Reducing the risk of Legionella

Bacteria can grow in cooling towers, potable water systems
Sunday, December 2, 2012
By Bernard C. Siedlecki

Legionellosis refers to two diseases caused by inhaling airborne bacteria of the Legionella family. It is almost always caused by bacteria grown in and released from a building water system and, therefore, is essentially a man-made environmental disease. The more serious of the two conditions, Legionnaires’ disease, is a lung infection (pneumonia).

Sources and distribution of the Legionella risk
Legionella enters buildings as a bacterium found in very low concentrations in lakes and rivers. It is not completely eliminated by chlorination or other water treatment systems.

Legionella grows in warm water (above 20 C). The ideal growth range is 35 C to 45 C. The organism is killed above 65 C.

It is believed the organism lives mostly as a parasite inside deposits of other microorganisms in plumbing systems. This deposit, termed biofilm, can be found inside many building water systems such as cooling towers, tanks or distribution piping, particularly where surfaces are corroded or subject to scaling from hard water. Resistant to most water treatments, the biofilm protects the Legionella bacteria, which can be distributed into the water if the systems are disturbed.

The Legionella bacterium must be first inhaled to cause a respiratory disease. Common plumbing systems such as faucet aerators, showerheads, hot tubs, decorative fountains and cooling towers can all produce fine droplets containing the bacteria. These fine aerosols can be inhaled deeply in the lungs, particularly when less than five micrometres in size.

The inhalation of Legionella bacteria can result in two separate respiratory infections: Legionnaires’ disease and Pontiac fever, collectively termed legionellosis. Pontiac fever is a non-fatal flu-like disease that can last to two to five days. Recovery is quick, normally without the requirement of medication. Legionnaires’ disease is a more serious disease, fatal in approximately 10 to 15 per cent of cases. Even those who recover may suffer long-term respiratory effects. Persons with compromised immune systems are more likely to be infected and die from this disease; however, healthy individuals can still become infected by Legionella.

Risk factors for Legionella growth in building systems
Any water source in a building that can be aerosolized may be a Legionella risk, particularly where the water is dirty, remains still or is not exposed to a disinfection regime. Although cooling towers are the most commonly recognized source of potential Legionella exposure, at least one study has indicated that potable water distribution systems and hot water tanks can be colonized with Legionella growth at a higher rate than cooling towers. Because of this, all warm building water systems should be considered at risk of Legionella growth.

Several factors in domestic hot water storage tanks can increase the risk of Legionella growth. If the heating elements or coils do not reach the base of the tank, cool spots can occur in the tank. Pipe connections above the base of the tank allow for sediment accumulation, potentially harbouring biofilm growth. These combined conditions allow Legionella to grow in the sediment and biofilm. Disturbances to this sediment and biofilm from water pressure, surges from shutdowns or water hammer, for example, can release the Legionella into the water supply from where it can be aerosolized.

Ideally, water temperatures in hot water storage tanks are maintained at 60 C to minimize Legionella growth. This temperature can be difficult to maintain without the risk of scalding. Some regulations, for example, Ontario regulation 79/10 made under the Long-Term Care Homes Act requires that hot water serving resident areas not exceed 49 C. Water temperatures can be controlled by usage of mixing valves at the point of delivery, mixing cold water with hot to reduce temperatures.

‘Dead legs’ in water supply systems are a major risk for Legionella growth and often implicated in outbreaks. These are sections of piping or equipment left connected to the water supply but no longer serving a device. The stagnant water left in these dead zones allows bacteria to flourish. This contamination can drain back into the water system in the event of a water pressure differential or a shutdown of the system.

Cooling towers are perhaps the best known source of Legionella infections. The Legionella in the re-circulated condenser water can be aerosolized as drift and enter buildings via outside air intakes or operable windows, or even cause an exposure to persons outside of the building.

Reducing the risk of cooling tower Legionella contamination requires careful maintenance, frequent cleaning, effective water treatment and ongoing surveillance.

Reducing the risk of Legionella infections
The risk of Legionella infection is higher in health care settings; however, experience teaches it is not limited to these settings. Building operators are advised to reduce their risk of Legionellosis and the associated liability by taking the following steps.

  • Immediately implement the risk management principles of ASHRAE standard 12-2000, Minimizing the Risk of Legionellosis Associated with Building Water Systems. ASHRAE 12-2000 addresses a wide range of water systems, including potable and emergency water systems, heated spas and decorative fountains, to name a few. As a minimum, evaluate systems to eliminate any ‘dead legs.’ While not a substitute for sound maintenance practices, testing with culture analysis can be useful to verify the effectiveness of water treatment.
  • Follow the cooling tower maintenance recommended by the Cooling Technology Institute’s Best Practices for Control of Legionella, 2008.
  • Consider implementing the currently proposed ASHRAE standard 188, Prevention of Legionellosis Associated with Building Water Systems, when promulgated. (It is expected in early 2013). Based on the Hazard Analysis and Critical Control Point (HACCP) system used extensively in the food industry, this standard provides a framework for the analysis of systems at risk and the implementation of controls and monitoring. Following ASHRAE standard 188 will help satisfy the due diligence requirements of the various authorities having jurisdiction for public and workplace safety.

Bernard C. Siedlecki is Pinchin Environmental Ltd.’s senior associate of indoor air quality and microbial contamination.

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