COSHH · Mandatory · Pre-Construction
COSHH Assessment — Legionella
This COSHH assessment covers legionella risks from water systems on construction sites and in newly completed buildings. Stagnant water in temporary supplies, dead legs, and cooling towers can harbour Legionella pneumophila bacteria. It requires written schemes of control, temperature monitoring, regular flushing regimes, and competent water treatment management.
Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. COSHH Regs / L8 ACoP remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.
| Legionella risk | Legionella pneumophila grows between 20-45 degrees Celsius (optimum 35-40 degrees Celsius). Transmitted by inhalation of contaminated water aerosols. Legionnaires’ disease has a case fatality rate of approximately 10 percent. |
| Duty holder | The employer, self-employed person, or person in control of the premises is the duty holder under COSHH 2002 and the Approved Code of Practice L8 (Legionnaires’ disease: the control of Legionella bacteria in water systems) |
| Risk assessment | A written Legionella risk assessment is required. Must identify water systems, people at risk, and control measures. Must be reviewed regularly and after any significant change to the water system. |
| Temperature control | Hot water must be stored at 60 degrees Celsius minimum, distributed at 50 degrees Celsius minimum (55 degrees Celsius in healthcare). Cold water must be maintained below 20 degrees Celsius. |
| Schematic | A written schematic diagram of the water system is required, showing all tanks, calorifiers, pipework routes, dead legs, outlets, and ancillary equipment. Must be kept up to date. |
1. Legionella Risk on Construction Sites
Legionella pneumophila is a bacterium that occurs naturally in freshwater environments but proliferates rapidly in artificial water systems where conditions favour its growth — specifically water temperatures between 20 and 45 degrees Celsius, stagnation, and the presence of biofilm, sediment, or scale. When contaminated water is dispersed as a fine aerosol (droplets small enough to be inhaled), Legionella can cause Legionnaires’ disease, a severe and potentially fatal form of pneumonia, or Pontiac fever, a milder flu-like illness.
Construction sites present specific Legionella risks that are frequently overlooked. Site welfare facilities — including showers, washbasins, and drinking water points — are often served by temporary water systems with long pipe runs, intermittent use, and variable temperature control. Buildings undergoing refurbishment may have water systems that have been dormant for extended periods, allowing bacterial colonisation. Cooling towers, spa pools, and ornamental water features on or adjacent to the site generate aerosols that can spread Legionella over considerable distances.
The legal framework is established by COSHH 2002 (Legionella is a biological agent within the scope of COSHH) and the HSE Approved Code of Practice L8 — Legionnaires’ disease: the control of Legionella bacteria in water systems. L8 has special legal status: compliance with its provisions is not mandatory in the strict sense, but failure to follow L8 is admissible in criminal proceedings as evidence that the duty holder did not comply with COSHH. The accompanying technical guidance HSG274 (Parts 1, 2, and 3) provides the detailed practical standards for hot and cold water systems, cooling towers, and other risk systems.
Commissioning water systems: flush and disinfect before first use
New or refurbished water systems must be thoroughly flushed and disinfected before they are put into service. Water that has been standing in newly installed pipework during the construction period will have been at ambient temperature for an extended period — ideal conditions for Legionella colonisation. Chlorination or thermal disinfection must be carried out in accordance with BS 8558 before the system is handed over for use. This applies equally to temporary site welfare water systems and to permanent building water systems being commissioned at practical completion.
2. Control Measures for Legionella
The primary control strategy for Legionella in hot and cold water systems is temperature management. Legionella does not survive above 60 degrees Celsius and does not multiply below 20 degrees Celsius. Maintaining hot water storage and distribution temperatures above the growth range, and cold water temperatures below it, is the most effective control measure. The following table sets out the key control measures required under L8 and HSG274.
| Control measure | Requirement |
|---|---|
| Temperature control — hot water | Hot water must be stored at a minimum of 60 degrees Celsius in the calorifier or water heater. Hot water must be distributed so that it reaches a minimum of 50 degrees Celsius at the nearest outlet within one minute of running. In healthcare premises, the distribution temperature is 55 degrees Celsius minimum. Thermostatic mixing valves (TMVs) used for scald prevention must be fitted as close to the point of use as possible to minimise the length of mixed-temperature pipework. |
| Temperature control — cold water | Cold water must be stored and distributed at below 20 degrees Celsius. Cold water tanks must be insulated, covered, and located away from heat sources. Pipework must be insulated and routed away from hot water pipes, heating services, and areas subject to solar gain. Where cold water temperatures cannot be maintained below 20 degrees Celsius, additional controls (e.g. more frequent flushing, biocide dosing) are required. |
| Flushing infrequently used outlets | Any outlet (tap, shower, hose) that is not used for 7 or more days must be flushed by running the water for a minimum of 2 minutes (or until the temperature stabilises) before it is used again. This removes stagnant water from the dead leg between the outlet and the circulating system. A flushing log must be maintained as a written record. |
| Dead legs and redundant pipework | Dead legs (sections of pipework that are capped off or serve removed outlets) must be removed back to the nearest circulating branch. Redundant pipework that cannot be removed must be flushed regularly. Dead legs allow water to stagnate at ambient temperature, creating ideal conditions for Legionella growth. |
| Calorifier inspection and maintenance | Calorifiers must be inspected annually. Drain valves must be operated to remove sediment from the base of the vessel. The stored water temperature must be checked monthly and recorded. Calorifiers that have been out of service must be cleaned, disinfected, and confirmed at the correct operating temperature before being returned to use. |
| Sampling and testing | Routine microbiological sampling of water systems is recommended by HSG274 to verify that controls are effective. Sampling points should include the calorifier, sentinel outlets (nearest and furthest from the water heater), and any outlets identified as higher risk. Where Legionella is detected above actionable levels, the system must be disinfected and re-sampled to confirm efficacy. |
| Written records | L8 requires written records of the risk assessment, schematic, monitoring results, remedial actions, and the identity of the responsible person. Records must demonstrate ongoing active management of the water system. The absence of records is treated by the HSE as evidence of non-compliance, regardless of what controls may actually have been in place. |
Stagnation is the enemy — if water is not flowing, Legionella is growing
Water that sits in pipework at ambient temperature is the single greatest risk factor for Legionella colonisation. On construction sites, temporary water systems, infrequently used welfare facilities, and buildings with dormant services all create stagnation risks. A proactive flushing regime, removal of dead legs, and temperature monitoring are the most effective controls. Do not wait for a positive Legionella test to act — by the time the result comes back, the exposure has already occurred.
