Factors affecting Legionella growth

Small amounts of Legionella bacteria contained in the network water pass into many water systems. The conditions in the water systems play a decisive role in what kind of levels the concentration of legionellae can reach in them.

It is hard to completely remove legionellae from water systems. In order to prevent illness, it is usually enough that the concentration of legionellae in the water system remains low.

This page describes factors that have a significant impact on legionellae in water systems: 

• Water temperature
• Degree of utilization of water systems
• Water system structure
• Materials in the water system
• Maintenance of water systems
• Other microbes
• Biocides

Water temperature

Temperature is an important factor in the growth of legionellae. Legionellae can multiply in water with a temperature between 20 °C and 45 °C. Above this temperature, the legionellae will begin to be damaged and the majority of legionellae will be destroyed within a few hours when the temperature is over 50 °C. 

In water systems built or repaired after 2007, according to the Ministry of the Environment Decree (1047 / 2017), hot water must be at least 55 °C throughout the water system. This means that the water from the heat exchanger must be significantly hotter, in some situations as hot as 65 °C.
Ministry of the Environment Decree on Water and Drainage Systems in Buildings (1047/2017) (in Finnish)

In properties built before 2007, the recommended minimum temperature of 50 °C is commonly complied with. However, it is not always enough to prevent the growth of Legionella and cases of legionellosis. In order to control legionellae, it is worth keeping the domestic hot water at around 55–60 °C regardless of the system's age. In the most problematic water systems, it may be necessary to increase the temperature of the domestic hot water up to 65 °C.

In addition, the temperature of household water and other cold water must be kept below 20 °C to reduce the growth potential for legionellae.

Degree of utilisation of water systems

The degree of utilisation of water systems has an impact on the growth potential for legionellae: water systems that are used little or not at all present the greatest difficulties for the prevention of legionellae.

All water points in water systems should be in regular use, meaning daily use where possible.

For example, if the humidification or cooling water system has been out of use, it should be disinfected with biocides that are also effective against legionellae. 

The safe sealing of the domestic hot water system requires that the system is chlorinated before and after sealing, the water is heated to 60–65 °C, and the effectiveness of the control measures is ensured by Legionella analyses before taking the system into use again.

The water in swimming pools and whirlpools should be sufficiently disinfected at all times. Whirlpools that are emptied after each use do not generally pose any legionellae threats provided that the hot water in the building's water system is sufficiently hot (at least 50–55 °C) and the cold water is sufficiently cold (below 20 °C). However, problems may arise from microbial growth in the pipe sections of the whirlpool during breaks between use. It can be prevented by chlorinating the water in the whirlpool on a regular basis.

Water system structure

The structure of the water system affects legionellae, for example, through the prevailing water temperature and the possibilities for the spreading of legionellae through aerosols.

In the water system, the areas where water exchange is slow are a good growth environment for legionellae. The distance from the circulating water to the water fixtures, known as the connecting section, should therefore be as short as possible.

Unused water fixtures should be completely removed, including the coupling sections.

The shorter, simpler and faster the circulation of water in the hot water system, the easier it is to control legionellae through the regulation of water temperatures.

The design and operation of cooling systems are usually such that the Legionella populations must be regularly controlled with biocides, either automatically or manually.

The safest humidifiers are injection steam humidifiers because of their hot operating temperature. For other humidifier types, disinfection treatments may be required, in addition to maintenance, to prevent the growth of legionellae.

Materials in the water system

The materials in the water system affect legionellae in various ways. Experiments have shown that the growth of legionellae on a copper surface is initially slower than on different kinds of plastic surfaces. For example, some of the rubber materials used in valves and fixtures may promote the growth of legionellae.

Maintenance of a water system

Water system maintenance is an essential part of Legionella control.

It has been observed that the formation of deposits and biofilm in places such as the bottom of tanks and the surface of water fixture components increases the growth potential of legionellae.

Regular cleaning and maintenance is important for filters and other accessories in water systems. New water systems can also quickly become contaminated with Legionella if the conditions are favourable for them.

It is essential to ensure to the maintenance and especially the disinfecting of all open and aerosol-forming water systems that contain hot water, even if they only hold the water for a short period of time.

Other microbes

Other microbes can increase or decrease the growth of legionellae.

Legionellae can use protozoa, especially amoebas, as their breeding sites. Legionellae are also found in the biofilm formed on the surfaces of pipes, which provides them with protection against substances such as microbial pesticides.

Legionellae concentrations cannot be assessed based on other microbes, but rather a specific legionellae analysis of the samples is always needed.

Microbial pesticides / biocides

The usefulness of microbial pesticides (biocides) or slimicides against legionellae varies depending on the type of water system. Biocides should not be used where they are not needed, which means that the Legionella situation and the efficacy of the pesticide used against legionellae should be confirmed by sampling.

Chlorine compounds are the most commonly used substances for the control of legionellae in domestic hot water and cold household water systems. Peracetic acid can be used to give cold drinking water systems an intensive clean.

Water in whirlpools and swimming pools must be chlorinated in any case to prevent the growth and infection of other microbes, and this is usually sufficient to also control Legionella bacteria. In whirlpools, successful control of legionellae usually requires slightly higher chlorine concentrations than those used in cooler pool water.

Many other chemicals can be used in industrial water systems.

Chlorine, chlorine dioxide and monochloramine can also be used to control Legionella, for example in more extensive water systems such as in hospitals, as continuous feed or shock treatment.

The biocides currently approved for use in cooling and other circulating water systems in Finland are listed in the Finnish Safety and Chemicals Agency list of permitted protective chemicals:
Biocides (list of permitted protective chemicals, Finnish Safety and Chemicals Agency Tukes, in Finnish)