Multiple questions on disinfection

I have multiple questions regarding disinfection.

1) The majority (>90%) of cases of Legionnaires’ disease reported in the US are caused by L. pnuemophila. Is it correct that L. anisa is frequently isolated from the environment but it almost never causes disease?


2) Are the normal rules and regulations (>55°C, no dead legs/stagnation, circulation loop maintenance, monitoring, etc.) sufficient to control the presence of Legionella in hot water systems?


3) What is the “infective dose” for contracting Legionnaires’ disease?


4) Is there a Legionella risk from a condensate drain of an air conditioner?


5) Does the scaling of pipes predispose to Legionella contamination?


6) Air conditioners are not a source of Legionnaires’ disease?


7) Can ozone be used as a disinfection for Legionella in water systems? Is ozone used at all to disinfect contaminated (or suspect) areas?


8) Should cooling tower workers use personal protective equipment?


9) When water samples are positive for Legionella, is it common or rare to find more than one species present?


10) Should we use aerators on sinks?

1) Correct. L. anisa is generally nonpathogenic and disinfection is not needed when this species is present.

2) These factors are insufficient and while often recommended, may not affect Legionella growth. The only way to be sure is to culture your water system. Download the Stout ASHRAE 2007 article for more information.

3) Quantitative cultures are not predictive, but percent site positivity is predicitive for water systems.

4) In our opinion, there is no risk.

5) Yes, due to the presence of biofilm.

6) No. This belief is a myth.

7) There is one published attempt with ozone in a Los Angeles hospital, but the results were inconclusive.

(Muraca, P. Stout, J. Yu, VL. Comparative assessment of chlorine, ozone, and ultraviolet light for killing of Legionella pneumophila within a model plumbing system. Appl Environ Microbiol. 1987; 53:447-453.)

8) No comment. Ask the CDC.

9) Presence of multiple species of Legionella in water is common.

10) We discourage their use because of their predilection for biofilms.