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Legionnaires' Disease

Section II:B. Humidifiers and Misters

Humidifiers and MistersMany HVAC systems supply humidified air to building occupants to maintain comfort. Improperly maintained humidifiers can be both amplifiers and disseminators of a variety of bioaerosols. However, the cool temperatures in HVAC systems generally are not conducive to growth of Legionnaires' disease bacteria (LDB).

Control Strategy:

TopHow do these systems operate?
  • Atomizing humidifiers use mechanical devices or pneumatic air to create a water mist that evaporates into the air stream. A contaminate-free water source is essential.

  • Heated pan humidifiers use a heat source to evaporate water from a pan open to the air stream but do not create water mists. Intermittent use of the device coupled with a warm pan of water may support microbial growth. Contaminate-free water is essential.

  • Direct steam-type humidifiers inject boiler-generated steam directly into the air stream. These systems do not create water mists and normally operate above 70°C (158°F), that is a temperature at which LDB cannot survive.

TopHow do I maintain humidifiers and misters?
Cold-water humidifiers require rigorous maintenance to ensure that the water source does not contribute to potential problems. 
  • Because humidifiers discharge water vapor or droplets into HVAC air distribution systems, inspect them for standing water and treat according to the HVAC Air Distribution System protocol as follows: 

    • Inspect the entire air distribution system (including return and exhaust systems) for visual evidence of water accumulation.

    • Eliminate all water leaks and remove any standing water found in the system. Replace or eliminate any water-damaged insulation in the system.

    • After cleaning or disinfecting humidifiers and misters, operate the HVAC system using 100 percent outside air for eight hours before returning the building to normal operation.
  • An adequate maintenance program should be in effect to reduce the growth of LDB

  • Water storage temperatures should be above or below the 20° - 50°C (68° - 122°F) range, and the system should be kept clean.

  • Cold-water humidifiers in HVAC systems should be connected to a potable domestic water source and provided with a drain line to remove excess water.
Note: Stand-alone, console-type humidifiers that recirculate water for humidification should not be used because the water in these systems becomes contaminated with micro-organisms rapidly. These stand-alone units have been linked to an outbreak of Legionnaires' disease in a hospital. Ideally, HVAC humidifiers should use steam injection systems that reduce potential microbial contamination.

TopHow to test humidifiers and misters for LDB following identification of contamination
Outbreak Protocol:

When water sampling has identified LDB contamination and the treatment protocol has been successfully completed, return the humidifier to operation and test the unit's water system to detect recontamination according to the schedule below:
  1. Weekly for the first month.

  2. Every two weeks for the next two months.

  3. Monthly for the next three months. If no contamination is detected after three months, end the monitoring program but continue the maintenance procedures as long as the humidifier is in use.

TopHow do I collect water samples?
Water Sampling Protocol: Sampling information specific to humidifiers and misters is provided below. For more information about water sampling, please refer to Section II:E. Water Sampling Guidelines.
  • Collect water samples before starting decontamination and at other times identified above.
  • Sample the incoming water supply if the plumbing provides access.
  • Sample water reservoirs.
  • If a biocide is used, follow the manufacturer's instructions for proper neutralization.
Interpretation Guidelines:
  • The suggested OSHA guideline for LDB in humidifier water systems is less than 1 colony forming unit per milliliter, CFU/mL.

    • If LDB concentration are below 1 CFU per milliliter and no LDB were detected in swab or other samples, no further monitoring for LDB is necessary. Continue the maintenance program as long as the system is in use.
    • If detectable concentration of culturable LDB are measured in water or other samples, take steps to identify the source of contamination or amplification and treat the system.
      See How to treat a contaminated water system.

      • Sample the water system monthly until the source of contamination is identified and adequately treated. Once LDB levels remain undetectable for a three-month period, sampling may be stopped.

TopHow to treat a contaminated water system
Where water in humidifiers has been sampled and shown to contain measurable LDB or where such water is assumed to be contaminated with LDB based on epidemiological evidence, use the following protocol:
  • Disinfect water in piping or reservoirs feeding the humidifier with chlorine or other effective biocide.

  • Sample the humidifier water to ensure the LDB are eliminated. See Section II:E. Water Sampling Guidelines.

  • Before using the humidifier, flush the piping or reservoir thoroughly to remove biocides.

Additional information (App II:A-1) on biocides is also available.

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