A recent report from the Centers for Disease Control and Prevention (CDC) sent waves of disgust through the public when they learned that a 2012 study found the filters of Atlanta area public pools were teeming with fecal bacteria. The CDC used the findings to underscore the importance of swimmer hygiene. A pre-swim shower with soap, says CDC, is essential to limiting fecal pathogens in pool water. Additionally, swimmers experiencing diarrhea should not be in the pool.
We agree and count showering and other hygiene measures, such as “don’t pee in the pool,” as important layers of protection used to guard swimmers against waterborne illness. Swimming pool sanitizers constitute another significant layer of protection and a well-maintained filter represents yet another. Nevertheless, based on the new report, the public may be wondering how fecal bacteria can be present in pools that are treated with sanitizer. The evidence suggests it comes from the swimmers themselves.
Fecal Bacteria: Dead or Alive?
New methods, known as quantitative polymerase chain reaction, or “qPCR,” where one breaks open the bacteria and uses the unique DNA to identify the organisms in the filter samples, were used by the CDC. This type of analysis can’t determine whether the bacteria are dead or alive (viable or infectious). This makes it impossible to determine the level of risk to swimmers from the accumulating organisms in the filter.
This is really a good news/ bad news story. Finding bacteria and pathogens in the filters means that the filters are doing their job of taking some pathogens out of the water. Many of the bacteria and pathogens identified in the study might have been dead, inactivated by a pool sanitizer, such as chlorine. Nevertheless, the point is made: Fecal bacteria are entering pool water on the bodies of swimmers, reinforcing the need for better swimmer hygiene.
How Long Does it Take Chlorine to Kill Pathogens in Pool Water?
CDC notes frequently that chlorine does not kill germs instantly as a reminder to swimmers not to treat pool water as sterile—to avoid swallowing water, for example. US Environmental Protection Agency-registered chlorine-based sanitizers, used according to label directions, kill 99.9% of the most common waterborne germs within seconds (or minutes for the hardy parasite Giardia) as the table below indicates. But for chlorine to do its job, the water chemistry must be appropriately maintained. For example, pool water pH should be in the range of 7.2 to 7.8 and the chlorine level should be within the range of one to three parts per million. Under those sustained conditions, the risk to swimmers of contracting waterborne disease is minimized and chlorine remains in the water as a safeguard against further contamination.
Pathogens Found in Atlanta-area Public Swimming Pool
|Pathogen Detected in CDC Report||Percentage of Filters Containing the Bacteria or Pathogen||Significance of Pathogen’s Presence||Kill Time for 99.9% at 2 ppm chlorine*|
|E. coli||58||A marker for fecal contamination||2 to 5 seconds|
|P. aeruginosa||59||Can cause skin rashes and ear infections||4 to 10 seconds|
|Giardia intestinalis||Less than 2||Causes diarrhea, Chronic infection||20 to 45 minutes|
*Free Chlorine, temperature 20 to 25o C
1Cryptosporidium was found in less than 2 percent of Atlanta area pools; to destroy Cryptosporidium with chlorine sanitizer, pool chlorine concentrations must be raised to 20 ppm chlorine for 12.75 hours in a pool evacuated of swimmers.
Joan B. Rose, PhD, is the Homer Nowlin Chair in Water Research at Michigan State University and a member of the Water Quality and Health Council.