the forty-four deaths had occurred in houses supplied by the Southwark and Vauxhall Water Company -- the company whose water came from a polluted source.

Snow decided to expand his survey, but he needed help to canvas all the homes where cholera deaths had occurred. He engaged Dr. Joseph J. Whiting to visit half of the homes, while he visited the other half. When the two men tallied their figures, they learned that in the four-week period between July 8 and August 5, 286 of the 334 victims had used Southwark and Vauxhall water whereas just 14 of the victims had used Lambeth water.

Snow took large statistical samples from other districts and discovered that deaths related to the two companies stood at a ratio of 71:5.  Excited by his results, Snow believed that he had obtained "very strong evidence of the powerful influence which the drinking water containing the sewage of a town exerts on the spread of cholera when that disease is present."

His critics were not impressed by the results of the survey. They continued to believe that cholera was caused by miasmas, not by germs or a waterborne poison, and they asserted that the enormous quantity of water in the Thames would sufficiently dilute any poison to render it harmless.

Snow tried to agree with his opponents as much as possible. He downplayed his belief that germs caused cholera, asking his colleagues only to believe that the disease was spread by a type of poison that could somehow remain potent after dilution. He wrote, "The poison consists probably of organized particles, extremely small no doubt, but not incapable of indefinite division, so long as they keep their properties."

Snow's critics pointed out that he had no evidence of the actual presence of the cholera "poison" in the water. Dr. Edmund A. Parkes wrote, "This array of evidence [does not prove] contamination of the water." In fact, Snow had accumulated statistical evidence that, by today's standards, would be considered worth acting upon, but because he was the first person to make use of a survey of the incidence and distribution of an epidemic in an effort to determine its cause, his evidence was seen as novel and unsound.

In late August of 1853, cholera broke out suddenly and devastatingly in a neighborhood just a five-minute walk from Snow's home in the west London district of Soho. Snow immediately turned his attention to the outbreak. He first learned about it on Sunday, September 3, but it appeared to have begun the Thursday or Friday before.  By the time Snow visited the affected neighborhood, many of the sick had already died. A yellow flag had been hung at the top of Berwick Street to alert the populace to the presence of cholera. People were fleeing the neighborhood as the dead were being hauled away in carts.

On Sunday evening, Snow took a sample of the water from the public water pump on Broad Street because most of the fatalities had occurred in its vicinity. He knew that the companies that supplied water to that pump had a clean source of water, but he suspected that the well under the pump might have become contaminated by nearby sewer pipes. The sample of water that Snow drew from the pump looked fairly clear to him. He had been expecting to see cloudiness, which would indicate organic impurities. For comparison, he took samples of water from four nearby pumps in Warwick Street, Bridle Lane, Vigo Street, and Marlborough Street. There were no visible differences between the four samples.

The next day Snow drew another sample from the Broad Street pump and took it to a microscopist, Dr. Arthur Hill Hassall, who reported that he saw a lot of organic matter in the water, but that this was not unusual.  Because the physical evidence of pollution was not conclusive, Snow decided to gather statistical evidence-to compile a map showing where the victims lived and where they got their water.

The following day Snow went to the General Register Office, where he copied records of the eighty-three cholera deaths that had occurred in the neighborhood for the week ending September 2. He obtained the name and address of each victim. He then returned to Broad Street, walked through the neighborhood, and calculated the distance from each victim's house to the nearest pump. He discovered that seventy-three of the eighty-three deaths had occurred in homes closer to the Broad Street pump than to any other pump. After visiting the homes of the ten victims who had lived nearer to another pump, he was told that eight of those ten victims had drunk from the Broad Street pump-some preferred that water and others, who were children, had drunk from the pump on their way to school.

Of the seventy-three victims who lived close to the pump, Snow learned that sixty-one of them had drunk the water.  He calculated that the number of cholera deaths that could be expected in that neighborhood, as part of the general outbreak in London, was just fourteen. Therefore, he concluded that the higher than expected number of victims must be associated with the water from the Broad Street pump.

Map showing areas within walking distance of the Broad Street pump. Snow gathered statistical evidence that showed a high incidence of cholera in homes and businesses within a short walk of the pump. Black bars indicate cholera deaths.

