The changeover in the water supply for Flint, Michigan, from Lake Huron to the Flint River exposed the city to dangerously high levels of lead, but Legionella pneumophila bacteria thrived. Rates of Legionnaires' disease rose sixfold until residents were encouraged to boil their water. Research published in the Proceedings of the National Academy of Sciences has now linked the 2014-15 outbreak to insufficient chlorination of the water supply.

In all the panic about fluoridation of water, it is often forgotten that municipal water suppliers add much more chlorine to their water as a way to control bacteria. There can be a price to pay in taste, but it usually works well enough for us not to notice.

Flint's 2014 change to its water source was quickly followed by a 6.3-fold increase in cases of Legionnaires' disease, which only dropped off when residents were encouraged to boil water before drinking. Rates returned to normal when the source was returned to Lake Huron. Sadly, the damage done to the city's water pipes in the meantime meant the new water was initially as filled with lead as the old, and only slowly became clean, something boiling cannot address.

However, disasters can represent scientific opportunities, and a team of scientists led by Sammy Zahran of Colorado State Univesity took the chance to compare the level of chlorine provided with the rate of the disease over time and across the city's eight water monitoring stations. “The risk of a Flint neighborhood having a case of Legionnaires' Disease increased by 80 percent per 1 part per million decrease in free chlorine,” Zahran and co-authors wrote. Adjacent communities also experienced outbreaks as commuters from other neighborhoods visited Flint for work.

Chlorine reacts with organic matter, ammonia, and some metal ions. If there is too much of these in the water supply, not enough free chlorine is left over to kill microbes. Water authorities can struggle to work out how much chlorine they need to add to leave sufficient free chlorine when the water reaches the tap.

The team concluded that the Flint River water's corrosive effects on the city's pipes didn't just release lead into the water, it also added various ions that, while harmless in themselves, soaked up chlorine erratically, confusing efforts to compensate with larger doses.

L. pneumophila causes fever, coughing, and shortness of breath. In the last 10 years, it has overtaken gastrointestinal bacteria as the main cause of drinking water-related disease outbreaks. Besides causing thousands of deaths in the US every year, Legionnaires' disease outbreaks strain health systems as they require an average of roughly 10 days' hospital care per affected person.

Two and a half years after the crisis was recognized, much of Flint still lacks drinkable tap water. Meanwhile, five officials have been charged with manslaughter over the delayed warnings.