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Understanding Legionnaire's Disease
Causes and Prevention
by Brenda E. Barry, Ph.D.
What follows are answers to some of the most frequently asked questions about Legionnaires’ disease and the bacteria that cause it. This information is based on the work experience and applied research activities of EH&E’s multi-disciplinary staff, including toxicologists, certified industrial hygienists, and professional engineers.
Q. What causes Legionnaires’ disease?
Legionnaires’ disease is caused by inhalation of Legionella bacteria. These bacteria, which were named for the noted 1976 outbreak at a legionnaires convention in Philadelphia, actually can cause two types of infections: Legionnaires’ disease and Pontiac fever. Legionnaires’ disease is a serious debilitating pneumonia that can develop two to ten days after inhaling the Legionella bacteria. It most often affects the elderly and those with compromised immune systems. Pontiac fever is a milder infection that produces flu-like symptoms that last for only two to five days. It usually occurs in generally healthy individuals who recover with no lingering effects. The Centers for Disease Control and Prevention estimate that 25,000 to 100,000 people develop Legionnaires’ disease each year and that an estimated 8,000 to 9,000 deaths result from this disease. Liability issues become important factors for companies to consider when thinking about Legionnaires’ disease and its potential effects on employees or building occupants. Because the symptoms of Pontiac fever are not as severe, it is suspected that most cases go unreported. While the liability issue is reduced, companies should consider the loss of productivity that occurs when workers’ performance is affected by even a minor illness
Q. What conditions are required for Legionella bacteria to grow?
Legionella bacteria are normal inhabitants of lakes, streams, and wet soils. They can also be present in very low numbers in the municipal water supply provided to homes and businesses. If both the warm temperatures and appropriate nutrient conditions required by the bacteria are present, such as in a cooling tower, they can multiply.
Q. Is a cooling tower the only source of Legionella bacteria?
No. Although large air-conditioning equipment, such as cooling towers and evaporative condensers, are common sources, Legionella bacteria can be found in numerous potable water sources, including hot-water heaters, showers, hot tubs, and whirlpool baths.
Q. How can Legionella bacteria in a cooling tower affect the health of a building occupant?
Legionella bacteria can become airborne within aerosolized water vapor droplets that escape from a cooling tower. The droplets can then travel into the outdoor air intakes of the building’s ventilation system and reach the indoor air. If the droplets are inhaled by a susceptible person, they may cause Legionnaires’ disease or Pontiac fever.
Q. How can I tell if Legionella bacteria are present in my building’s cooling tower?
You cannot detect Legionella bacteria by simple visual inspection. Microbiological testing of the cooling tower water must be conducted. It is recommended that a trained professional collect the water samples using the equipment and personal respiratory protection. The samples are then sent to a reputable laboratory that can test for the presence of the Legionella bacteria.
Q. Our air-conditioning equipment is properly maintained and periodically cleaned. I have inspected the equipment and do not detect any algal or slime growth. Can I assume there are no Legionella bacteria?
No. There are more than 36 species of Legionella bacteria, and none are detectable by the human eye. In our experience, we have seen cases where cooling towers have significant algal growth and no Legionella bacteria, while other towers had little algal growth yet tests confirmed the present of Legionella bacteria. Therefore, the only way to be certain is to conduct microbiological testing. There are, however, conditions that can increase the chances of bacterial growth in a cooling tower: slime and algal growth, deterioration of materials, damage to components, blockages, corrosion, and faulty operation of fans, motors and pumps. So, it is important to continue to keep your cooling tower and its equipment properly maintained and cleaned.
Q. How can I prevent the growth of Legionella bacteria?
You cannot completely prevent the growth of Legionella bacteria. You can, however, reduce the risks of growth and subsequent Legionella bacteria outbreaks through appropriate cleaning, maintenance procedures, and regularly-scheduled testing of the cooling tower water during its seasonal operation. Early detection is the key to controlling the growth of any bacteria.
Q. Can building design affect the likelihood of a Legionella outbreak?
Yes. While you cannot absolutely prevent the growth of Legionella bacteria, you can take precautions to reduce the risks for bacterial growth. Proper building design and commissioning are important factors to consider. You should be sure that any building design includes proper location of cooling towers with respect to prevailing winds and outdoor air intakes. If your cooling towers are closely situated to your outdoor air intakes, you are at increased risk for inhaling Legionella bacteria in your indoor air. For this reason, we strongly recommend that if your building’s cooling towers are less than 30 feet from your outdoor air intakes, you should conduct microbiological testing monthly during the summer season as a precautionary measure. Additionally, when selecting a cooling tower, you should include ease of maintenance and cleaning, accessibility, reliability and drift eliminators in your list of considerations. If you are planning new construction or major renovations, EH&E’s professional engineering staff can provide advice concerning the location of cooling towers and the potential impact improperly placed towers can have on occupant health and safety.
Q. What is done if Legionella bacteria are found?
Depending upon the extent of growth, a proper remediation strategy would be developed and implemented immediately. For example, for a cooling tower, decontamination procedures would be conducted and a water testing protocol established. For a potable water system, two possible approaches are flushing the hot water system outlets with hot water (>150 degrees F) or flushing all outlets with water containing elevated levels of chlorine.
Dr. Barry is a Senior Associate and toxicologist at EH&E specializing in indoor air pollutants, their related health effects, and risk communication. She also specializes in laboratory safety and biosafety issues in the biotech industry.
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