What Is Legionella?
Legionella is a naturally occurring bacteria found in water. It can multiply in man-made water systems such as taps, showers, tanks, and pipework when conditions allow — particularly where water is warm, stagnant, or poorly managed.
People are exposed to Legionella through inhalation of fine water droplets (aerosols), not by drinking the water. In some cases, exposure can lead to Legionnaires' disease, a potentially serious form of pneumonia.
Legionella bacteria cannot be detected by sight, smell, or taste. Laboratory testing is the only way to confirm whether it is present in a water system.
Why is Legionella testing important?
Routine testing provides a snapshot of the bacterial condition of a water system at the time samples are taken. It can help identify whether Legionella bacteria are present in hot and cold water outlets and inform whether further investigation or action may be required.
Legionella testing is commonly used in:
- Residential rental properties
- Care and healthcare settings
- Commercial buildings
- Low-use or infrequently occupied premises
Testing does not replace risk assessment or ongoing water management, but it plays an important role in understanding potential risks within a water system. Learn more about Legionella prevention and control.
How Legionella testing works
Water samples are collected from selected outlets and analysed by an independent laboratory. Results are issued in a formal report showing whether Legionella bacteria were detected and at what level. View our water sample collection and results guide.
This service provides laboratory testing and reporting only. Interpretation, risk assessment, and remedial advice should be considered separately where required.
When should Legionella testing be considered?
Legionella testing may be considered:
- As part of routine water monitoring
- Following periods of low water use
- After changes to a water system
- Where reassurance or confirmation is required
The frequency and necessity of testing will depend on the specific building, water system, and usage.
For information on general water quality testing, see our guide to Total Viable Count (TVC).
Related Resources
Learn more about Legionella:
- What is Legionnaires' Disease?
- Legionella Prevention and Control
- Who needs Legionella testing?
- Legionella Testing for Landlords
- Legionella Water Sampling Guide for UK Buildings
Ready to test your water?