Year Review 2021

New Legionella regulations to be rooted in latest scientific understanding

Scientific research into Legionella has yielded many new insights over the past twenty years that are now being used to improve the relevant legislation and regulations. The Minister of Infrastructure and Water Management has asked KWR and Berenschot to make a scientific and legal evaluation of the current rules in preparation for their amendment. KWR researcher Frank Oesterholt: “That is a sensible and necessary step. Scientific understanding is constantly developing. So if you want effective and meaningful laws and regulations based on sound science, you have to update those rules from time to time in line with the latest thinking.”

After several major outbreaks of legionellosis – also known as legionnaires’ disease – legislation and regulations relating to Legionella and Legionella prevention were tightened up early this century. They were based on the state-of-the-art knowledge at the time, with input from, among other sources, research by KWR and the collective research programme of the water companies, BTO.


In the meantime, worldwide scientific knowledge about Legionella (and its prevention) has been further expanded. The Ministry of Infrastructure and Water Management therefore asked KWR and Organisatieadviesbureau Berenschot last year to review the regulations for the prevention of Legionella in mains water systems on the basis of seven scientific questions, and where necessary to make proposals for scientifically based amendments. Berenschot was responsible for project management and the legal evaluation of the legislation and regulations, while KWR looked at the regulations on the basis of the latest scientific understanding.

Lower House

The Minister of Health, Welfare and Sport submitted this report to the Lower House of the Dutch Parliament on 16 November. For a comprehensive overview, please read the full report (in Dutch) on the government website, which consists of the minister’s letter to the Lower House, the KWR-Berenschot report on the latest scientific insights entitled Met recht naar een doeltreffender legionellapreventie – Een toekomstgerichte evaluatie van de regelgeving over legionellapreventie in leidingwaterinstallaties op basis van een wetenschappelijke en juridische analyse and a report from the supervisory committee with members from a wide range of backgrounds bringing together viewpoints and experiences relating to 23 questions and difficulties in the area of Legionella prevention based on their practical experience. Below, we will discuss some of the new insights that may lead to amendments in the regulations with Paul van der Wielen and Frank Oesterholt of KWR, who were closely involved in the scientific evaluation.

Mains water systems are not only the sources of Legionella

Oesterholt: “It has become clear in recent years that, among other things, Legionella in mains water systems is the cause of only a limited proportion of cases of legionnaires’ disease. Wastewater treatment plants and cooling towers result in more identifiable risks than mains water systems. Nevertheless, it is important to tailor the regulations for mains water systems properly as well in line with the latest scientific understanding so that we adequately protect vulnerable people but also deploy people and resources as efficiently as possible.”

Focus on Legionella pneumophila

One of the most important latest scientific insights is that, when monitoring mains water systems, the primary focus should be on the presence of the most pathogenic Legionella species (Legionella pneumophila). It is only in systems housed in buildings where there are large numbers of vulnerable patients that it is important to keep an eye on less pathogenic Legionella strains as well. Van der Wielen: “There are over seventy different strains of the Legionella bacteria. The current regulations are based on understanding from around 2000, when we still believed that the presence of any Legionella strain was a cause for alarm because it was thought to indicate that even the most pathogenic Legionella strains could grow in the plant. We now know that this is not the case: different Legionella strains need different conditions to grow and the presence of any random Legionella species does not necessarily mean that the most pathogenic strains can also grow well in that plant.”

In her letter to the Lower House, the Minister has already stated that she is willing to work with this understanding and wants a broader standard to be maintained only in locations where there are large numbers of people with compromised immune systems. This requires a sound specific detection method for Legionella pneumophila that can be included in the regulations. The Minister has asked the Dutch National Institute of Public Health and the Environment to check whether methods of this kind have been adequately standardised and validated.”

Temperature of pipe systems cannot be lowered

An important reason for the research request from the Ministry of Infrastructure and Water Management was the need to save energy, for example by lowering the temperature to which hot water is heated. Van der Wielen: “From the point of view of preventing Legionella, that is not a good idea. We actually advise making the regulations stricter and raising the temperature of the hot water in the heating unit to at least 60ºC everywhere and at all times in domestic systems and collective systems. That is because research has shown that, at temperatures below 60ºC, more pathogenic Legionella bacteria are found than when you stay above that limit. In turn, the Minister is now advising NEN, the Royal Dutch Standards Institute, to adopt our recommendations and she will actually take them into consideration for the purposes of implementing the EU Drinking Water Directive.”

Heat shock

In the current guidelines, it is possible to submit collective mains systems to weekly heat shock treatment in order to tackle any Legionella that may be present. However, research in recent years has shown that heat shocks as a preventive measure can lead to the heat-resistant Legionella pneumophila becoming predominant in the mains water system – and heat shocks are not effective against this strain. Van der Wielen: “Indeed: these heat-resistant strains can actually grow more after a heat shock because they can reproduce using the nutrients released when the heat shock kills biomass in the mains network. In that case, heat shock treatment actually has an adverse effect. In the coming year, the Minister wants a more detailed investigation of the effects of heat shock on Legionella and, where necessary, amendments to the regulations in line with that investigation.”


Current legislation requires the weekly flushing of collective mains water systems to prevent the growth of culturable Legionella. Van der Wielen: “Research in other countries has shown that this can be effective when disinfection residues are distributed with water because those residues react during long periods of stagnation, Legionella can grow. However, in the Netherlands, we distribute drinking water without disinfection residues. That is possible because the Dutch water companies ensure that the water contains very few nutrients on which micro-organisms can grow. No evidence has been found indicating that flushing is also effective for Dutch drinking water without disinfection residues, largely because most research is conducted in systems with disinfection residues. Experience in Dutch practice varies: flushing has also been seen to result in adverse effects. Moreover, flushing requires a lot of manpower in, for example, healthcare facilities and a lot of drinking water is wasted in the process.” The scientific report therefore proposes research into the effectiveness of flushing in Dutch situation with the aim of maintaining the requirement in the regulations to flush systems if it actually proves to be effective. The recommendation for flushing will be maintained in the legislation (through NEN 1006) as a measure to address problems with the taste and smell of drinking water during long periods of stagnation. The Minister has also adopted this recommendation.

In the coming year, the required research will be carried out and work will take place on the amendments to the Drinking Water Decree, the Legionella Prevention Regulations and the Drinking Water Regulations. The intention is for the new regulations to come into effect on 1 January 2023.