Understanding Swimming Pool Chemistry

By Fred Reiff, P.E.

It’s no secret that swimming pools, although fun and refreshing, are essentially communal bath tubs. To help keep water clean and safe, pool operators must adjust pool chemical levels. Maintaining proper pool chemistry can be a challenging task, especially in community pools where the number of swimmers fluctuates wildly from hour to hour.  Add to that the fact that many swimmers are unaware of the role of personal hygiene in keeping waters safe, and one begins to understand the magnitude of the pool operator’s responsibilities.

Chlorine and pH Get it Done

Chlorine is strongly associated with swimming in popular culture.  Although there are persistent myths about chlorine in swimming pools, chlorine is used as a disinfectant in the majority of pools to help prevent waterborne illnesses such as diarrhea, swimmer’s ear and skin infections.  Alternative sanitizers have been introduced in the marketplace over the last several years, but chlorine continues to offer the most effective and economical option to helping maintain safe pools.

Chlorine actually serves two purposes: it destroys algae and most waterborne germs, and it reacts with—oxidizes—small bits of organic debris and impurities introduced into pool water by swimmers.  Chlorine does this work in the form known as free chlorine, a combination of hypochlorous acid and hypochlorite ion.  Free chlorine is produced in pool water when chlorine disinfectant is added.  When free chlorine reacts with nitrogen-bearing or organic substances, the product is known as combined chlorine, a much weaker disinfectant and oxidant. The World Health Organization recommends free chlorine levels up to 3 mg/l be maintained in swimming pools.

As pool operators know, hypochlorous acid1 is a more effective disinfectant and oxidant than the hypochlorite ion 2, and their relative proportions fluctuate with the pH (acidity) of the water in the pool (low pH is more acidic and high pH is more basic).   To maintain optimal levels of hypochlorous acid for germ and algae destruction while at the same time keeping the water comfortable for swimmers, pool operators should maintain pH in the slightly basic range of 7.2 to 7.8.

Monitoring, Monitoring, Monitoring

One of the most important tasks of the swimming pool operator is vigilantly monitoring the pH and free chlorine level of pool water to ensure germs are being destroyed.  This is critical because chlorine may be depleted, for example, by a heavy “bather load”.  A crowded swimming pool adds more organic debris (e.g., perspiration, body oils, trace urine and fecal matter), which can lower the chlorine level or even deplete it, leaving little or no protection against waterborne germs.

A 2010 report from the Centers for Disease Control and Prevention (CDC) found one in eight public pool inspections conducted in 13 states in 2008 resulted in pools being closed immediately due to serious code violations, including insufficient disinfectant.  That is why CDC encourages swimmers to take matters into their own hands and (1) shower and thoroughly wash their bottoms before entering the pool and (2) use portable pool test strips to monitor for adequate free chlorine and pH.  As a public service, the Water Quality and Health Council is making these strips available free to the public this summer at www.healthypools.org/freeteststrips.  Swimmers will have to wash their own bottoms.

Fred Reiff, P.E., is a retired official of the Pan American Health Organization.

1Hypochlorous acid is HClO.

2An ion is an atom or molecule with a net electric charge due to the loss or gain of one or more electrons. Hypochlorite ion is OCl-.