Legionella
A bacterium that causes Legionnaires’ disease and is a building water systems problem rather than a source water or mains problem. It proliferates in warm stagnant water in hot and cold services, cooling towers, spa pools and showers, and is acquired by inhaling aerosol rather than by drinking.
Key facts
- Contaminant group
- Biological
- European Union — legal limit
- 1,000 cfu/lIntroduced by the recast Drinking Water Directive for building water systems in priority premises.
- United Kingdom — legal limit
- Risk assessment and control duties under ACOP L8A duty to assess and control rather than a numerical limit in most premises.
Where it comes from
- Hot water systems held between roughly 20 °C and 45 °C
- Cooling towers and evaporative condensers
- Spa pools and hot tubs
- Rarely used outlets and dead legs where water stagnates
How it behaves in water
Multiplies within amoebae and biofilms, which shield it from disinfection. Growth is fastest between about 25 °C and 42 °C, is suppressed below 20 °C, and the organism does not survive above about 60 °C. Temperature control is therefore the primary engineering control.
How it is measured
Cultured from water samples and reported as colony-forming units per litre. Results take up to ten days, which is why routine management relies on temperature monitoring and system design rather than on sampling alone.
Regulatory position
| Jurisdiction | Value | Status | Notes |
|---|---|---|---|
| European Union | 1,000 cfu/l | legal limit | Introduced by the recast Drinking Water Directive for building water systems in priority premises. |
| United Kingdom | Risk assessment and control duties under ACOP L8 | legal limit | A duty to assess and control rather than a numerical limit in most premises. |
Table sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- European Commission — Drinking Water Directive (EU) 2020/2184 and reporting. Open — attribution required · European Commission reuse policy (Decision 2011/833/EU)
- Drinking Water Inspectorate — Drinking water quality in England and Wales. Open — attribution required · Open Government Licence v3.0
- United States Environmental Protection Agency — Safe Drinking Water Information System. Open · US Government work — public domain
Health context
Causes Legionnaires’ disease, a severe pneumonia with substantial mortality in vulnerable groups, and the milder Pontiac fever. Risk is concentrated in older people, smokers and the immunocompromised. Drinking contaminated water is not the route of infection; inhaling aerosolised droplets is.
What removes it
| Row | Effective | Not reliably effective |
|---|---|---|
| 1 | Keeping hot water above 60 °C and cold water below 20 °C | Point-of-use carbon filters, which can themselves harbour growth |
| 2 | Eliminating dead legs and flushing infrequently used outlets | Water softening |
| 3 | Thermal or chemical disinfection of the system | — |
| 4 | Copper-silver ionisation or chlorine dioxide dosing in large buildings | — |
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted