Legionnaires' disease

Technical guidance

This guidance is for dutyholders, including employers, those in control of premises and those with health and safety responsibilities for others. It will help them comply with their legal duties to control the risk from exposure to legionella.

Applies to:
March 2024 (Second edition)
Series code:

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You can order hard copies of Legionnaires' disease: Technical guidance (HSG274)


The guidance gives practical advice on:

  • the legal requirements of the Health and Safety at Work etc Act and the Control of Substances Hazardous to Health Regulations concerning the risk from exposure to legionella
  • guidance on compliance with the relevant parts of the Management of Health and Safety at Work Regulations

The guidance is in three parts. Part 1 has been updated to provide improved guidance on a number of areas when managing the legionella risk presented by evaporative cooling systems. These changes include:

  • further information on DPD No 1 testing methodology
  • the effect that cooling water pH has on the efficacy of halogen-based biocides such as chlorine and bromine

Many organisations rely on this test to ensure correct dosing of their systems. The pH correction of free-halogen test results is a vital part in ensuring you have sufficient biocide in your system to prevent the growth of legionella. Check your most recent results and ensure they are pH corrected.

Please note that the second edition has been reviewed and updated in Part 1 only – The control of legionella bacteria in evaporative cooling systems.

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Updated 2024-06-05