Today's post is by History of Vaccines intern Alexandra Linn.

When the New York Times announced that Franklin Delano Roosevelt was diagnosed with polio on September 16, 1921, fear swept the nation. Polio, a little-understood illness at the time, had suddenly disabled a wealthy and prominent politician on the cusp of his career, making it clear that any American, irrespective of social status, was potentially susceptible to the disease. With the national spotlight focused on the issue, the search for a cure or vaccine began, and the defeat of the dreaded illness became an important health objective in America almost overnight.[1]

Polio is an enterovirus that infects the gut, and in certain cases, can travel up to the nervous tissue causing neuron death and ultimately paralysis. After the first U.S. cases were identified in the late 1800s, the numbers grew to a high of nearly 60,000 in 1952. The virus seemed to thrive in the summer months, with a “polio season” peaking in mid- to late-summer, and receding with the cool weather.[2]

FDR’s sudden illness occurred during a trip to a vacation house and lake in New Brunswick, Canada. Fearful parents jumped on this association, and began warning their children against swimming in pools, lakes, or any area with open water. Pools were vacated, lake homes were avoided, and swimming became seen as a dangerous exercise.[3]

How did this scare change swimming pools, and how did their association with polio fade away?

Swimming pools date back to the time of the ancient Greeks and the Roman Empire, when public baths were a comfortable place for socializing and relaxing. The first public swimming pool in the United States was built in Massachusetts in 1887. Baths and public swimming areas became common in large cities by the 1920s, with thousands of pools open throughout the United States.[4]

With the growth in popularity came the need for better sanitation measures. Originally pools used archaic filtration systems that required that water filters, and the actual water itself, to be changed frequently. Chlorine was discovered and produced prior to WWI in the early 1900s, but it was not until the war invigorated its manufacture that its use truly came into vogue. By the time of the polio scare in the late 1930s and 1940s, chlorine was used widely in public swimming pools as a sanitation measure.[5]

Still, this didn’t prevent the panic that arose over the public’s fears that children could be exposed to the poliovirus in community swimming pools.

In 1946, however, a study showed that chlorine was actually one of the few known chemicals that could inactivate the virus.[6] Although polio is resistant to common soaps and chemicals of low pH – which is one of the reasons it was able to spread so efficiently -- it can be rapidly inactivated by chlorine, as well as by heat and formaldehyde. Formaldehyde was the chemical ultimately used to inactivate the virus in Jonas Salk’s inactivated polio vaccine, or IPV. [7]

The problem of polio transmission had not been solved, but swimming pools regained popularity as a fun and exciting summer venue for families. Moreover, chlorine, as a polio disinfectant, became the new face of sanitation, with strict regulations on chlorine in pools in place by the early 1960s. [8]

So the next time you hear someone complaining of the smell of chlorine, or even the fact that it might turn some people’s hair a greenish hue, tell them about the role of chlorine in the public health crisis that panicked the United States in the mid-1900s and emptied community pools.