On Thursday, September 6, Snow attended the meeting of the Board of Guardians. He showed them his evidence and recommended that they remove the Broad Street pump handle, so that no more people could become infected. The Board members were not persuaded by his argument but agreed to have the handle removed as one of several precautionary measures. They were chiefly concerned about miasmas, which they tried to eliminate by ordering the extensive spreading of lime in the streets.

After the removal of the pump handle and the spreading of lime, the local cholera outbreak quickly ended.

To determine what had caused the outbreak, the Board of Health appointed inspectors to look at atmospheric conditions in the neighborhood and sanitary conditions in the homes of victims. One prevalent theory, articulated in a letter to the Times of London, was that cholera was caused when a new sewer was constructed: "It must have disturbed the soil, saturated with the remains of persons deposited here during the great plague [of 1665].... A deadly miasmatic atmosphere has been for some months arising . . . poisoning the surrounding atmosphere."

Inspectors examined the homes of victims on the assumption that they would find unsanitary conditions. The following Tuesday, September 11, they reported that, to their surprise, many of the victims had had clean homes.

Meanwhile, Snow was conducting his own investigation. He visited a small coffee shop near the Broad Street pump where the proprietor mentioned that she normally served water from the pump with dinner and that nine of her customers had died.

Snow began looking at other ways that people in the neighborhood might have drunk the water without drawing it directly from the pump. He found that local pubs mixed the water with spirits they served, and several shops put an effervescing powder into the water and sold it as "sherbet."

Snow learned that eighteen of two hundred workers at a percussion cap factory had died; the factory supplied drinking water that came from the pump. At a dental supply house all seven workmen had died after drinking - pump water.

Snow also tracked down people who had fled the neighborhood in the first few days of the epidemic, and he collected names and addresses of victims who had been taken to hospitals outside the neighborhood. Once he tallied the numbers, he realized that instead of the 83 deaths first reported, there were 197 deaths, all having occurred among people who had lived within a three-minute walk from the pump.

Snow was able to gather hard evidence about cases that, at first, did not appear to be connected to the epidemic. He learned from Dr. David Fraser, a medical inspector for the General Board of Health, that the death of Susannah Eley, a widow in Hampstead, a few miles distant from Broad Street, was also linked to the Broad Street pump.  The widow had insisted on having water from the pump brought to her every day. Her niece, who was visiting when the widow became ill, also drank the water and died a few days after her return to her home in Islington.

Snow was curious about places in the neighborhood where there was a low incidence of cholera. He found that in a workhouse in the neighborhood, only 5 of 535 inmates had died-a low figure compared to nearby houses. The miasmists at the Board of Health had expected a higher than average rate at the workhouse since the inmates were poorly nourished, unclean, and of low morals -- indicators, by their standards, of susceptibility to disease. Snow learned, however, that the workhouse had its own source of water and therefore did not draw water from the Broad street pump.

Miasmists also would have expected a high death rate at the Lion Brewery where workers drank beer -- the Board of Health advised that drinking alcohol was linked to cholera. In fact, none of the seventy-odd workers at the brewery had perished. Snow learned that they only drank beer and never touched the water from the pump.

On September 25 inspectors from the Board of Health inspected the brick lining of the well under the Broad Street pump to be sure that no drainage from the sewer could enter it. After a cursory inspection inside the well, they found the lining apparently intact. A local surveyor, Jehoshaphat York, told them that the sewer was ten feet away and deeper than the bottom of the well. This seemed to disprove Snow's theory.

Satisfied that the well had not been polluted and that the epidemic was over, the Board of Health reopened the Broad Street pump. People who had fled the neighborhood during the epidemic began returning to their homes by the last week in October.

In November, Snow was asked by the Reverend Henry Whitehead to join a committee of the St. James Parish to investigate the causes of the outbreak. Snow was pleased by the prospect of working with Whitehead, who seemed to know everybody in the parish.  Whitehead disagreed with Snow's theory about the cause of the epidemic, but he liked Snow's honest and straightforward method of investigation.

