Risk assessment for legionella

BSRIA, Legionnaires' Disease, legionella
Understanding the basic principles of legionella control is still the key to minimising the risk — Reg Brown.

With new guidance on risk assessment for Legionnaires’ Disease about to be published by BSRIA, Reg Brown takes a pragmatic view of the issues.

Legionella bacteria are ubiquitous in nature, but it is water systems within the modern built environment that provide ideal conditions for them to multiply to hazardous concentrations. Inhalation of legionella pneumophila bacteria in microscopic water droplets dispersed from these systems can then result in Legionnaires’ Disease, a potentially fatal form of pneumonia.

There are typically 300 to 400 notified cases of Legionnaires’ Disease each year in the UK, but many more cases of legionellosis with less-serious symptoms may go unreported as they recover without medical intervention.

European statistics for reported cases indicate that 44% of cases are in persons aged 65 or over and that men are 2.6 times more likely to contract the disease than women. Very few reported cases occur in the under 25s. The increase in susceptibility with age, as well as the severity of the symptoms, is largely a consequence of impaired lung function due to smoking and pre-existing medical conditions.

The relatively low incidence of Legionnaires’ Disease in the UK (fewer than one in 100 000 of the population) is only possible because of the continuing vigilance of building and facilities managers. Where this breaks down, particularly in the case of cooling towers, deaths can result.

A recent outbreak near Lisbon in Portugal (November 2014) resulted in 311 cases and 7 deaths. Another near Barcelona in Spain (September 2014) resulted in 40 cases and 8 deaths.

No large outbreaks have occurred in the UK since Edinburgh and Stoke on Trent in 2012, involving a total of 120 cases and four deaths.

Minimising the risk of Legionnaires’ Disease is therefore a key public-health concern of building and facilities managers in all buildings — particularly those with cooling towers and those supporting vulnerable populations in hospitals and care homes. The question is whether we have learned anything in nearly 40 years since the Philadelphia outbreak in 1976 that named the disease.

It is not impossible to keep legionella out of water systems, just impractical for the majority. Where substitution of water systems with dry equivalents is not feasible, the focus is on avoiding the development of hazardous concentrations. In many respects the early guidance has stood the test of time. The correct use of biocides and regular cleaning will help to prevent colonisation of cooling towers. Avoiding temperatures between 25 and 50°C and stagnation will help to prevent colonisation of domestic water systems. Almost all other practical guidance is a development of these principles.

Unfortunately, when general principles are turned into simple rules the original point may be lost. The risk-assessment mantra for domestic water services is ‘greater than 50°C in one minute or less than 20°C in two minutes’.

Achieving a temperature greater than 50°C is more important than the arbitrary one minute. One minute is actually a rather long time to be stood next to a running tap with a thermometer, yet 55 s would be classed as compliant and 65 s as non-compliant. In reality many non-residential hot-water systems are recirculatory and taking longer than 15 s to achieve 50°C would suggest some kind of problem.

BSRIA, Legionnaires' Disease, legionella
Due to be published this month (March), BSRIA’s guidance on legionella risk assessment.

Literal application of the rule for cold water is even more questionable. Heat picked up in the cold-water distribution pipework, possibly running in close proximity to hot water or heating, can mean that the temperature of water from the outlet increases and then decreases during the two minutes. That slug of warm water in the pipework may be more significant to the risk of Legionnaires’ Disease than the fact that the water drops below 20°C in two minutes. This is not a call for abandonment of simple rules but that risk assessment should also take the context into consideration and keep the underlying principles in mind.

One change in the last 40 years is that there are now guidelines for assessing legionella concentrations in water samples from both cooling tower systems [HSG274 Part 1 [2]] and domestic water systems [HSG274 Part2 [3]]. Both agree that greater than 1000 cfu/l is bad news (cfu is colony forming unit), and something must be done, but this is based on the presumption of loss of control rather direct consideration of infection routes and consequent risks to people.

In the case of cooling towers it’s easy to dose with extra biocide, but the disinfection of domestic water systems is more difficult and disruptive and may not eliminate the underlying cause of the problem. Understanding the basic principles of legionella control is still the key to minimising the risk.

BSRIA has recently revised its publications concerning legionella risk assessment in line with changes in HSE and other guidance. ‘BG 57/2015 Legionella risk assessment’ is an aid to carrying out a legionella risk assessment and developing the record-keeping system that is required to comply with health and safety legislation. It provides a framework and checklists to help the risk assessor assess and record the condition of a wide range of risk systems and situations found in buildings and where necessary to make recommendations to reduce and control that risk.

The risk assessor may be the building manager, a competent member of staff or specialist consultant. Documentary evidence of the risk assessment allows the responsible person to demonstrate they have complied with the relevant health-and-safety legislation, otherwise they risk prosecution by the HSE. A separate BSRIA guide for documenting the operation and maintenance of risk systems (BG58) will be published later in 2015.

Reg Brown is head of energy and environment at BSRIA.


[1] ECDC Annual epidemiological report. Respiratory tract infections 2014

[2] HSG 274 (2013). Legionnaires' disease. Technical guidance. Part 1 The control of legionella bacteria in evaporative cooling systems

[3] HSG 274 (2014). Legionnaires' disease. Technical guidance. Part 2 The control of legionella bacteria in hot and cold water systems

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