First off, always look at
the original CDC paper since news organizations often take things out of context. The CDC took 1 liter samples from filters and conducted quantitative polymerase chain reaction (qPCR) assays. This was NOT a test for live or active microorganisms. It was only a test to identify what types of pathogens are introduced into pool water and gives no indication whatsoever as to whether the pool is safe. The report explicitly states:
qPCR results alone cannot be used to determine whether the detected microbes were viable or infectious or determine the level of swimmer risk; qPCR detects viable microbes as well as those inactivated by disinfection.
and
CDC does not recommend testing the water or the filter backwash of treated recreational water venues (e.g., pools and hot tubs/spas) for microbes unless the venue is at least suspected to be associated with a waterborne disease outbreak. Maintaining proper disinfectant level and pH should prevent transmission of chlorine-susceptible pathogens.
Since qPCR is so sensitive to finding (via amplification) extremely small amounts of DNA, I think that this study was pretty pointless. The study states:
Detection of a study microbe was defined as a qPCR cycle threshold value <40.
Each PCR cycle doubles the quantity of the targeted DNA segment and eventually there is enough DNA to bind with dyes so that it will fluoresce and can be quantified. Read
this document regarding cycle thresholds where you will see that using < 40 "are weak reactions
indicative of minimal amounts of target nucleic acid which could represent an infection state or environmental contamination." They should have at least reported the cycle threshold profile (histogram) for their samples so we could see if any were 30-37 "indicative of moderate amounts of target nucleic acid" or <= 29 "indicative of abundant target nucleic acid in the sample." Some other sources consider 35 or more cycles as weak. Some of this depends on the initial dilution of the sample that is sometimes done to avoid interference, but the CDC study did not indicate dilution so it is possible that just ONE microbe's DNA in the 1-liter backwash sample was enough to count for detection.
So of course there was fecal matter in the pool, but it doesn't mean that there were significant lapses of hygiene or incidents of diarrhea. All it shows is that even a tiny amount of fecal matter got into the pool and then into the filter and did not get removed from backwashing. The CDC is concerned about the rise in recreational water incidents, especially with the protozoan oocyst
Cryptosporidium parvum which is highly chlorine-resistant and is introduced by diarrhea from infected persons. I think they should focus more on making sure that people with intestinal illness (especially with diarrhea) are told not to swim. They seem to be trying to use scare tactics to change behavior, but I think it will just turn off people from swimming in pools and that's not a good thing given the health benefits from swimming.
As for kill times, I show in the table in
this post time for a 3-log reduction (99.9% kill) when the Free Chlorine (FC) level is roughly 10% of the Cyanuric Acid (CYA) level. Bacteria such as
E. coli, at least when in planktonic (free-floating) form, are killed very quickly by chlorine in less than 1.2 minutes. Basically, half are killed every 7 seconds or faster. For public/commercial pools, I'd recommend an FC that was 20% of the CYA level so kill times roughly half those shown in the table.