Snow was at that time putting the final touches on the second edition of On the Mode of Communication of Cholera, and although he still had no physical evidence of pollution in the pump's water, he was convinced that the pump was the source of the epidemic. Hoping that a closer examination of the well beneath the pump would support his theory, Snow urged the parish committee to ask the Paving Board (responsible for water pumps) to reexamine the interior of the well. They did so and reported "that there was no hole or crevice in the brickwork of the well by which any impurity might enter." Snow had to admit in his publication that there was no direct evidence of any contamination of the Broad Street pump.

After Snow's pamphlet was published, he gave copies to all the members of the parish committee. Whitehead read his copy and was impressed by Snow's array of facts, but he could not see how the pump could have been contaminated by the sewer line for just the few days when most of the deaths had occurred. Whitehead reasoned that if Snow's theory was correct, "the outbreak ought not so soon to have subsided, when much larger quantities of cholera excretions must have been continually pouring into the well through the ... sewers." Whitehead was not convinced about the pump, but he was as eager as Snow to solve the mystery.

Broad Street, circa 1850, showing the pump in front of #40

In March of 1855, Whitehead was reviewing reports by the Registrar General about cholera deaths from the week ending September 3, 1854, when he came across an interesting item.  He read, "At 40 Broad Street, 2nd September, a daughter, aged five months, [died of] exhaustion, after an attack of diarrhea four days previous to death."  Whitehead remembered the case, but he had neglected to inquire at the time about the date of the attack.  He now realized that if the child had first showed symptoms four days before September 2, then she had been the first cholera case in the neighborhood.  He also realized that the Broad Street pump was located right in front of that house.

Whitehead immediately went to see the infant's mother, Sarah Lewis.  She described how when her baby was sick, she had cleaned the child's diapers in a pail, and then emptied the pail into the drain about a cesspool in front of her house.  Whitehead was surprised to hear that there was a cesspool there.  He had been under the false impression that all the cesspools on the street had been replaced by drainpipes that emptied directly into the recently installed sewer system. The cesspool that Mrs. Lewis had used was within a few feet of the Broad Street pump and the well beneath it. Whitehead informed the other members of the committee. Snow was especially interested in the news.

On April 23 surveyor York supervised excavation of the cesspool, drains, and pump well. He found that, although it had not been obvious to previous inspectors, the brickwork of the pump well lining had decayed from the outside. The cesspool, which had been built to act as a trap that would overflow into the sewer drain, was so poorly constructed that it actually blocked the drain, forcing sewage to back up. The brickwork lining was loose, and the distance between the leaking cesspool and the pump well was just 2 feet 8 inches. The soil between the cesspool and the well showed signs of a steady percolation of waste from the cesspool into the well.

Two previous inspections of the well had not uncovered this problem, because the soil outside the well had not been examined, and no one had known that there was a cesspool less than three feet away.

The dates when the diaper water had been emptied into the cesspool corresponded to the early days of the outbreak, when most victims came down with cholera. After the child's death, no more diaper water had been emptied into the cesspool, and the epidemic had ended. As far as Whitehead and Snow were concerned, they had found the cause of the outbreak. The committee unanimously backed them in its report, but the General Board of Health dismissed Snow and Whitehead's conclusion. When John Snow died of a stroke on June 16, 1858, his theory about the spread of cholera had not gained any ground. The miasmists still prevailed.

Snow died without knowing that the bacillus that causes cholera had been discovered in 1854 by the Italian anatomist Fillipo Pacini, who used a microscope to examine the intestinal walls of people killed by cholera. Pacini did not prove that the bacillus that he had discovered caused the disease, so his work remained obscure, and his reports were not translated into English.

The germ theory of disease was not widely accepted until the 1860s, when the French chemist Louis Pasteur demonstrated by experiments that microscopic organisms can cause illness. Snow's final vindication came in 1884, when German microbiologist Robert Koch rediscovered, isolated, and cultured the cholera bacillus, Vibrio cholerae.

For his persistent efforts to determine how cholera was spread and for the statistical and mapping methods he initiated, John Snow is widely considered to be the father of epidemiology.


Shephard, David A. E. John Snow: Anaesthetist to a Queen and Epidemiologist to a Nation. Cornwall, Prince Edward Island: York Point Publishing, 1995.

Vinten-Johansen, Peter et al. Cholera, Chloroform, and the Science of Medicine: A life of John Snow. Oxford: Oxford University Press, 2003.

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