Water system risk scoring per AS/NZS 3666 and ASHRAE 188. Temperature, flow, deadleg, and system configuration.
How to use
Select the conditions that apply to the water system being assessed. Each factor is scored according to its contribution to Legionella proliferation risk, consistent with the risk matrix approach in AS/NZS 3666.3 and ASHRAE 188:2021. The cumulative risk score determines the required management response.
System type
Water system type
The type of system determines baseline risk and applicable standard requirements
Temperature conditions
Hot water storage temperature
Measured at the calorifier/storage vessel
Hot water at point of use (after 1 min flush)
Measured at tap / outlet
Cold water temperature
Measured at point of use
Flow and stagnation
Deadlegs / blind ends present
Sections of pipework with no through-flow
Infrequently used outlets
Outlets not used in previous 7 days
Stagnation during shutdown
System was shut down or building unoccupied
System condition and design
Scale / sediment / corrosion
Observed in system components, strainers, or at outlets
Water treatment status (cooling towers / systems)
Biocide dosing, conductivity and pH control
Thermostatic mixing valves (TMVs)
Present and maintained to reduce scald risk
Population and consequence
Population at risk
Most susceptible users of the facility
Aerosol generation potential
Likelihood of inhalable aerosol at point of use
Risk score and required response
Risk score
0
Completing assessment...
0 — Low | 10 — Medium | 20 — High | 30+ — Critical
Required actions
Complete all selections above to generate the risk score and required response.
Risk factor scores
Legionella temperature relationship
Temperature is the most critical factor governing Legionella proliferation. The relationship between temperature and bacterial growth is well-characterised and forms the basis of all major control standards.
Temperature zones and Legionella risk
Temperature range
Effect on L. pneumophila
Risk level
Control requirement
≥70°C
Rapid kill (99.9% in seconds)
None
Thermal disinfection set-point
60–70°C
Kill within minutes; pasteurisation zone
Negligible
Calorifier storage target ≥60°C
50–60°C
Growth inhibited; gradual inactivation
Low
Maintain ≥50°C at all points of use
35–46°C
Optimal growth (peak ~42°C)
Very high
Must not sustain these temperatures
25–50°C
Survives and multiplies
High
Avoid sustained occupancy of this zone
20–25°C
Survives; slow or no growth
Moderate
Cold water target ≤20°C at point of use
<20°C
Survival only; no multiplication
Low
Cold water target — safe zone
Standard compliance requirements
Standard
Hot water (storage)
Hot water (outlet)
Cold water
Cooling tower
AS/NZS 3666.1
≥60°C
≥50°C within 1 min
≤20°C
Biocide programme required
ASHRAE 188:2021
≥60°C
≥50°C within 2 min
≤25°C
Water management programme
UK HSG274
≥60°C
≥50°C within 1 min
≤20°C
Biocide dosing + monitoring
WHO (2007)
≥60°C
≥50°C
≤20°C
Water safety plan approach
Temperature data: Barbaree et al. (1986); Wadowsky et al. (1985); Stout et al. (1985). Standard references: AS/NZS 3666:2011 (Parts 1–3), ASHRAE 188:2021, UK HSG274 (2014). Healthcare settings require a more conservative approach — refer to HTM 04-01 (UK NHS) or facility-specific infection control guidelines.