The Broad Street Pump

Part One · The City

In the summer of 1854, the district of Soho in central London was one of the most crowded neighborhoods in the largest city in the world. Thousands of people lived there in narrow houses and cramped apartments, and the streets were filled with workshops, taverns, markets, and small factories. There were no pipes carrying clean water into the homes, and most families collected their water from public pumps on the street corners. Sewage was normally drained into cesspools beneath the cellars, which were pits that were emptied only occasionally. In an age before germs were understood, these arrangements seemed to trouble hardly anyone, and nobody imagined that the ordinary habit of fetching a bucket from the pump could possibly turn out to be dangerous. Historians estimate that the capital contained more than two million inhabitants at that time, and its explosive growth had far outpaced the construction of any adequate system for removing waste, so that the foul contents of the cesspools frequently seeped into the surrounding ground.

Cholera was unquestionably among the most feared diseases of the nineteenth century. It causes severe sickness in the stomach and a rapid loss of water from the body, and in those days it frequently killed within a few days. Epidemics had swept through Britain in 1831 and 1848, and each time thousands of people had died. The prevailing opinion among doctors and officials was that the illness was caused by miasma, a poisonous vapor that supposedly rose from rotting garbage and swampy ground and drifted through the air. People therefore tried to protect themselves by avoiding bad smells, and the authorities concentrated on cleaning the streets. It was a reasonable theory, given what was known at the time, but it was mistaken, and the mistake cost many lives. Because the disease struck so suddenly, it inspired such dread that families sometimes abandoned their homes at the first rumor of an outbreak.

One of the few physicians who doubted the theory of miasma was a London doctor named John Snow. He had been born in the city of York in 1813, the eldest son of a laborer, and he had trained as a surgeon's apprentice before settling in London. By the 1850s he had become well known for his work with anesthesia, the use of gases such as ether and chloroform to prevent pain during operations, and he had even attended Queen Victoria when she gave birth. Snow was careful, methodical, and persistent, and he preferred to rely on measurements rather than guesses. During the epidemic of 1848, he had formed the suspicion that cholera was spread by something that people swallowed, probably in contaminated drinking water, and he had published a short book that explained his reasoning. His modest background made his achievements all the more remarkable, since the medical profession of the period was dominated by wealthy gentlemen from the most prestigious universities, who were generally reluctant to listen to a newcomer.

Part Two · The Map

At the end of August 1854, a terrible outbreak of cholera began in the streets around Broad Street in Soho. Within three days, more than one hundred twenty residents of the neighborhood had died, and within a week many families had fled in terror, leaving their houses empty. Snow lived only a short distance away, and he quickly resolved to find the source of the trouble. Instead of relying on theories, he began to walk the streets and to knock on doors. He asked the people who were left a simple series of questions about who had been sick, where each person lived, and where each family had obtained its drinking water. Then he wrote the answers down carefully, house by house, and street by street. It was an exhausting and often distressing task, since nearly every household had lost someone, but Snow understood that only a complete and accurate record could reveal the underlying cause of the epidemic.

Snow soon noticed a pattern. Nearly all of the people who had fallen ill lived close to the public pump on Broad Street, and many of them were certain that they had used its water. He drew a map of the area and marked each death with a short black bar at the address of the house, and the bars piled up thickly around the pump, while the streets farther away showed far fewer of them. A map that displays data in this manner makes a pattern visible at a glance, and it has remained a classic example in the study of public health. The idea of mapping a disease was not entirely new, but Snow's map was designed to answer one specific question, and that focus made it unusually persuasive. Still, Snow knew that a map alone did not prove his case, so he sought out the unusual cases that did not appear to fit.

The exceptions turned out to be the most convincing evidence of all. A workhouse on nearby Poland Street housed hundreds of poor residents, but only a handful of them became ill, and Snow discovered that the building had its own private well. A large brewery on the same street had no cases among its workers, who were given free beer to drink and who also had a well of their own. On the other hand, a widow who lived far away in Hampstead had died, along with her niece, and Snow learned that she had liked the taste of the Broad Street water so much that she had a bottle of it delivered to her house every day. Each of these stories pointed to the pump. Taken together, the details demonstrated that the illness followed the water rather than the air, which was precisely the opposite of what the supporters of the miasma theory would have predicted.

On the evening of September 7, 1854, Snow presented his evidence to the local board of guardians, the group of officials who were responsible for the parish. Although the members were skeptical of his claim that a single pump was spreading the disease, they agreed to take action, and the next day the handle was removed from the pump, so that nobody could draw water from it. The number of new cases was already falling by then, partly because so many residents had fled, and some people have argued that the removal of the handle made little difference. Even so, it was an important moment, because it was one of the first times that an official decision had been based on a scientific map. Snow was asking the officials to take only a simple and inexpensive precaution, which could do no harm even if his theory proved to be wrong.

The final piece of the puzzle was supplied by a local clergyman named Henry Whitehead, who had at first believed in the theory of miasma and who set out to disprove Snow. Whitehead was well known and trusted in the neighborhood, and he interviewed many of his parishioners. In the process he learned that the first victim of the outbreak had been a baby girl who lived at 40 Broad Street, and that her family had emptied the water from her soiled diapers into a cesspool close to the front of the house. When workers dug up the ground, they discovered that the cesspool had been leaking, and that its brick lining lay only a few feet from the well. Whitehead became convinced that Snow was right, and his willingness to change his own mind when the evidence demanded it is one of the qualities that people most admire in the story.

At first, most of the medical establishment would not accept Snow's conclusion, and the official report on the outbreak declined to support it. Snow died of a stroke in 1858, at the age of forty-five, without knowing that his work would be vindicated. A few decades later, scientists proved that cholera is caused by a microscopic bacterium that spreads through contaminated water, and the importance of his investigation became obvious. Today he is regarded as one of the founders of epidemiology, the study of how diseases spread in populations. His careful method of collecting facts, mapping them, and testing explanations against the exceptions still guides the work of health officials today, and clean water has since saved millions of lives. London itself began an enormous project to build modern sewers in the years that followed, and cities all over the world eventually adopted similar improvements.