Safe pool water depends on two systems working together: circulation and chemistry. Pumps draw water from skimmers and drains, push it through filters that remove particles, and return it to the basin, with the full volume turning over several times a day. Disinfectants — most often chlorine — then inactivate pathogens between bathers.
Guidance such as the US CDC's Model Aquatic Health Code recommends free chlorine of at least 1 ppm and a pH of roughly 7.0 to 7.8, a range that keeps disinfectant effective and water comfortable for eyes and skin. Swimmers have simple checks of their own: the water should be clear enough to see the main drain, surfaces should not feel slimy, and a well-run facility will post its inspection or test results.