Chlorine: Inexpensive and easily available, chlorine is the chemical most commonly used in swimming pools and hot tubs to kill bacteria that can spread disease. Used in concentrations of 1 to 4 parts per million, chlorine helps keep pool water at the recommended pH level of 7.2 to 7.8. However, chlorine breaks down into different chemicals that have been blamed for itchy skin, difficulty breathing, asthma and other ailments. If released into the environment, chlorine can cause low-level harm to organisms living in the water and soil, according to the U.S. Environmental Protection Agency.
Saltwater: A saltwater pool is kept clean by hydrolysis, a process that converts water and food-grade salt into pure sodium hypochlorite, the primary ingredient of chlorine, without the chemical additives or byproducts typically used to stabilize or store chlorine, said David Goldman, director of product development for Zodiac Pool Systems.
Ozone gas: More expensive than chlorine, ozone is a short-term disinfectant that rids pool water of organic material to keep it clear. Ozone gas is generated by combining oxygen with electricity. It's added to pool water after it goes through the filter and heater.
Ionizer: This device uses a low electric current to create copper and silver ions that attract and kill algae, bacteria and viruses.