Quick Take: Why is Legionnaire’s Disease Making a Comeback?

Charles Haas
Charles Haas, PhD, a noted leader in water treatment and management research, supported the NAS’s report on how to reduce the spread of Legionella.

Legionnaire’s disease, a waterborne bacterial infection that got its name after an outbreak at a large American Legion gathering in Philadelphia just over 40 years ago, had been near elimination across much of the world for decades. Since 2000 there has been a five-fold increase of infections from 2000-2017 in the United States. Researchers are looking at why we’ve seen the spike and how we can reduce the prevalence of the disease. The National Academies of Sciences, Engineering and Medicine, with support from researchers like Drexel’s Charles Haas, PhD, who study how diseases spread in water, recently presented a set of guidelines for water treatment and management that could send Legionnaire’s back into remission.

Haas, who is a leading researcher in the link between water management and Legionella, notes that the mid-Atlantic region currently has the highest rates of reported Legionnaire’s disease in the country, so the report could be of particular pertinence to the organizations that set guidelines for how water is used and cleaned in Philadelphia and across the state.

“The is a problem not so much related to the nature of water sources and treatment,” Haas said. “But more so to the appropriate management of water in buildings, cooling towers, and the like, that allow the organisms to grow in the water between the treatment facility and your faucet or fixture.”

The report highlights the shortcomings of the Safe Drinking Water Act, which created the standard for municipal water treatment in 1974, but does not include regulations about managing water once it’s inside buildings. It calls for stronger federal policies and oversight of building water management to help prevent the spread of waterborne diseases like Legionella.

Many of the guidelines relate to recent developments in how we use water, including technologies and practices that help reduce water use in homes and conserve electricity — including low-flow fixtures and lower-temperature hot water heaters. The bacteria that causes Legionnaire’s is known to thrive in stagnant water that is not too hot.

“The Legionella bacteria that cause these illnesses grow inside amoeba that are part of biofilms – thin layers of microbes that coat wet surfaces. For example, biofilms can form in drinking water distribution systems and building plumbing systems and their associated faucets and showerheads, cooling towers, hot tubs, and fountains. Conditions that promote the growth of biofilms and Legionella include warm temperatures, stagnant water, lack of chemical disinfectants, and the presence of nutrients in the water because of pipe corrosion,” the authors of the report write.

The guidelines include:

  • Low-flow fixtures should not be allowed in hospitals and long-term care facilities, due to these buildings’ high-risk occupant populations.
  • Guidance is needed for homeowners about practical steps that can be taken to combat Legionella For example, flushing stagnant water from taps after periods of disuse and increasing water temperature in household hot-water heaters to 140 degrees Fahrenheit.
  • Criteria for certifying green buildings, energy-conserving features, and water conservation features should be modified to take into account risk factors for growth of Legionella in building water systems
  • Cooling towers should be registered and monitored. Building- and industrial-scale cooling towers – which remove heat from recirculating water used in water-cooled chillers, heat pumps, and other HVAC equipment — have been implicated in many outbreaks of Legionnaires’ disease. These towers may generate bacteria-laden aerosols that drift away from the facility and then are inhaled by people working and living in the building or by passersby.
  • All public buildings, such as hotels, businesses, schools, apartments and government buildings should be required to have a water management plan.
  • A minimum disinfectant level should be required throughout public water systems, along with monitoring for Legionella.

You can access the full NAS report “Management of Legionella in Water Systems” here.


Charles Haas, PhD, is LD Betz Professor of Environmental Engineering in Drexel University’s College of Engineering

Charles Haas
Charles Haas, PhD, a noted leader in water treatment and management research, supported the NAS’s report on how to reduce the spread of Legionella.

and head of its Department of Civil, Architectural and Environmental Engineering. Haas studies how pathogens and toxins can persist and be disbursed in water systems and is a noted authority in quantitative microbial risk assessment.


For media requests, contact Britt Faulstick, assistant director of media relations, bef29@drexel.edu, 215.895.2617

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