Legionella Assessment and Risk Management

Princeton Engineering Services, PC  specializes in Legionella Risk Assessment and in the development of a Legionella Risk Management Plan.

The Risk Management Plan (RMP) addresses the following critical risks:

   •  Stagnant water including the lack of water recirculation in a cooling tower system and the presence of dead-end pipework and other fittings in a system.
   •  Nutrient growth including the presence of biofilm, algae and protozoa in a cooling tower system, water temperature within a range that will support rapid growth of microorganisms in a system and the exposure of the water of a system to direct sunlight.
   •  Poor water quality including the presence of solids, Legionella and high levels of microorganisms in a cooling tower system.
   •  Deficiencies in the cooling tower including deficiencies in the physical design, condition and maintenance of the system.
   •  Location of and public access to a cooling tower or cooling tower system including the potential for environmental contamination of the system and potential for exposure of people to the aerosols of the system.

Princeton Engineering Services assessment steps

The steps involved in risk assessment and in the development of Risk Management Plan are as follows:

   1. Assessment of Water Systems

The first step as recommended in several guidelines to minimize the risk of Legionnaires' disease and resultant litigation is an engineering assessment of building water systems. Our assessment includes a thorough review of building water system design, operation and maintenance to ensure compliance with ASHRAE and other applicable standards.

   2. Development of Risk Management Plans

We work with you to develop a site-specific Risk Management Plan to ensure that your facility is in compliance with applicable standards. The Plan is a written Standard Operating Procedure (SOP), which details documentation, maintenance, operation, and water treatment practices.

   3. Personnel Training

Training programs are designed to improve your team's understanding of the root causes and aerosolization in potable and process water and cost effective maintenance and operation practices to control these pathogens.

About the Legionnaires' disease

The 'bug' that causes Legionnaires' disease

Legionnaires' disease is a type of pneumonia caused by Legionella bacteria. The bacterial species responsible for Legionnaires' disease is L. pneumophila. Major symptoms include fever, chills, muscle aches, and a cough that is initially nonproductive. Definitive diagnosis relies on specific laboratory tests for the bacteria, bacterial antigens, or antibodies produced by the body's immune system. As with other types of pneumonia, Legionnaires' disease poses the greatest threat to people who are elderly, ill, or immunocompromised.

Legionella bacteria were first identified as a cause of pneumonia in 1976, following an outbreak of pneumonia among people who had attended an American Legion convention in Philadelphia, Pennsylvania. This eponymous outbreak prompted further investigation into Legionella and it was discovered that earlier unexplained pneumonia outbreaks were linked to the bacteria. The earliest cases of Legionnaires' disease were shown to have occurred in 1965, but samples of the bacteria exist from 1947.

Exposure to the Legionella bacteria doesn't necessarily lead to infection. According to some studies, an estimated 5-10% of the American population show serologic evidence of exposure, the majority of whom do not develop symptoms of an infection. Legionella bacteria account for 2-15% of the total number of pneumonia cases requiring hospitalization in the United States.

There are at least 40 types of Legionella bacteria, half of which are capable of producing disease in humans. A disease that arises from infection by Legionella bacteria is referred to as legionellosis. The L. pneumophila bacterium, the root cause of Legionnaires' disease, causes 90% of legionellosis cases. The second most common cause of legionellosis is the L. micdadei bacterium, which produces the Philadelphia pneumonia-causing agent.

The bacteria is found in natural water bodies such as rivers, lakes, creeks and hot springs. The bacteria are also found in spas, potting mix, warm water systems and artificial systems that use water for cooling, heating or industrial processes, such as cooling towers.

Catching the disease

A person may catch Legionnaires' disease by breathing in fine droplets of water that contain the bacteria. You cannot catch it from another person or by drinking contaminated water.

Some people are at greater risk of infection than others

Although this is a common bacteria in the environment, only a few people who come in contact with the bacteria become infected. Some people are at greater risk, such as:

   •  Men over 50 years of age.
   •  Smokers.
   •  People with chronic illness
   •  People with medical conditions that impair their immune system.

The physical effects

The symptoms of Legionnaires' disease usually begin to appear within six days of being exposed to the bacteria. Early symptoms are like the flu and include:

   •  Headache.
   •  Fever.
   •  Chills.
   •  Muscle aches and pains.
   •  A dry cough and shortness of breath.

Sometimes other systems in the body are affected. This can cause:

   •  Diarrhea.
   •  Mental Confusion.
   •  Kidney failure.

Preventing Legionnaires' disease

There is no vaccine to prevent the disease. However, the growth of the bacteria in spas and cooling towers can be controlled. You can also take care to avoid exposure to Legionella bacteria from other sources.


Most cases of Legionella pneumonia show improvement within 12-48 hours of starting antibiotic therapy. The antibiotic of choice has been erythromycin, sometimes paired with a second antibiotic, rifampin. Tetracycline, alone or with rifampin, is also used to treat Legionnaires' disease, but has had more mixed success in comparison to erythromycin. Other antibiotics that have been used successfully to combat Legionella include doxycycline, clarithromycin, fluorinated quinolones, and trimethoprim/sulfamethoxazole.

The type of antibiotic prescribed by the doctor depends on several factors including the severity of infection, potential allergies, and interaction with previously prescribed drugs. For example, erythromycin interacts with warfarin, a blood thinner. Several drugs, such as penicillins and cephalosporins, are ineffective against the infection. Although they may be deadly to the bacteria in laboratory tests, their chemical structure prevents them from being absorbed into the areas of the lung where the bacteria are present.

