Legionella – an ongoing threat

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.

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 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 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.

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.

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.

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.

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.

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.

¹National Academies ²Kaplan, Sarah. “Legionnaires’ Disease, once ‘the Greatest Epidemiological Puzzle of the Century, Kills Seven in N.Y.”, Wash. Post, Aug. 3, 2015. ³Kaplan, Sarah. “Legionnaires’ Disease”. ⁴Centers for Disease Control and Prevention ⁵PubMed ⁶PubMed ⁷PubMed ⁸USEPA ⁹ASHRAE ¹⁰Centers for Disease Control and Prevention