3. Common Mistakes
No Legionella risk assessment for site welfare showers and washbasins
Construction site welfare facilities are served by water systems that fall within the scope of L8 and COSHH 2002. Temporary showers, washbasins, and drinking water points require a Legionella risk assessment. The assessment must identify the water system configuration, the risk factors (stagnation, temperature, biofilm), and the control measures in place. Many principal contractors fail to carry out a Legionella risk assessment for site welfare facilities, leaving a significant compliance gap.
Not flushing infrequently used outlets weekly
Any water outlet that is not used for 7 or more days must be flushed before it is used again. On construction sites, this commonly applies to showers or taps in welfare units that are not used over weekends, holiday periods, or during phases when a particular area of the site is unoccupied. A flushing log must be maintained as a written record. Failure to flush creates stagnant water at ambient temperature in the pipework — the exact conditions in which Legionella proliferates.
Bringing a mothballed building water system back into service without disinfection
Buildings that have been vacant or mothballed for extended periods will have water systems in which water has been stagnating at ambient temperature — potentially for months or years. Bringing such a system back into service without first draining, cleaning, disinfecting (either thermally or chemically), and confirming that water temperatures meet L8 standards is extremely dangerous. Several outbreaks of Legionnaires’ disease in the UK have been traced to water systems that were recommissioned without adequate disinfection.
Not maintaining written records of monitoring and maintenance
L8 requires the duty holder to maintain written records of the Legionella risk assessment, the water system schematic, temperature monitoring results, flushing logs, maintenance and inspection records, and any remedial actions taken. The absence of records is treated by the HSE as evidence of non-compliance. In the event of an outbreak investigation, the first thing the HSE will request is the L8 records. If they do not exist, the duty holder will be assumed to have failed to manage the risk, regardless of what actions may actually have been taken.
Not identifying a named responsible person for water system management
L8 requires the duty holder to appoint a named responsible person with day-to-day responsibility for managing the water system and implementing the control measures identified in the risk assessment. On construction sites, this is typically a site manager or facilities manager. If no one is specifically appointed and trained, the monitoring, flushing, and maintenance tasks will not be carried out consistently. An unnamed, unassigned responsibility is an unmanaged risk.
5. Frequently Asked Questions
When does L8 apply on a construction site?▾
L8 applies whenever a construction site has a water system that could create a risk of Legionella exposure. This includes site welfare facilities with showers, hot water systems for washbasins, temporary water supplies with long pipe runs, and any work on or near existing building water systems (particularly during refurbishment or commissioning). It also applies where the site is adjacent to cooling towers, spa pools, or other aerosol-generating systems that may be under the control of a neighbouring premises. The duty holder for site welfare facilities is typically the principal contractor. If the site has welfare showers or hot water, L8 applies.
What temperature should hot water be stored and distributed at?▾
Hot water must be stored at a minimum of 60 degrees Celsius in the calorifier or water heater. It must be distributed so that it reaches a minimum of 50 degrees Celsius at the nearest outlet within one minute of turning on the tap (55 degrees Celsius in healthcare premises). These temperatures are above the range in which Legionella can survive and are the primary control measure for hot water systems. Where thermostatic mixing valves (TMVs) are fitted for scald prevention, they must be positioned as close to the point of use as possible to minimise the length of mixed-temperature pipework downstream of the valve.
How are cooling towers regulated for Legionella?▾
Cooling towers and evaporative condensers are high-risk systems for Legionella because they generate large volumes of aerosol that can travel considerable distances. They are subject to notification requirements under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992 — the duty holder must notify the local authority of the location and nature of the system. HSG274 Part 1 provides the detailed technical guidance for Legionella control in cooling towers, including water treatment regimes, drift eliminators, regular cleaning and disinfection, and routine microbiological monitoring. Cooling towers require specialist water treatment management and should not be managed without competent professional input.
What happens if Legionella is detected in water samples?▾
The response depends on the level of colonisation detected. At low levels (100 to 1,000 colony-forming units per litre), the system should be reviewed for deficiencies in temperature control, stagnation, or biofilm, and corrective action taken. At higher levels (above 1,000 cfu/l), immediate disinfection of the affected system is required — either by thermal disinfection (raising the water temperature to 70 degrees Celsius and flushing all outlets for a minimum of 2 minutes) or by chemical disinfection (hyperchlorination to 50 mg/l free chlorine for one hour). The system must be re-sampled after disinfection to confirm that the treatment was effective. Where Legionella serogroup 1 is detected at any level, or where a case of Legionnaires’ disease is suspected, the local Health Protection Team must be notified immediately.
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