Disinfection & Testing

Chlorine is added to the water to kill germs. But it does not work right away. If used properly, free chlorine* can kill most germs within a few minutes. CDC recommends pH 7.2–7.8 and a free chlorine concentration of at least 1 ppm in pools and at least 3 ppm in hot tubs/spas.

If using cyanuric acid, a chlorine stabilizer, or chlorine products with cyanuric acid (for example, products commonly known as dichlor or trichlor [see product label]), CDC recommends pH 7.2–7.8 and a free available chlorine concentration of at least 2 ppm in pools. CDC recommends not using cyanuric acid or chlorine products with cyanuric acid in hot tubs/spas.

* Free available chlorine is the more active form of chlorine that kills germs.

Three reasons. First, as pH goes up, the ability of free chlorine to kill germs decreases, especially if pH is >8.0. Second, as pH goes down, especially if pH is <7.0, the ability of free chlorine to kill germs increases but the pool or hot tub/spa pipes are more likely to corrode or break down. Third, keeping the pH in the 7.2–7.8 range helps keep swimmers comfortable in the water by helping to prevent eye and skin irritation. This means keeping the pH in the 7.2–7.8 range best balances killing germs to prevent recreational water illness, the lifespan of the pipes, and swimmers’ comfort.

pH and its Effects

All sorts of things can reduce chlorine concentration in pool or hot tub/spa water. Free available chlorine breaks down pee, poop, sweat, and dirt from swimmers’ bodies instead of killing germs and uses up chlorine, which means there is less to kill germs. The sunlight and hot tub/spa jets creating droplets or mists from the water also uses up free chlorine. That’s why the free chlorine concentration must be routinely tested. And remember, the time it takes for free chlorine to kill germs is also affected by the other member of the disinfection team, pH.

For most accurate testing results, pool and hot tub/spa owners should use a DPD ^† test kit to measure free chlorine and pH. Of note, if the free chlorine concentration of the water sampled from the pool or hot tub/spa is more than 10 ppm, the test might partially or totally bleach out, resulting in a false low or 0 ppm free chlorine result. Pool and hot tub/spa owners alternatively can use test strips, with the understanding they are less accurate than the DPD test kit. Regardless of what test is used, owners should always follow manufacturer’s directions.

^† DPD is short for N,N Diethyl-1,4 Phenylenediamine Sulfate.

Test free chlorine concentration and pH at least twice per day and more often when the pool or hot tub/spa is being used often.

Free chlorine kills most bacteria, such as E. coli 0157:H7, in less than a minute if its concentration and pH are maintained as CDC recommends. However, a few germs are moderately (Giardia, Hepatitis A) to very (Cryptosporidium) chlorine tolerant. The table below shows the approximate times it takes for free chlorine to kill these germs.

Notes:

• Times based on 1 ppm free chlorine at pH 7.5 and 77°F (25°C)
• These disinfection times are only for pools and hot tubs/spas that do not use cyanuric acid. Disinfection times are longer in the presence of cyanuric acid.

The best way to kill germs is by properly maintaining the free chlorine concentration and pH. To do this, pool and hot tub/spa owners must routinely test and adjust both the free chlorine concentration and pH. Since a few germs can survive for long time periods in even the best maintained pools, it is also important for swimmers to follow the steps of healthy swimming.  Combining good chlorine and pH control and encouraging swimmers to follow the healthy swimming steps will help prevent the spread of germs that cause recreational water illnesses.