Miljøprojekt, 897 – Forekomst af Legionella - risikovurdering

Findings of Legionella - Risk Assessment

Summary and conclusions

The basis for preparing this report was, first, to evaluate the relevance of laying down official limits for the content of Legionella in water (quality criteria) defined by the authorities and secondto design a tool for case officers and consultants for evaluating of findings of Legionella.

The report is introduced with a short survey of knowledge of the clinical picture, routes of infection, occurrence and high-risk groups. The report focuses on the risk analysis itself, giving an evaluation of the individual sources that are known or suspected to present a risk of contamination with Legionella. The technological aspects of pool water, bathing water, drinking water, hot water, cooling towers, humidifiers and fountains are described, indicating the estimated risk and the possibility of limiting any undesirable factors.

It is concluded that the areas below are considered particularly problematic:

Poorly maintained spa-pools
Hot water supplies – particularly showers – operating at too low temperatures
Poorly maintained ice cube machines
Pre-mixing tanks for tempered water
High-pressure cleaning with water containing Legionella or flushing of surfaces with biofilm
Cooling towers
Certain humidifiers

Danish drinking water is not considered a risk, since the temperature is too low for the growth of Legionella. Likewise various other areas are considered unproblematic such as:

Public indoor swimming pools (cold water < 28°C)
Public indoor swimming pools (hot water > 30°C), with documented high maintenance and control standards of water treatment, including disinfection
Bathing water (fresh water and coastal water)
Hot water supplies, which are designed with documented high maintenance and control standards of temperature management.

The report concludes that the results of microbiological analyses of Legionella should always be related to the conditions of the places such as water temperature, risk of stagnant water, formation of aerosols and persons' staying time / time of exposure.

In this light and because of missing dose response correlation it is concluded that specific limit values should not be determined – only guiding levels. With specific limit values there is a risk of very rigid interpretations which in some cases may be far too comprehensive an in others too lenient. The detection of very low numbers of Legionella or even negative results, for instance, is no guarantee that a system or plant is safe. The results may be a sign that the plant or system is unproblematic, but may on the other hand merely indicate intermittent liberation of the organisms from the biofilm. If the conditions for growth are present, there is a potential risk. One of the final sections of the report, thus, indicates guiding reaction levels of microbiological detection in hot water supplies and in cooling towers.

Action plans and risk analyses of specific plants or systems are considered as useful tools for the prevention of Legionella and – in particular – of growth of Legionella. Preventive actions of this type should be taken in connection with the establishment of new plants as well as in the evaluation of existing plants. The work involved, however, is very limited once compliance with building regulations is ensured.