Rob Rottersman, Chris Donohue

April 1, 2023

Legionella – an ongoing threat

Rob Rottersman and Chris Donohue take a look at the history, biology, occurrence, and control of this deadly disease.

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Legionnaires’ disease remains a public health threat nearly 50 years after its first appearance, with the number of cases increasing almost 10-fold between 2000 and 2018. A 2019 report from the National Academies estimates that between 52,000 to 70,000 Americans suffer from Legionnaires’ disease each year, and thousands die.1 It kills more people in the US than any other reported waterborne disease.
A deadly discovery
In addition to celebrating the United States Bicentennial, the summer of 1976 is known for the headlines made by a mystery illness when more than 200 attendees at the American Legion convention in Philadelphia became ill, and 34 died.2 It wasn’t until 1977 when the cause was identified as gram-negative bacteria growing in the cooling tower of the hotel. The bacteria was named Legionella, and the disease was named Legionnaires’ disease, after the convention where the first documented outbreak occurred.
Legionnaires’ disease made headlines again in 2015 when more than 70 people became ill and seven died in an outbreak in New York City. Legionella was found in five cooling towers around the South Bronx.3
The biology of Legionella
The disease manifests as a serious lung infection, a type of pneumonia, after inhaling aerosolized droplets of water containing the bacteria. While Legionella is a waterborne bacteria, it does not cause disease by drinking. It infects people through inhalation of mist, or water droplets small enough that they may not be seen with the naked eye but large enough to transport the bacteria into a person’s lungs.4
Legionella species occur naturally in surface waters including lakes, ponds, rivers, and in mud. They are known to have symbiotic relationships with other bacteria and even parasitize and multiply in certain protozoa and amoeba species.5
Legionella and buildings
Legionella species are a public health concern when they colonize the water system within the built environment, such as a building cooling tower or plumbing distribution system. The problem is amplified when a building provides the right environment for the microorganism to live, multiply, and become aerosolized into a mist.
Often, the bacteria persist within biofilms inside these structures where they are largely protected from harsh environmental conditions including water treatments. Loose organisms along the outer layer of the biofilm can slough off and be transferred to other locations by the water running over it.6
Considering the multiple environmental survival tactics of Legionella, it is not uncommon to find low “background” concentrations of the bacteria in a building, where small amounts survive drinking water treatment and make their way into a building’s plumbing system and equipment.
Mitigation and treatment
Legionella in buildings can be mitigated by avoiding conditions that promote bacterial growth. The proper strategy depends on multiple factors and can include controlling water temperature, water disinfectant concentrations, and treatment frequency; evaluating and mitigating dead legs in the piping system; and taking measures to prevent water from stagnating, or aging, in the system.
Water temperature is a key factor in preventing Legionella growth. According to the Centers for Disease Control and Prevention (CDC), the ideal temperature range for the growth of Legionella is between 77⁰F and 113⁰F, and the organism will begin to die at temperatures above 135⁰F. Ideally, Legionella growth could be prevented by maintaining higher water temperatures, but this is not always practical. For example, water used in cooling towers and hot tubs is often in the “ideal growth range” and cannot be increased. In addition, potable water in buildings can be heated to kill bacteria, but proper controls are needed to avoid scalding water users.
Disinfectants and filtration are critical water treatment components. Disinfectants are typically added by the municipal water supplier at concentrations that help control bacteria. However, Legionella also grows by multiplying within organisms that make up the biofilm on the inside surfaces of pipes. While a disinfectant may be able to reduce free-swimming bacteria, the biofilm protects Legionella and once it gets a foothold in the pipe biofilm, it can be very challenging to control.
Increased Legionella risk after the Covid-19 pandemic
Early in the Covid-19 pandemic there were statements made by the CDC and American Industrial Hygiene Association (AIHA) regarding the potential increased risk of Legionella growing in building water systems. These statements were made somewhat naively, when buildings were thought to reopen by mid-April 2020.
When a building is not used or rarely used for an extended period of time, the stagnant water ages within the pipes, water temperatures drop below minimum thresholds, and disinfectant in the system will dissipate over time. The overall building water health is negatively affected and the risk of Legionella growth increases.7
As the pandemic dragged on some proactive building engineers would periodically flush water systems to bring in fresh water. Around this time, it was believed the primary route of transmission for Covid-19 was through touching surfaces contaminated with the SARS-Co-V2 virus. Therefore, many buildings began installing touchless, low flow faucets. However, to adequately flush a system fresh water needs to be drawn from the main pipe to each fixture. Facilities rarely have enough staff with the time needed to wave their hands in front of every touchless faucet long enough to draw fresh water to the end of each pipe.
