Pool care articles
How fast does chlorine kill pool germs?
At 1 ppm free chlorine, pH 7.5 and 77°F, in water without cyanuric acid, chlorine kills the E. coli strain in the CDC table in under a minute, but Cryptosporidium takes about 15,300 minutes, or 10.6 days. pH and stabilizer change those times. Sullivan's Pool Service maintains backyard pools across Austin.
If you own a backyard pool in Austin, the CDC's guidance on chlorine comes with a timetable worth knowing. Chlorine is put into pool water to kill germs, but the CDC states plainly that it does not act immediately. Used the right way, it can kill most germs within a few minutes. That word "most" is the whole story, because a few germs tolerate chlorine far longer than the rest, and for one of them the wait is measured in days.
The CDC timetable, germ by germ
The clearest window into this is the timetable the CDC publishes for how long chlorine needs to kill specific germs. The E. coli strain in the CDC table, a bacterium, is killed in less than 1 minute. Hepatitis A, a virus, takes approximately 16 minutes. Giardia, a parasite, takes approximately 45 minutes. Then there is Cryptosporidium, also a parasite, which takes approximately 15,300 minutes. Run the math and that comes to 10.6 days.
These times come with conditions attached. They assume 1 ppm free chlorine at pH 7.5 and 77°F, and they apply only to pools and hot tubs that do not use cyanuric acid. When cyanuric acid is present, disinfection takes longer still.
This is why a single diarrheal incident in a pool matters so much. The CDC notes that one such incident can release millions of diarrhea-causing germs, and that Cryptosporidium can survive in properly treated water for more than 7 days.
Why pH is such a big lever
The CDC recommends keeping pH in the 7.0 to 7.8 range, which best balances killing germs, the lifespan of the pipes, and swimmers' comfort. As pH climbs, the ability of chlorine to kill germs decreases, especially once it passes 8.0. As pH falls, especially under 7.0, chlorine kills germs more effectively, but the pipes are more likely to corrode or break down. The CDC also notes that pH which is too high or too low can cause skin and eye irritation in swimmers and damage pool equipment.
Chlorine concentration and cyanuric acid
The CDC recommends at least 1 ppm free chlorine in pools and at least 3 ppm in hot tubs. If cyanuric acid is in the picture, either as a stabilizer added on its own or inside chlorine products such as dichlor or trichlor, the recommendation shifts to pH 7.0 to 7.8 and at least 2 ppm in pools. The timetable applies only to water without cyanuric acid; the CDC notes that disinfection takes longer when it is present. For hot tubs, the CDC recommends not using cyanuric acid or chlorine products that contain it.
What wears your chlorine down
Even at the right concentration and pH, chlorine does not fight alone against everything swimmers bring in. Pee, poop, sweat, and dirt from swimmers' bodies all pull the chlorine level down, as does sunlight. Hot tub jets that churn water into droplets or mists play a part too. When chlorine mixes with dirt, sweat, pee, and poop, less of it is left to kill germs, and the CDC notes the same reaction creates the chemicals that make swimmers' eyes red and sting.
Testing is not optional
Because so many things drag the chlorine level down, the CDC says to test chlorine concentration and pH at least twice per day, and more often when the pool or hot tub is being used by a number of people. For the most accurate results, the CDC recommends a DPD test kit, which measures both chlorine and pH. Test strips can also be used, but they are less accurate than a DPD kit. One quirk is worth knowing: if the sampled water has more than 10 ppm chlorine, the test might partially or totally bleach out, giving a false low or zero reading. Whatever you use, the CDC advises following the manufacturer's directions.
This is routine work, and it is the kind of thing we handle on our scheduled weekly routes, where one technician services each pool on the same day every week. Our water testing covers free and total chlorine, pH, total alkalinity, calcium hardness and cyanuric acid, plus salt level and cell output on salt systems, with corrections dosed on site and written down. Homeowners in South Austin, Hyde Park and Round Rock can have those readings checked on a regular weekly day; the CDC's twice-daily checks still apply between visits.
Austin tap water, for comparison
Austin Water publishes a quarterly table of average readings for the finished water leaving its Davis, Ullrich and Handcox plants. The pH of the finished drinking water was 9.70 at Davis, 9.74 at Ullrich and 9.65 at Handcox. Total hardness at the plant tap was 76 mg/L as CaCO3 at Davis and 77 mg/L at both Ullrich and Handcox. Total alkalinity as CaCO3 was 52 mg/L at the Davis tap, 51 mg/L at Ullrich and 60 mg/L at Handcox. The same table lists total chloramines of 2.69 mg/L at Davis, 2.52 mg/L at Ullrich and 2.60 mg/L at Handcox. Those are averages for tap water, not readings of your pool, so test the pool itself after a top-up rather than assuming where it sits.
Swimmers carry part of the load
The CDC notes that a few germs can survive for long periods even in well-maintained pools, so swimmers also need to follow the steps for preventing swimming-related illnesses. Stay out of the pool if you have diarrhea. Shower for at least 1 minute before getting in to remove dirt or anything else on the body. Do not swallow the water. Do not pee or poop in the water. Take kids on bathroom breaks and check diapers every hour, and change diapers away from the poolside. Drying ears thoroughly with a towel after swimming helps prevent swimmer's ear.
Because Cryptosporidium can survive more than 7 days in properly treated water, swimmer habits matter alongside testing and chlorine levels.
Sources
In Austin
Fill water matters here. Austin Water reported finished water leaving its three plants in the second quarter of 2026 at pH 9.65 to 9.74, total hardness of 76 to 77 mg/L as CaCO3, total alkalinity of 51 to 60 mg/L, and 2.52 to 2.69 mg/L of chloramines. Those are averages for tap water, not readings of your pool, so test the pool itself after a top-up.
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- How often should I test my pool in Austin?
Sullivan’s Pool Service maintains pools in and around Austin, including Northwest Hills, South Austin, West Lake Hills, Tarrytown, Great Hills, River Place, Crestview, East Austin, North Austin, West Campus, Hyde Park, Downtown Austin and St. Johns, and in Bee Cave, Cedar Park, Georgetown, Leander, Pflugerville and Round Rock. See every service area.
Weekly residential service has one fixed monthly price, set by pool size: see pool service pricing for what each size tier covers.
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