Sustain Summer Shield Chlorine Extender provides water with an extra level of protection against algae. It does this by creating a reserve, or backup, of chlorine fighting power that is released on-demand when routine sanitizing levels drop (below 1ppm), such as after a rainstorm or pool party. Sustain Summer Shield chlorine extender also contains a clarifier to gather suspended particles that can cloud your water, making them easier to be removed by the pool filter. Summer Shield Chlorine Extender is a “good form” of combined chlorine. It is measured by subtracting the Free Available Chlorine (FAC) reading from the Total Chlorine (TA) reading. Proper application of the Sustain 3-Part system will prevent “bad” combined chlorine (weak, foul smelling water) from forming. So, remember, what is measured as combined chlorine is actually the Summer Shield concentration in water when using the Sustain System.