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COVID-19 Guidance for Legionella and Building

September 03, 2020 0 Comments

COVID-19 Guidance for Legionella and Building

The focus of this guidance is to reduce Legionella growth and other pathogen and corrosion concerns that occur when there is a large drop in building water use. The guidance focuses on potable water systems inside larger buildings with complex plumbing and the information is general in nature. Each building is different and will require different actions based on its plumbing systems, use patterns, and source of water supply.

Please see the department’s Guidance for Shutting Down and Reopening Water Recreation Facilities for information about how to maintain and treat pools and spas. More information about cooling towers and HVAC systems can be found on the New York State’s Department of Health webpage. You may also want to make sure that the buildings p-traps do not dry out. You can do this by flushing water down all drains to prevent sewer gas intrusion into the building.


What is the problem and who does it apply to?

Many buildings are closed to the public or have limited access in order to help slow the spread of COVID-19. The resulting drop in building water use increases the risk for Legionella growth in building plumbing and associated equipment like cooling towers, pools, decorative fountains, hot tubs and other equipment. To prevent Legionella growth, these systems must be actively managed and maintained.

If Legionella grows during low use periods, building users have a higher risk of contracting Legionnaires’ disease and Pontiac Fever during the shutdown and when full use resumes. While Legionella is a primary risk, other opportunistic pathogens (Mycobacterium avium) and corrosion concerns (lead scale release) are increased by closure or reduced use situations.


Building closures and reduced occupancy affect all environmental systems operating inside buildings including 1) potable and non-potable water systems, 2) cooling towers and 3) heating, ventilation and air conditioning (HVAC) that regulate interior relative humidity and control mold. These systems must be actively managed and maintained to protect the health of building users. In addition to managing systems during shutdown periods, building owners and operators need to implement well thought out start up protocols to ensure public health protection. By implementing procedures now, you can protect the public and minimize the steps needed to safely re-open closed or partially closed facilities.


Key points about Legionella risk and building water systems in Washington State

•     Prior to the start of the COVID-19 pandemic, Legionnaires’ disease sickened and killed more people in the United States than any other reportable waterborne disease.4

•    Legionellatransmission occurs primarilythrough breathingin contaminatedaerosols into the respiratorysystem.


•       The optimum growth range for Legionella is between 77°F (25°C) and 108°F (42°C),

however, Legionella can survive and grow outside of this temperature range. To reduce bacteria growth, keep hot water systems above 122°F (50°C) at all fixtures and cold water systems below 74°F (23°C). Always follow plumbing and safety standards to prevent scalding at the point of use.

•    Loss of free and total chlorine residual in building plumbing as a result of stagnant or very low water use is a significant factor that allows Legionella bacteria to flourish.

•    Nearly all large and medium sized municipal water systems in Washington State use free chlorine for both primary disinfection and for distribution disinfectant residual (secondary disinfection). The only exception uses chlorination for secondary disinfection, which is also very effective against Legionella. If you are not sure if your water system is disinfected, please contact your water utility.

•    Microbial disinfection of water using free chlorine depends primarily upon the concentration of free chlorine and time. Temperature and pH also impact chlorine disinfection efficiency against microbes. ‘CT’ is the term used in the drinking water industry to quantify disinfection. C stands for ‘concentration of free chlorine in mg/L units’.T stands for timein minutes. Example: Free chlorine residual of 50 mg/L held in piping for 120 minutes (2 hours) equals a CT of 6,000 mg-min/L.



Recommended Monitoring and Management of Building Water Systems

Monitoring building water quality

Because temperature and chlorine residual are primary factors affecting Legionella presence in building plumbing, DOH strongly recommends that temperature and chlorine residuals be accurately measured and used to manage building water age. All building water systems should have an accurate digital chlorine residual test kit that uses an EPA-approved test method for use in drinking water compliance.6,8 Test strips and color wheels are not accurate and not recommended testing methods for water systems. Thermometers should be accurate to within

+/- 1°C. Operators should measure daily chlorine residual and temperature of water entering the facility from the water utility supplier. Chlorine concentrations in building plumbing will be

Less than or equal to utility concentrations. Building cold water temperatures will be equal to or higher than utility temperatures.

