How long after chlorination should a second follow-up test for Legionella be performed according to CDC recommendations?

The correct timeframe for performing a second follow-up test for Legionella after chlorination is 14 days, as recommended by the CDC. This interval allows sufficient time for the residual effects of chlorination to be assessed accurately. Immediately after chlorination, testing may not provide reliable information, as chlorine levels may still be fluctuating, and bacteria may not have sufficiently responded to the treatment.

Testing too soon would not capture the effectiveness of the chlorination in reducing Legionella levels, while waiting longer than 14 days could give the opportunity for re-colonization of the bacteria within the plumbing system. A follow-up at this two-week mark helps ensure that the disinfection process was successful and provides a clearer picture of the water system's safety concerning Legionella presence.