Since 1976, outbreaks of Legionnaires' disease have often been surrounded by intense publicity and concern from the general public looking for answers as well as protection against future outbreaks. Many of the outbreaks associated with Legionnaires' disease could have been avoidable with a comprehensive understanding of the factors involved along with appropriate actions to test and maintain clean water systems.
The largest outbreaks of legionnaires' disease have originated in cooling towers from legionella bacteria. Though we strive to maintain clean systems for optimum heat transfer, standard biological control of cooling systems for operational performance does not entirely protect from the threat of Legionella bacteria. As long as there is potential for Legionella and any risk of Legionnaires' disease, engineers and building owners should take seriously the obligation to ensure that exposure is minimized.
Legionella bacteria can be present in the bulk recirculating water of a cooling tower and the structural supports. Legionella actually live in protozoan microorganisms and grow within the biofilm and mass of dead microorganisms. The bacteria are often in the biomass and neither identified with or killed by biocide additions.
To contact Legionnaires' disease, the bacteria must be inhaled. The water droplets may be as small as 3-5 microns in size. The incubation period is 5-10 days. The symptoms mirror pneumonia and the typical treatment of penicillin is ineffective; erythromycin is recommended.
Low bacteria counts in the bulk water do not guarantee system cleanliness. An apparently clean system with low bulk water bacteria counts may encourage Legionella bacteria by eliminating the competition of other bacteria. Systems high in suspended solids are especially suspect because they collect bacteria and provide a substrate for growth.
There are serious liability factors associated with the do-nothing approach towards Legionella control. Facilities have been sued for lacking Legionella programs and it appears that litigation will only increase in the future. The Occupation Health and Safety Administrative (OSHA) has classified Legionella as a preventable workplace hazard. Other countries such as the United Kingdom and Australia have issued standards for acceptable and unacceptable levels of Legionella in cooling towers and have severe penalties for non-compliance.
The Cooling Technology Institute (CTI) and the American Society of Heating, Refrigerating, and Air-Conditionong Engineers (ASHRAE) have recently published standards which now detail "best practices" for Legionella prevention. Even so, they do not currently recommend routine testing for Legionella. However, certain states and counties have supported tests to identify the presence of Legionella and verify disinfection following an outbreak. The JCAHO has, as of January 1, 2001, required hospitals to develop a program for managing pathogenic biological agents in cooling towers, domestic hot water and other aerosolizing water systems.
Because Legionella is an actual parasitic and can live within biofilm and masses of dead algae, the key to controlling Legionella is control of the biofilm, or simply stateda good microbiological control program. The most widely accepted program to control Legionella is continuous feed of an oxidizing biocide.
Intermittent feed of non-oxidizing biocides can be effective to control Legionella if they have been field-proven to keep a specific system clean. Non-oxidizing biocides may be used alone, alternated in a dual biocide program or used to supplement the oxidizing biocide to keep the system free of biofilm. Bacteria counts should be frequently measured in order to evaluate general system cleanliness. These tests do not usually identify a specific organism and are no insurance against the presence of legionella. Field tests are now available that accurately indicate the presence of the most critical Legionella strain. In conjunction with the biocide program, the cooling tower owner should also have systematic additions of an effective deposit and corrosion inhibitior. Tower water filtration and scheduled cooling tower cleanings should also be a part of the ongoing program.
Some cooling water systems are more susceptible to grow Legionella bacteria and some present more risk based on their location. Some hospital domestic water loops and ornamental fountains are maintained less than 120° F whereas temperatures greater than 140° F are required to kill the bacteria. Towers that could release mist near air intakes to the host buildings pose a high risk for the disease if the bacteria is present. Systems that contain stagnant water lines or are high in suspended solids and towers with algae (even dead algae) are likely to contain Legionella bacteria. New construction areas are susceptible to Legionella bacteria where dirt could be scrubbed into the cooling tower and systems are left stagnant.
The Legionella bacteria can be present in a wide variety of water using systems and equipment. In order to protect building occupants, facility engineers and building managers should evaluate their properties in relation to the existing water systems. Both general health and liability issues should provide the motivation to follow the newly published guidelines and provide for a safe building environment. Trident Technologies has available a number of special services to help address the Legionella issue at all levels.