Although the pneumonia typically known as Legionnaires’ disease is considered rare, with only about 5,000 U.S. cases annually, it’s potentially deadly, with the elderly and those with compromised immune systems being the most at risk.

A U.S. Centers for Disease Control and Prevention study involving 196 cooling towers nationwide found that 84 percent contained Legionella DNA, indicating that the dangerous bacteria were present or had been at some point. This means the question is not if there will be another outbreak but where and when it will occur. 

The disease is named for the event where it was first discovered, the July 1976 American Legion convention at the Bellevue-Stratford Hotel in Philadelphia. About 4,000 members of the veterans organization were attending the event. A week after the convention, more than 200 attendees fell ill, complaining of chest pains and fever. Thirty-four of them eventually died. 

An intense investigation by the CDC determined the illness was caused by bacteria that bred in the cooling towers of the hotel’s HVAC system.

According to the CDC’s website, “During 2000–2014, passive surveillance for Legionellosis in the United States demonstrated a 286 percent increase in reported cases per 100,000 population.”


Keeping Legionella out of water systems in buildings, with cooling towers a noted risk, is critical in preventing infection, the CDC says.

In response, some HVAC construction contractors now are pairing chillers and high-performance plastic cooling towers with new antimicrobial options that can significantly reduce infection risk. 

Cooling towers are used in many large HVAC systems.

Cooling towers are used in many large HVAC systems.

Controlling pathogens

Throughout the U.S. and most of the world, the mainstay of large cooling systems remains the traditional HVAC combination of chillers, air handlers and cooling towers.  Cooling towers have a long history of effective use in expelling heat from the water used in many commercial and industrial applications that involve chillers.

However, it is well established that under typical operating conditions, cooling towers can propagate the bacteria that causes Legionnaires’ disease.  The design of many cooling towers creates pockets where water may stagnate, a condition that can lead to microorganism development. 

This has recently led the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ to publish a new standard, “Legionellosis: Risk Management for Building Water Systems,” which documents requirements for the HVAC design of new buildings and the renovations to existing structures.

“All facilities with HVAC or process cooling systems need to be aware of Legionnaires’ disease and handle any concerns about it,” said Rick Hill, facilities director at Arkansas Surgical Hospital, a physician-owned hospital specializing in joint and spine surgery in North Little Rock, Arkansas. “There have to be good procedures in place to prevent or control it.”

When it was time to replace an air-cooled chiller at Arkansas Surgical Hospital, Steve Keen, president of Powers of Arkansas, the HVAC contractor responsible for the hospital project, recommended and installed a water-cooled chiller, paired with an advanced cooling tower with unique antimicrobial properties.

“Legionella is always a concern for HVAC systems using a cooling tower and anywhere you have water exposed to the atmosphere,” Keen said.  “The Delta Cooling Towers’ antimicrobial properties will help prevent that type of growth and exposure to patients and staff.”

Delta Cooling Towers Inc., which pioneered the HDPE (high-density polyethylene) plastic cooling tower in the 1970s, recently introduced a line of towers constructed of antimicrobial resin, which is fully compounded into the base cooling tower structural material and casing. The cooling tower fill-and-drift eliminator, which captures large water droplets in the air stream, is also made from antimicrobial polyvinyl chloride.

The antimicrobial resin contains wide-spectrum additives that operate on a cellular level to continuously disrupt and prevent uncontrolled growth of microorganisms and biofilm within the cooling tower.  Efficacy tests were performed by Special Pathogens Laboratory, which bills itself as “The Legionella Experts.” 

Cooling tower design and materials can be very significant in the prevention of pathogen growth.  To avoid problems of stagnant water leading to pathogen growth, experts recommend cooling tower designs feature a sloped basin and/or basin sweeper system.

While some cooling tower manufacturers now market a tower with an antimicrobial fill (the medium over which the hot water is distributed as it is being cooled), another option is to have a cooling tower featuring the fill, structural casing and sump all composed of antimicrobial material.

“We decided that since we were replacing the air cooled chiller with a water cooled chiller, we wanted a cooling tower that aligned with our philosophy of protecting patient safety,” said Hill, who advocated for the antimicrobial technology and found support for it among his hospital’s leadership.  “We want to maintain one of the lowest infection rates among hospitals in the country.”

cooling towers Legionnaire's disease

Energy savings

By proactively making the switch to an advanced antimicrobial tower cooler, paired with a very high efficiency HVAC chiller, the hospital is also significantly reducing energy costs, which was also a prime consideration. 

“Our previous air-cooled HVAC system required a lot of electricity,” Hill added. “With the water-based cooling tower and very efficient chiller, however, we expect to save tens of thousands of dollars annually in energy costs.”

Durability and longevity of the cooling tower were additional issues that Hill considered.

Metal-clad cooling towers are vulnerable to corrosion from salt-laden air, industrial gases and even the chemicals used to treat the recirculating water.  The best water treatments for Legionella prevention, in fact, are oxidizing biocides which react aggressively toward metal surfaces, effectively attacking metal-clad cooling towers and shortening service life.

As a result, metal cooling towers require increasing patching, maintenance, costly downtime and eventual replacement.

Hill said he considered a metal cooling tower, but ultimately decided it would be too difficult to maintain. 

“Maintaining a metal cooling tower is more work than we wanted and the units have a shorter lifecycle because metal will rust and require mending and repair,” he said. 

In choosing Delta’s cooling tower, which features a fill, drift eliminator and shell all constructed of corrosion-proof antimicrobial plastic, Arkansas Surgical Hospital now has a cooling tower that is impervious to the corrosive effects of ambient air and water treatment chemicals, as well as oxidizing biocides — all of which plague metal-clad cooling towers. 

In addition, since the engineered, molded plastic cooling towers are one piece, there are no problems with seams, welds and patches that wear prematurely.  Therefore, the plastic models offer extended longevity and require far less downtime for cleaning, repair or replacement.

“In terms of lifecycle, the Delta cooling tower has a 20-year warranty on construction,” Hill said.  “You don’t get that with a metal cooling tower.” 

This article and its images were supplied by Delta Cooling Towers Inc.