Controlling the Risk of Legionella in Spa Pools


Identifying and assessing the risk

The purpose of the assessment is to enable a decision on: the risk to health, ie whether the potential for harm to health from exposure is reasonably foreseeable, unless adequate precautionary measures are taken; the necessary measures to prevent, or adequately control, the risk from exposure to legionella and other infectious agents.

There are a number of factors that create a risk of someone acquiring an infectious disease from a spa pool:

  1. presence of infectious agents (eg legionellae) in the spa pool;
  2. suitable conditions for growth of the infectious agents, eg a
    temperature of 20–45 °C;
  3. a source of nutrients (eg organic matter from bathers and from the
  4. a means of creating and spreading breathable droplets, eg the
    aerosol created by agitated water;
  5. presence of people who could be exposed to the infectious agents,
    eg users, those working on or near the vicinity of the spa pool, or
    passing near one.

Bather load

The design bather load is the maximum number of bathers using the
spa per hour (ie each hour is three periods of 15-minute bathing followed
by a 5-minute rest period) and this should be stated and not exceeded.
The design bather load should be approximately 10 times the capacity of
water in the spa-pool system when measured in cubic metres, with a
minimum of 250 litres per bather (for example a 5 m3 pool would have a
design bather load of 50 bathers per hour).


In a commercial setting, as a minimum the total water volume should be replaced with fresh water when the bather load equals 100 x the water
capacity measured in cubic metres since the last water replacement.


The turnover time is the time taken for the entire spa-pool water volume to pass through the filters and treatment plant and back to the spa pool. The maximum design water turnover time for lower bather loads is 15 minutes, and 6 minutes for commercial-type spa pools (where there are higher bather loads).


At pH 7, free chlorine residual measured by DPD1 should be 3–5 mg/l, or total active bromine 4–6 mg/l. The effectiveness of the disinfectant is directly related to the pH of the water.  Determine pH value, and residual disinfectant daily at opening and every two hours thereafter.


The risk from exposure to legionella and other infectious agents should be controlled by maintaining the cleanliness of all parts of the system and the water within it to ensure the pool remains free from nutrient sources arising from contamination and corrosion. In addition to the spa pool, balance tank and its associated components, cleaning should take account of other areas where
contamination may occur, for example the ingress of dirt into the spa pool from the surrounding walkways. 


Backwashing is the process of reversing the flow of water through the filter to clean the filter media. Where sand filters are used, backwashing will be necessary. In busy commercial spa pools it should be done daily.

Inspection & Maintenance

Filter inspections (quarterly) and media change (annually).

Microbiological testing

The monitoring programme should include the routine monthly sampling and testing for the presence of bacteria and should include an ACC (or total viable or total colony count), coliforms, E coli, and Paeruginosa, in addition to quarterly tests for legionella.