The 2014-15 Legionnaires’ disease outbreak in Flint, MI, was caused by low chlorine levels in the municipal water system, according to a pair of new studies published recently in the Proceedings of the National Academy of Sciences and the journal mBio.

The outbreak, attributed to the ongoing Flint water crisis, killed 12 people and sickened nearly 90. The number of deaths produced by the water crisis increased to 13 in December. 

The studies were conducted by Michele Swanson and colleagues at the University of Michigan in Ann Arbor, Sammy Zahran of Colorado State University, and a team of researchers at Wayne State University in Detroit. Swanson has been studying Legionnaires’ disease for 25 years.

The research suggests that a complex set of factors is responsible for low chlorine levels during the crisis. Chlorine is responsible for killing microbes in the water. Lead, iron and organic matter in the water supply, however, can lead to decreases in the amount of chlorine available to kill bacteria.

A switch of Flint’s water source in April 2014 from Lake Huron to the Flint River introduced such heavy metals into Flint’s water system, causing the chlorine levels to lower and leading to the deadly outbreak.

“The really striking finding from our research is that the amount of chlorine that needs to be present is actually influenced by other factors in this large municipal water system,” says Swanson. “So, for example, during the Flint water crisis, the amount of chlorine that needed to be present to reduce the risk of disease was much higher than normal.”

The outbreak ended when the city switched back to its original water source.

According to NPR, the results of the study may be referenced in the ongoing court cases against six state and local water officials facing charges of involuntary manslaughter related to the Legionnaires’ disease deaths during the Flint water crisis.


Flint Water Treatment Plant

Chlorination explained

Chlorination is the process of adding chlorine to drinking water to disinfect it and kill germs, according to the Centers for Disease Control and Prevention (CDC). Different processes can be used to achieve safe levels of chlorine in drinking water.

Chlorine is available as compressed elemental gas, sodium hypochlorite solution (naOCI) or solid calcium hypochlorite (Ca(OCI)₂. While the chemicals could be harmful in high doses, when they are added to water, they mix and spread, resulting in low levels that kill germs but are safe to drink.

There are drawbacks to chlorination, according to the CDC:

  • relatively low protection against protozoa
  • lower disinfection effectiveness in turbid waters
  • potential taste and odor objections
  • potential long-term effects of chlorination by-products.

The benefits of chlorination, however, are numerous:

  • proven reduction of most bacteria and viruses in water
  • residual protection against recontamination
  • ease-of-use and acceptability
  • proven reduction of diarrheal disease incidence
  • scalability and low cost.