Controlling Legionella in Hot Tubs

Key points

  • Hot tubs have been associated with Legionnaires' disease outbreaks.
  • All hot tubs—including display models—should be operated with proper disinfectant residuals and pH.
  • Public hot tubs should use automatic feed and control systems for disinfectant and pH.
An illustration of 2 people sitting in a hot tub.


Use this document to:

  1. Help evaluate hazardous conditions associated with all types of hot tubs and whirlpool spas (including display models)
  2. Implement Legionella control measures for hot tubs per ASHRAE Guideline 12
  3. Complement existing resources for water management programs
  4. Support environmental assessments conducted during public health investigations

Key factors affecting Legionella growth‎

Sediment and biofilm, temperature, water age, and disinfectant residual are the key factors that affect Legionella growth.


Hot tubs maintain water temperatures within the most favorable range for Legionella growth (77–113°F, 25–45°C). They also create aerosols and accelerate the decay of disinfectants. These conditions make the following design recommendations critically important for preventing disease.

Design recommendations

Use automatic feed and control systems to maintain proper disinfectant residual and pH.

Ensure easy access to all mechanical and filtration components for routine and preventive maintenance and service.

Look at basin design: Ensure the hot tub basin can be easily, quickly, and completely drained and refilled. Ensure the hot tub basin is suitable for regular scrubbing and cleaning.

Control spread of aerosols: Consider locating indoor hot tubs in rooms with isolated air handlers and dehumidifiers. Also locate building air-handling system outlets and returns to reduce the potential for transmission of aerosols.

Post signage warning of increased health risks to individuals who are immune compromised or who have chronic lung disease.

Water management program

Use a water management program to establish, track, and improve operation and maintenance activities.

Operation, maintenance, and control limits

Safe operation and regular hot tub maintenance protect staff, bathers, and bystanders from exposure to Legionella. Operate and maintain hot tubs of all types and sizes with the following guidelines in mind:

Follow requirements of the authority having jurisdiction. Areas without local regulations can use CDC's Model Aquatic Health Code (MAHC).

Follow all manufacturer recommendations, including

  • Routine backwashing of sand and diatomaceous earth filters
  • Regular replacement of cartridge filters

Monitor and maintain an appropriate chlorine or bromine disinfectant residual and pH.

Test disinfectant residual and pH at least twice per day (as often as hourly when in heavy use).

Ensure steady water flow across the filter 24 hours per day.

Clearly post the maximum bather load (CDC's MAHC recommends 10 ft2 per bather) and rules for appropriate use.

Remove from service daily to carry out disinfection with a higher-than-normal disinfectant residual. For example, one of the following (whichever is greater) is commonly used for at least one to four hours:

  • A free residual of 10 mg/L
  • 10 times the combined chlorine level

Drain, scrub, clean, and fill hot tubs.

Train staff: Ensure all staff involved in hot tub operation and maintenance are trained appropriately.

Maintain complete operating records for hot tubs. Review records for trends of disinfectant residuals, pH, and maintenance activities.

Consider testing for Legionella in accordance with the routine testing module of this toolkit (linked below).

Remediation during suspected illness

Role of testing

If the authority having jurisdiction suspects an outbreak or illness, test in conjunction with public health in order to:

  • Confirm the presence of Legionella before performing remediation.
  • Confirm Legionella elimination after remediation activities.

Note: The public health authority having jurisdiction determines whether there are associated illness(es) or an outbreak.

Steps to take

Remove the hot tub from service.

Drain the hot tub after collecting samples using public health instructions.

Scrub and clean all surfaces using water with a minimum free chlorine concentration of 5 ppm. Surfaces should include skimming devices and weirs.

Rinse all hot tub surfaces with fresh potable water and drain as needed.

Replace filters or filter media (if applicable).

Repair parts as needed.

Refill the hot tub with fresh potable water.

Hyperchlorinate the water to 20 ppm free chlorine. Circulate hyperchlorinated water with jets off for 1 hour. Circulate hyperchlorinated water with jets on for 9 additional hours. Maintain a minimum free chlorine residual of 20 ppm for a total of 10 hours.

Flush the entire system with fresh potable water and refill.

Return the hot tub to the routine disinfectant residual level.

Resume service in coordination with public health authority.

Remediation absent illness

Indications to test

Absent any illness, consider the remediation options described below if control measures are ineffective or routine results indicate poor Legionella control.

Steps to take

Remove the hot tub from service.

Add disinfectant. Maintain 10 ppm free chlorine for 1 hour.

Drain the water.

Scrub, clean, and rinse all hot tub surfaces with fresh potable water and drain as needed.

Clean and service filters according to manufacturer recommendations.

Refill with fresh potable water.

Return the hot tub to the routine disinfectant residual level.

Resume service. The hot tub can be used once performance indicators are consistent with Legionella control and are within control limits.

Managing risk in display hot tubs

If a hot tub is on display and contains water, then it also requires disinfectant.


Monitor residual disinfectant and pH twice a day.

  • Free chlorine: 3–10 ppm
  • Bromine: 4–8 ppm
  • pH: 7.2–7.8

Maintain complete operating records for display hot tubs, including disinfectant residual levels, pH, and maintenance activities.

Legionella control measures

Described below are control measuresA and recommendationsB for each water parameter.

Sediment and biofilm

Control measures: Cleaning frequency

Recommendations: Vigorously scrub all surfaces each time tub is drained.


Control measures: Control limits unlikely met due to operating conditions. Hot tubs operate within the favorable growth range for Legionella (77–113°F, 25–45°C).

Recommendations: Additional measures are required to control Legionella. Water should not exceed 104°F (40°C) to prevent scalding.

Water age

Control measures: Bather load, frequency of use

Recommendations: Water replacement frequency in days = (Spa volume in gallons/3)/average # users per dayC

Disinfectant residualD

Control measures: Control limits


  • pH: 7.2–7.8C
  • Free chlorine: 3–10 ppmC
  • Bromine: 4–8 ppmC
  1. The listed control measures were last updated according to ASHRAE Guideline 12 and CDC’s 2018 MAHC. View the current versions of ASHRAE Guideline 12 and MAHC for the most up-to-date recommendations.
  2. See Managing Legionella Risk in Display Hot Tubs section for recommendations for controlling Legionella in display hot tubs.
  3. Recommendation based on guidance from MAHC, accessible at:
  4. Cyanuric acid or stabilized chlorine products should not be used in hot tubs as they slow disinfection.