The recent Bronx, NY outbreak of Legionnaires infections is a grim reminder of the deadliness of this strain of pneumonia. Like many of the outbreaks, cooling towers caused this one. Twenty have been identified but the search is still going on. Over 120 people were infected and 12 have died. Tragically it affects the most vulnerable people--people with one or more immune system deficiencies.
Legionella bacteria thrive in cooling towers because food sources blow in and water is a great breeding environment. Wind gusts blow through the tower and aerosolize the contaminated water and disperse it in all directions. This is why legionnaires is an IAQ problem.
Cooling towers aren't the only source of Legionnaires. People become infected when they inhale microscopic water droplets containing legionella bacteria. This might be the spray from a shower, faucet or whirlpool, air conditioners, evaporative condensers, potting mixes, and decorative lobby fountains or...drip pan water dispersed through the ventilation system in a large building.
All HVAC systems should first prevent Legionella living in drain pans and getting into its air stream. Source prevention using routine treatments involves various kinds of chemical disinfection, biocides or high temperatures for containers, pipes, surfaces, and so on. Typical MERV7-8 prefilters are not sufficient to stop the droplets. Air purification using UV light followed by activated carbon should destroy bacteria laden mist before moving it through air conditioning. You cannot wait to get test culture results since it takes 8-14 days. HVAC needs to be part of the solution to halting spread of Legionnaire's.
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