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1. What is legionella?

Legionella is a bacteria that can cause Legionellosis, also known as Legionnaires’ Disease (a serious lung infection). There are close to 60 species of the bacterial genus Legionella, and Legionella pneumophilais is responsible for the majority of human cases. Legionella multiplies and survives within a temperature range of 77°F to 108°F (25°C to 42°C).

2. What are the sources of legionella?

Legionella is ubiquitous in the environment and is found in hot and cold water taps, creeks, ponds, and surrounding soil. Potting soil has also been associated with cases of Legionellosis, though the mechanism is unknown. Legionella does not transmit from person to person. 

For water, the route of exposure is through aerosolization or inhalation. Exposure has been associated with hot water systems, air conditioning cooling towers, evaporative condensers, mist machines, decorative fountains, respiratory therapy devices, humidifiers, hot tubs, and whirlpool spas.

3. What are the risks?

For many people exposure to Legionella will have no effect; however, for others it may cause mild symptoms, such as headache and fever. A small percentage of people exposed will become seriously ill. The elderly, smokers and persons with chronic lung disease are at higher risk for infection. Also at increased risk are immunocompromised individuals such as those with cancer, diabetes, or AIDS. Symptoms of the disease will usually appear 2 to 10 days after exposure. The annual incidence of Legionellosis nationally is between 20 and 25 cases per million people since 2015. More regionally, in San Francisco, it is between 1 and 5 cases per million people since 2015.

4. How does the City of Napa control and monitor for legionella in drinking water?

The Environmental Protection Agency (EPA) has an established Maximum Contaminant Level Goal (MCLG) of zero for the presence of Legionella in drinking water. 

The City of Napa water system meets this non-enforceable guideline through compliance with EPA’s Surface Water Treatment Rule, which requires water systems to filter and/or disinfect water so that microorganisms are removed and/or inactivated. 

The City of Napa monitors it’s residual disinfectant in the distribution system. Even with these processes, however, small amounts of Legionella can survive, and, given the proper conditions, can colonize in on-premise plumbing systems. 

Consequently, a key element for controlling Legionella is the design and maintenance of cooling towers and on-site plumbing systems to limit Legionella growth and aerosolization. Increasing the temperature of hot water systems and ensuring proper biocide treatment of cooling towers may limit exposure to Legionella.

5. How can legionella be controlled in residential homes?

Residential customers can reduce or eliminate potential exposure to Legionella by setting their water heater at or above 140 F (60 C ). This is in accord with U.S. Centers for Disease Control (CDC) guidelines and Standard 188 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), which specifies how to set up and implement a water management program to control Legionella. However, maintaining such high water temperatures can introduce a scalding risk, so thermostatic mixers may be used to minimize this risk. 

6. What are large building best practices for control of legionella?

The most common location for getting Legionellosis is in large buildings, such as hotels and hospitals. Large building owners should follow the ASHRAE Standard 188.