Maintaining systems under low use conditions (but not closed)

  1. Flush cold water systems to maintain temperature and chlorineresidual.
  2. Monitor and maintain temperatures in hot water systems at the farthest (distal) fixtures. Or turn off the heating system, drain and flush the hot water tank and refill with cold water. Then flush the hot water system with the cold water supply to maintain temperature and chlorine residual similar to the cold watersystem.
  3. Measure and record temperature and chlorine residual of the supply water from the utility every day. Use these values, not time, as your optimum target for flushing at your distal measurements sites. Individual buildings have too much plumbing variability for time to be a useful flushing parameter.
  1. Maintain cold water distribution free chlorine residuals at or above 0.2 mg/L (mg/L is the same as parts per million (ppm) at low concentrations). Measure it with an approved device. The supplying utility’s chlorine levels may limit your ability to maintain this minimum plumbing distribution residual. Contact your utility to better understand their operating parameters.
  2. Document all measurements and maintenance actions in a daily log.



Starting up building water systems after closures and sustained low use


To ensure you and your employee’s safety from both chemical and biological exposure while disinfecting and flushing the building plumping, appropriate training and PPE should be considered. You can find guidance on worker safety for Legionella control and prevention on the OSHA website.

  1. Flush the entire water system to replace all water. Use an approved chlorine testing device to measure residual chlorine, flush until measured levels are equal to or slightly less than the supplying utility’s chlorine residuals. Some flushing considerations are listed below. Additional flushing guidance can be found at the Environmental Science, Policy, and Research Institute (ESPRI)12website.
  2. Verify that testing for back flow assemblies is up to date prior to flushing.
  3. Create a list of all plumbing fixtures that will need to be flushed including ice machines, dish washers, and point of use (POU) treatment devices to ensure that no fixture is overlooked. Point of use filters should be replaced.
  4. Make sure that fixture drains are functioning and can handle expected flows without overflowing.
  5. During flushing operate all valves in the fully open position so that any particulate matter can be flushed through. Pay close attention to float-operated or other restrictive valves which need to be manually opened to clear particulates and prevent fouling of the valves.
  6. Remove all aerator screens before flushing. Clean or replace aerator screens to get rid of scale deposits that may contain harmful metals (lead) or microbial biofilms. Disinfect, heat sterilize, or replace shower heads especially if vulnerable populations have access to the showers.
  7. Adjust valves back to normal operating positions to ensure that the system is rebalanced.
  8. Return hot water systems to normal operating temperatures. 9. Document all start up actions in the daily maintenancelog.
  9. Evaluate the effectiveness of your startup procedures relative to Legionella by testing for Legionella using an approved culture method or one cited by the National

Academies of Science4 for testing of Legionella in water. Collect samples at least 48 hours after final flushing and return of normal operation of the water system. Samples collected

too early may give false negative results1. Collect samples from critical locations9. (CDC

toolkit) The CDC maintains a list of labs certified for legionella testing called ELITE labs. Other labs may be available in your area that are not on the CDC ELITE list of labs. Discuss with the labs if their method is suitable for conducting environmental testing of water for Legionella.

  1. Since there are no Legionella drinking water standards in Washington State,you may use the European Union Action levels for Legionella in Potable Hot and Cold Water Systems5to evaluate test results and identify additional steps needed. See Table 6 below.
  2. If testing reveals widespread or high levels of Legionella in the water system, or you have other concerns, then remedial measures may be needed.Two remedial measures recommended by CDC and EPA are shock chlorination and thermal disinfection.
  3. For shock chlorination, fully disinfect the cold-water system by flushing water through all fixtures and outlets with chlorinated water achieving a CT of no less than 3,000 mg-min/L. The chlorine concentration used in calculating CT is the lowest free chlorine residual measured throughout the building at the end of the hold time using an approved measuring device. For high chlorine doses, the sample will have to be diluted to accurately measure the residual with typical approved devices. A CT target of 3,000 mg-min/L is a minimum recommendation. Systems with heavy biofilms or extensive Legionella colonization may need to use higher CT targets.Refer to the department’s publication entitled Shock Chlorination Guidance for Building Water Systems for a more complete descriptionof this process.
  4. b. For thermal disinfection, start by flushing the cold water from each outlet. This method involves raising the temperature of hot water tanks to between 160 to 170°F (71 to 77°C) and maintaining the water temperature at 149°F (65°C) or higher at outlets during flushing is important to provide effective disinfection. Theoptimal flush time varies from 10 to 30 minutes depending on the characteristics of the premise plumbing system.
  5. Limitations of this approach include that it only targets the hot water piping, and the hot water supply must be large enough to maintain a high temperatureat the fixtures being flushed. In addition, these hightemperature present a scalding risk in less than 1 second, so such remediation effort should only be performed when access to the plumbing system is limited and controlled. If thermal disinfection is inadequate to address Legionella risk, chemical disinfection should be considered.
  6. c. After any remedial measures, it is important to sample again in accordance with Step 10 outlined above.