Predictably, we have seen a substantial increase in Legionella concentrations in buildings with decreased occupancy and water use.
Rules and regulations
The US Environmental Protection Agency (USEPA) ensures that public water systems have proper disinfection systems in place through the Surface Water Treatment Rules (SWTR).8 It does not regulate premise plumbing systems unless a secondary treatment is added to the system.
Since large facilities tend to have conditions that are high risk for Legionella growth, facility owners tend to add filtration and disinfectants to the system, often redefining the system from a premise plumbing to a public water system.
However, USEPA’s Office of Pesticide Programs regulates antimicrobial products (chemicals and devices) used to treat water including mitigating Legionella. These antimicrobial products are considered pesticides and are regulated under the Federal Insecticide Fungicide and Rodenticide Act (FIFRA). Antimicrobial chemical pesticides require approval by both USEPA and each state where the product is used or offered for sale.
Guidelines for building managers
While building water management, particularly for Legionella, is largely unregulated, there are guidelines building managers can implement to reduce the risk of disease in their buildings.
Many states provide information to help facility managers make science-based decisions about different ways to control Legionella in premise plumbing systems.
The American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) Standard 188 provides guidance on the prevention of Legionella.9 The ASHRAE standard was written with language intended to be adopted into building code which can be challenging to follow. The CDC developed a user-friendly water management toolkit based on the ASHRAE standard.10 By following the steps in the toolkit a building manager could develop a program to prevent Legionnaire’s disease that aligns with the ASHRAE standard.
USEPA regulatory status
There are currently no federal standards for testing or management of Legionella in buildings; however, a few states and municipalities mandate Legionella testing in cooling towers – but these do not address water in other building equipment or plumbing. Further, there is no guidance for disinfectant manufacturers to register and sell water treatment products that can kill Legionella in water cooling towers.
USEPA is aware of this problem and is working with the Biological and Economic Analysis Division (BEAD), which has added Legionella in cooling tower water to its list of public health priorities. BEAD is currently working with external stakeholders to develop a test method for determining the efficacy of antimicrobial products against Legionella pneumophila in cooling tower water. This collaborative work is ongoing and involves four independent laboratories. The statistical analysis of the data from this research will inform USEPA’s anticipated new guidance on the matter.
BEAD is also working with the American Chemistry Council’s Center for Biocide Chemistries (CBC) and Office of Pesticide Program’s Antimicrobial Division to address proposed label claim language and product performance standards.
It is anticipated that USEPA will release this new interim guidance in 2023 which will allow companies a path forward in obtaining disinfectant claims against Legionella for cooling tower water treatment on product labeling.
Ramboll can help
Our experts respond when cases of disease are reported. We work with facility management and engineers to quickly perform risk assessments as well as design and implement testing protocols to identify or rule out potential sources of Legionella bacteria in buildings. When an outbreak is suspected there is uncertainty and anxiety. We manage communications with public health officials, building occupants, insurance companies, the media, and stakeholders. Our experts help facilities recover from outbreaks, creating a safe environment and preserving their reputation and brand.
Scientific and regulatory expertise in antimicrobial pesticide product development, testing, and registration with USEPA and states. Our scientists and regulatory experts have extensive experience in all phases of product development, product testing, and obtaining federal and state registration.
Legionella testing services and a team of health scientists skilled in risk communication. We help our clients obtain and interpret data for Legionella monitoring. We differentiate between low concentrations and species types that are commonly found in most buildings versus higher concentrations or Legionella species that increase the risk of disease. We provide balanced assessments and are skilled at communicating results and risk to affected stakeholders. We also help to diagnose potential sources of Legionella and determine the parameters supporting its growth in the water system, providing targeted guidance for achieving and maintaining Legionella control. We help building managers and engineers achieve healthy building water by developing water management programs that are specific to the building characteristics and use patterns and also practical to implement.
Expert support and testimony in litigation related to Legionnaires’ disease. Our experts have been retained to provide opinions associated with some of the most highly publicized outbreaks in the US. This has included the evaluation of facts and information related to potential sources of disease and the appropriateness of efforts to prevent or mitigate bacteria growth in water systems.

Want to know more?

  • Robert Rottersman

    Principal, Industrial Hygiene

    +1 312-288-3857

    Robert Rottersman

  • Chris Donohue

    Senior Managing Consultant

    +1 703-516-2310

    Chris Donohue