Understanding the Persistent Threat of Legionella in U.S. Water Systems

An outbreak of Legionnaires' disease in New York City this August resulted in over 100 illnesses and claimed five lives. Over the past 25 years, reported cases of this severe pneumonia have steadily increased nationwide, especially during summer and fall. Experts from Virginia Tech, Amy Pruden and Kirin Furst, explain the dynamics behind the disease and its connection to hot weather.

Legionnaires' disease is caused by inhaling tiny water droplets containing the bacteria Legionella. This bacteria can thrive within the biofilms of plumbing systems and fixtures that supply water to taps, showers, and especially in hot water lines. Additionally, Legionella proliferates in water cooling towers, which are used in HVAC systems of large buildings.

The bacteria prefer warm temperatures, typically between 90°F and 105°F, making hot water systems an ideal breeding ground. Often, water heaters do not reach temperatures high enough to kill Legionella, allowing the bacteria to flourish. During summer, the temperature of water in pipes can rise to optimal levels for Legionella growth while disinfectant levels, like chlorine, decline faster, increasing the risk of bacterial amplification.

To prevent outbreaks, vigilance is crucial. Water system managers, particularly in hospitals, nursing homes, apartment complexes, and hotels, should regularly monitor water temperatures and disinfectant concentrations. Virgina Tech experts emphasize the importance of consistent testing and maintenance to keep Legionella at bay.

Individuals can also take steps to reduce their risk. Using water filters in refrigerators to ensure cool, safe drinking water is recommended. For at-risk populations, taking baths instead of showers can minimize inhaling aerosols containing Legionella. Setting water heaters above 140°F can help destroy bacteria, but caution should be exercised to prevent scalding by mixing with cooler water.

Further, shading water pipes and tanks can help keep water cooler by reducing surface and subsurface temperatures. Households may also advocate for municipal efforts to improve insulation and shading of water distribution systems. Proper household plumbing, especially in uninsulated areas, should be checked and maintained regularly.

This ongoing threat underscores the importance of stringent water system management and personal precautions, especially during warmer months, to prevent Legionella outbreaks and protect public health.

Source: https://medicalxpress.com/news/2025-08-hot-hidden-experts-legionella-persists.html