Interpretation of Legionella
Culture Results

(not applicable for New York samples)

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There are no federal action levels for Legionella, however some states, agencies, and organizations have published interpretive guidelines for Legionella quantitative culture results. The full text of the guidelines below can be found by accessing the provided links. It is ultimately the responsibility of a qualified professional to select actions that are appropriate for the situation being evaluated.

OSHA Guidelines

OSHA Technical Manual, Section III, Chap. 7, Legionnaire’s Disease (which was adapted from George K. Morris, PhD, and Brian G. Shelton, Pathcon Technical Bulletin 1.3, Legionella in Environmental Samples: Hazard Analysis and Suggested Remedial Actions, June 1991

https://www.osha.gov/dts/osta/otm/legionnaires/sampling.html, accessed 8-18-17

Legionella colony forming units per milliliter (CFU/mL)

Action Cooling Tower / Evaporative Condenser Potabe Water Humidifiers & Misters
1 ≥ 100 ≥ 10 ≥ 1
2 ≥ 1000 ≥ 100 ≥ 10

Action 1: Cleaning followed by biocide treatment of the system, if appropriate.
Action 2: Cleaning and/or biocide treatment. Take immediate steps to prevent employee exposure.

AWT Guidelines

Legionella 2003: An Update and Statement by the Association of Water Technologies (AWT), https://www.awt.org/pub/035C2942-03BE-3BFF-08C3-4C686FB7395C, accessed 8-18-17

Cooling Tower Legionella Count, colony forming units per milliliter (CFU/mL)

>0-10 >10-100 >100-1000 >1000
A B C D
  1. Increase biocide addition/s
  2. Increase biocides; review program, retest till <10
  3. Disinfect/clean within 30 days; review program
  4. Disinfect/clean within 7 days; review program
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