Legionella
Of the many concerns associated with indoor air quality, one of the most feared yet preventable is the presence of Legionella bacteria. This bacteria is associated with Legionnaires' disease as well as Pontiac fever, both of which are increasingly common and serious health concerns. Although these two illnesses are caused by the same organism there are major differences in attack rate, incubation time, and severity of symptoms. The attack rate for the exposed average population is 5% for Legionnaires disease and 90% for Pontiac fever. Generally, severity can range from a mild cough with a low fever to a flu-like illness or in the case of Legionnaires disease, pneumonia. Heavy smokers, the elderly, and immuno-compromised individuals are at a higher risk than the general population.

Legionella bacteria, when inhaled, can cause Pontiac fever as well as Legionnaires disease. The incidence of Legionnaires disease is quite low when compared to the incidence of Pontiac fever. Pontiac Fever is a less dangerous illness, but still a significant health concern. It is also less well known than Legionnaires disease and is often mistaken for the common flu. Engineering staffs should consider wearing proper respiratory protection when conducting any type of work near cooling towers or its discharge mist.

Cooling towers and any body of warm stagnant water can provide ideal conditions for growth amplification. The best conditions for growth are:

Since the total eradication of Legionella from a cooling tower system is often difficult and temporary, a continually monitored prevention strategy is needed to minimize the number of organisms present in the water source. Periodic maintenance is required to prevent the buildup of scale, sediment, and bio-fouling. A periodic re-evaluation of the type and dosage of biocides in a cooling tower system is critical.

The effectiveness of any water treatment program depends on the use of clean water. High concentrations of organic matter and dissolved solids in the water reduce the effectiveness of any biocidal agent. Cooling towers should be cleaned and disinfected semi-annually.

Airborne sampling for this bacterium is not effective and not recommended. Sample analysis of the water system is the only way to know of the presence and quantity of Legionella bacteria. Understanding the number of colony forming units per milliliter of water and exposure potential provides guidance as to recommended remedial procedures.

It is especially important for water samples to be analyzed by a certified independent microbiological laboratory.

The majority of the risk to building occupants depends on the ability of aerosolized bacteria to enter a building's air distribution system. Keep in mind, the engineering staff is often at greater risk due to job requirements.

A wise protection strategy for every building includes:

Engineering staffs should be particularly careful when working near cooling towers. Since the optimum growth temperature for Legionella bacteria is 95 - 115 degrees there is an increased probability of growth in cooling towers during hot weather - even when they are chemically treated. Microbiological labs that typically find 8% of all cooling tower samples to be positive for Legionella, have noted an increase to 12% positive samples, during chronic hot weather. With this in mind, it is even more critical that samples be analyzed by a microbiological laboratory that is an AIHA EMPAT participant to insure an accurate result. Culture analysis is a must. DFA analysis is not recommended.

EDC collects hundreds of Legionella samples per year and delivers each directly to an independent certified microbiological laboratory. A chain of custody follows each sample.