Legionnaires’ disease is a deadly pneumonia caused by inhaling Legionella pneumophila or other species of Legionella bacteria that lurk in the organic matter lining drinking water pipes. The disease has been on the rise in the US, with cases quadrupling over the last 10 years. Headline-grabbing outbreaks often occur from contaminated ventilation or hot water systems in hospitals, which is where monitoring and prevention efforts have been focused. However, the majority of Legionnaires’ disease cases in the US arise from private homes with no common link except their water supplies, underscoring that drinking water distribution systems are the ultimate source of outbreaks.


Several conditions of piping systems influence the possibility of Legionella or other disease-causing organism growth [1]:

  • Age: Corrosion, leaks, biofilm, and scale buildup are more common in older pipes.
  • Corrosion-resistance: Industrial processes involved in pipe-manufacturing often introduce corrosion-increasing ingredients like iron-oxidizing bacteria, sulfate-reducing bacteria, and acid-producing material.
  • Material: Smooth pipes generally harbour less biofilm than rougher ones, with studies showing CPVC, stainless steel, and polypropylene pipes to experience 50% less biofilm-growth than copper and PEX. 
  • Chemicals: Not all pipe materials can tolerate continuous exposure to high temperatures or chlorine. Different pipe materials require different ranges of treatment to withstand pH-changes from chloramine reactions.
  • Size: Oversized pipes slow water flow, reduce helpful turbulence within waterways, and increase surface area for biofilm growth in the produced stagnant/low-flow areas.

Influence of Corrosion

Legionella flourishes in warm-water environments containing the nutrients needed for survival, which are present in abundance within corroded water pipes. Reactions with acidic water cause the protective linings of piping to dissipate, allowing iron to leach into water and any present Legionella to thrive on the micronutrient [2]. The metal also reacts with and inactivates chlorine disinfectant that otherwise would kill the bacteria.

Outbreak in Flint, MI

The infamous Legionnaires’ disease outbreaks in Flint, Michigan during the summers of 2014 and 2015 were directly correlated to the city’s switching of water sources to the Flint River without any adjustment of corrosion-control measures. The Legionnaires’ outbreak patterns were tied to the interactions of new water chemistry with the city’s pipe materials caused by the switch [3]. With no federally-mandated corrosion-control measures in place, the switch caused rampant corrosion within the city’s lead and iron pipes, low chlorine residuals and elevated bacteria. High iron concentrations and warm water caused the ideal breeding ground for Legionella within the city-wide water distribution system [3]. 

Preventative Measures and Testing

For future water system design, persistent disinfection using chlorine is critical in reducing the risk of Legionella growth. Pipe material has to be strategically chosen to work in tandem with water chemistry and disinfection plans. Although copper pipes have natural antimicrobial properties, a lack of additional corrosion-control leads to elevated iron and reduced chlorine residual, which in turn enhances Legionella growth [3]. Interactive effects of system components make frequent testing especially important. The detection of Legionella can be done directly using a legionella-specific water test, or indirectly by testing for excessive levels of lead or iron within piping systems. ExactBlue Technologies Inc. offers accessible and simple Legionella water tests which, if used regularly, can help curb potential waterborne disease outbreaks.