A breath of fresh air? Quantitative microbial risk assessment (QMRA) in Legionella risk management

The airborne pathogen Legionella pneumophila thrives in building-associated water systems such as cooling towers, hot water systems, and air conditioning units. Inhalation of aerosols containing Legionella bacteria can lead to a severe and potentially fatal form of pneumonia known as Legionnaire’s disease.

The UK has a well-established set of guidelines for the control of Legionella risk, primarily the Health and Safety Executive's (HSE) Approved Code of Practice (ACOP) L8, and the associated technical guidance HSG274¹. Nevertheless, the control of Legionella risk within engineered water systems remains a formidable challenge for water safety management professionals. While not commonplace, the UK still reports hundreds of Legionella cases each year, with an average of 459 cases per annum between 2017 and 2019². Legionella outbreaks also occur sporadically across the globe. The most recent was in Poland, which resulted in over 150 cases and 23 deaths³. These statistics prompt the question: can we reduce the risk of Legionella exposure by refining our approach to microbial risk assessment?

Quantitative Microbial Risk Assessment (QMRA) is a systematic and data-driven risk assessment approach to evaluating microbial risk, with four key steps: hazard identification, exposure assessment, dose-response assessment, and risk characterisation. The World Health Organization (WHO) recognises QMRA as a valuable risk assessment tool within its water safety planning guidelines⁴. The application of QMRA to assess Legionella risk has garnered particular attention as it necessitates evidence-based and objective analysis, allowing water safety managers to make more informed decisions with regards to water safety and public health risk prevention.

Reclaimed water and Legionella

One area of increasing interest within the realm of Legionella risk assessment is the use of reclaimed water. Often referred to as recycled, or treated wastewater, reclaimed water is gaining prominence globally in response to the escalating challenge of water scarcity. Its applications span diverse sectors, including agriculture, industrial processes, and non-potable uses in residential and commercial settings.

The application of QRMA becomes especially important when considering the potential risks associated with Legionella in reclaimed water systems. 

While reclaimed water undergoes treatment processes to meet stringent quality standards, there remains a need to thoroughly evaluate the safety of this resource, particularly when used in aerosol-generating systems such as cooling towers and irrigation systems. QMRA methodology has been employed in the assessment of Legionella risk in reclaimed water applications worldwide. Recent applications include understanding the risk associated with the use of reclaimed water for crop irrigation in countries like France⁵ and the USA⁶. In South Africa, QMRA has been used to examine the risk of Legionella infection through solar pasteurisation of rainwater⁷. In urban settings, QMRA has been employed to assess Legionella exposure risk from public fountains that use reclaimed water⁸.

WRc to launch new Legionella risk QMRA service

At WRc, we understand the importance of comprehensive and robust microbial risk management. We look forward to announcing soon a new QMRA service, which will give us the capability to provide clients with an in-depth evaluation of Legionella risk in inhalation scenarios. Our service also extends its scope to accommodate ingestion scenarios for other pathogens.

1. Health and Safety Executive (2014). Legionnaires’ disease - What you must do. Available at: https://www.hse.gov.uk/legionnaires/what-you-must-do.htm.
2. Public Health England (2014). Legionnaires’ disease: guidance, data and analysis. GOV.UK. Available at: https://www.gov.uk/government/collections/legionnaires-disease-guidance-data-and-analysis.
3. World Health Organization (2023). Legionellosis – Poland. Available at: https://www.who.int/emergencies/disease-outbreak-news/item/2023-DON487.
4. World Health Organization (2016). Quantitative Microbial Risk Assessment: Application for Water Safety Management.
5. Massiot, G., Courault, D., Jacob, P. and Albert, I. (2022). Monitoring the risk of Legionella infection using a general Bayesian network updated from temporal measurements in agricultural irrigation with reclaimed wastewater. Environmental Science: Water Research & Technology, 9(1), pp.176–192.
6. Mori, J. and Smith, R.L. (2022). Risk of Legionellosis in residential areas around farms irrigating with municipal wastewater. Risk Analysis, 43, pp.1115-1123.
7. Reyneke, B., Khan, S. and Khan, W. (2023). Social perception and risk assessment of domestic uses of solar pasteurized rainwater in an informal settlement community. Journal of Environmental Chemical Engineering, 11(5), p.110532.
8. Chatziprodromidou, I.P., Savoglidou, I., Stavrou, V., Vantarakis, G. and Vantarakis, A. (2022). Surveillance of Legionella spp. in Open Fountains: Does It Pose a Risk? Microorganisms, 10(12), p.2458.