In severe cases with complications, antibiotic therapy may be joined by respiratory support. If renal failure occurs, dialysis is required until renal function is recovered.


Appropriate medical treatment has a major impact on recovery from Legionnaires' disease. Outcome is also linked to the victim's general health and absence of complications. If the patient survives the infection, recovery from Legionnaires' disease is complete. Similar to other types of pneumonia, severe cases of Legionnaires' disease may cause scarring in the lung tissue as a result of the infection. Renal failure, if it occurs, is reversible and renal function returns as the patient's health improves. Occasionally, fatigue and weakness may linger for several months after the infection has been successfully treated.

Public Awareness

In November 1999, CNN & TIME, a TV news magazine, aired a segment on Legionnaires' disease in hospitals. CNN reported that "every year thousands of patients contract [Legionnaires'] disease from contaminated hospital water systems." The show included an interview with Dr. Victor Yu, a Legionnaires' expert from the VA Medical Center in Pittsburgh: "These outbreaks--they're actually occurring all through the country every week," said Yu. "It's an explosive problem to realize that people are dying, to realize that the means are there for preventing all this from happening, and that those means are not being exercised." Indeed, more people are learning that Legionnaires' disease is widespread, and that preventive measures are not implemented in most buildings.

How are Legionella bacteria spread in indoor environments?

Cooling towers are part of large modern air-conditioning systems. They are used to cool water and dissipate unwanted heat to the atmosphere through water evaporation. Warm water flows into the top of the cooling tower through spray nozzles (as shown in the schematic Figure 1). While the water passes through the nozzles, tiny airborne droplets are formed, providing maximum contact between the water and the air moved through the tower by fans. To prevent droplets from fusing into larger ones, splash bars are placed below the nozzles.

While falling through the tower, some of the water evaporates. Because evaporation consumes heat, the remaining water is cooled. Air pushed through the tower by fans also cools the water. Some droplets, known as drift, are carried out of the tower by the air stream produced by the fans. This water loss is reduced by a drift eliminator positioned at the top of the tower. The cool water collects at the bottom of the tower and is pumped back for another cycle.

Evaporative condensers (Figure 2 below) are similar in their construction and operation to cooling towers.

Cooling towers and evaporative condensers may contain Legionella and other microorganisms brought in by circulating air or water.

Legionella grows easily in the water, especially if algae and scale are present. Legionella can be dispersed with aerosolized drift or with the evaporate, but it may enter the air-conditioning system if there is a break between its ducts and those of the cooling tower or evaporative condenser.

Legionella has been found in hot water tanks, hot water propelled from shower heads and faucets and in whirlpool spas. It is not known whether Legionella enters a building's water from municipal feeder systems or adjacent contaminated cooling towers. The use of hot water with production of aerosols allows Legionella, if present in the water, to get into the lungs.

How can we prevent Legionnaire's disease?

Since the bacteria thrive in warm stagnant water, regularly disinfecting ductwork, pipes, and other areas that may serve as breeding areas is the best method for preventing outbreaks of Legionnaires' disease. Most outbreaks of Legionnaires' disease can be traced to specific points of exposure, such as hospitals, hotels, and other places where people gather. Sporadic cases are harder to determine and there is insufficient evidence to point to exposure in individual homes.

The likelihood of Legionella infection can be best reduced by good engineering practices in the operation and maintenance of air and water handling systems. Cooling towers and evaporative condensers should be inspected and thoroughly cleaned at least once a year. Corroded parts, such as drift eliminators, should be replaced. Algae and accumulated scale should be removed. These measures will not only control the growth of germs, but will also maintain operating efficiency. During cleaning operations in confined spaces, safety procedures for entry into confined spaces should be applied.

Cooling water should be treated constantly. Ideally, an automatic water treatment system should be used that continuously controls the quality of the circulating water.

The disinfectants commonly used in cooling towers are chlorine, dibromonitrilopropionamide, and ammonia derivatives called quaternary ammonium compounds. They appear to be effective in laboratory tests, but their value in practice has not been completely proven. It is important that any disinfectant used in a cooling tower water system be compatible with the tower's construction materials.

Fresh air intakes should not be built close to cooling towers since contaminated aerosols may enter the ventilation system. Air filters should be examined, cleaned and/or replaced periodically and tested for leaks. Cooling towers should be positioned so the drift or evaporate does not enter the fresh air intake. Hot water tanks, which might provide ideal conditions for the growth of Legionella, should be cleaned regularly. The water system should be flushed out on a regular basis to prevent the water from stagnating.



     A molecule created by the immune system in response to the presence of an antigen. It serves to recognize the invader and help defend the body from infection.


     A molecule, such as a protein, which is associated with a particular infectious agent. The immune system uses this molecule as the identifying characteristic of the infectious invader.

Colony Forming Units

     Legionella is reported as the number of Colony Forming Units per Millilitre (CFU/mL) of sample.


     A laboratory system for growing bacteria for further study.

DNA probe

     An agent that binds directly to a predefined sequence of nucleic acids.


     A molecule created by the immune system in response to the presence of an antigen. It serves to recognize the invader and help defend the body from infection.


     The protein molecule that serves as the primary building block of antibodies.

Immunosuppressive therapy

     Medical treatment in which the immune system is purposefully thwarted. Such treatment is necessary, for example, to prevent organ rejection in transplant cases.


     A disease caused by infection with a Legionella bacterium.


     Substance which contains all the nutrients necessary for bacteria to grow in a culture.


     The "ingestion" of a piece of matter by a cell.

We can perform a thorough Legionella Assessment & Risk Management