  1. EuropeanSociety of Clinical Microbiology andInfectious Disease- Guidance for

Managing Legionella during the Coronavirus Pandemic 2020.

  1. Recoveringfrom COVID-19 BuildingClosures. AIHA 2020 Guidance Document(Prepared by Indoor Environmental Quality Committee of the American industrial Hygiene Society).
  2. SafeManagement of Water Systems in Buildings During the COVID-19 Outbreak,25th March 2020, Legionella Control Association, Staffordshire,UK
  3. National Academies of Sciences, Engineering and Medicine2019. Management of




Legionella in Water Systems. Washington, D.C. The National Academies Press.

  1. EuropeanTechnical Guidelines for the Prevention, Control andInvestigation, of

Infections Caused by Legionella species. June2017

  1. Deem,S. & Feagin, N., 2016. “Disinfection Data Integrity in WashingtonState.” Journal AWWA, 108:10:24.
  2. Emergency Disinfection of Small Water Systems, Publication331-242 revised2/17/2020,

Washington State Department of Health

  1. Measuring FreeChlorine, Publication 331-442 revised January 2020, Washington State

Department of Health


  1. Developing a Water Management Programto Reduce Legionella Growth and Spreadin Buildings: A Practical Guide to Implementing Industry Standards June 5, 2017. U.S.

Department of Health and Human Services CDC

  1. Technologies for Legionella Control in Premise Plumbing Systems: Scientific Literature Review. EPA Report USEPA. 2016 - EPA 810-R-16-001. USEPA, Washington DC.
  2. Restoring Water to Medical, Residential, and Commercial Buildings, Shutdowns, Unsafe

Water, Purdue University Center for Plumbing Safety

12 Building Water Quality and Coronavirus: Flushing Guidance for Periods of Low or No Use, Environmental Science Policy and Research Institute


Section 3.185 Table 6: European Union Action Levels for Legionella in Potable Hot and Cold Water Systems


Legionella CFU/Liter

Action Required


Not Detected < 100 to 1,000


Acceptable – continue monitoring

Refer to responsible person and assure water quality values are within target

> 1,000 to < 10,000

i)      Resample ifsmall percentage(10-20%) are positive; review control measures

ii)      If >20% positive may indicate low level colonization, disinfection of system,and risk assessment to determine additional actions

≥ 10,000

Resample, immediate review of control measures, disinfection of whole system

Source: EU (2017).


The EU guidelines emphasize the goal to achieve no cultural Legionella, but acknowledge that





occasional detection (<20%) of low levels of Legionella (< 1,000 CFU/L) may be acceptable

provided that other water quality values (e.g., temperature, disinfectant) and operational parameters are within the water management plan guidelines. Intermediate levels (> 1,000 to

< 10,000 CFU/L) and high levels (≥ 10,000 CFU/L) trigger a series of actions including resampling, remedial measures such as disinfection, and overall review of the water management plan program.



More COVID-19 Information and Resources

Stay up-to-date on the current COVID-19 situation in Washington, Governor Inslee’s proclamations, symptoms, how it spreads, and how and when people should get tested. See our Frequently Asked Questions for more information.

The risk of COVID-19 is not connected to race, ethnicity or nationality. Stigma will not help to fight the illness. Share accurate information with others to keep rumors and misinformation from spreading.



•    WA State Department of Health 2019 Novel Coronavirus Outbreak(COVID-19) •      WA State Coronavirus Response (COVID-19)

•    Find Your Local Health Department or District •         CDC Coronavirus(COVID-19)

•    Stigma ReductionResources


Have more questions about COVID-19? Call our hotline: 1-800-525-0127. For interpretative services, press # when they answer and say your language. (Open from 6 a.m. to 10 p.m.) For questions about your own health, COVID-19 testing, or testing results, please contact your health care provider.

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