Intro to Chlorine

Chlorine is regularly fed into the pool water and should be tested daily—at a minimum—for proper disinfection. Routine chlorination kills harmful microorganisms that can cause health-related problems, such as gastroenteritis, Legionnaires disease, ear infections and athlete’s foot. Learning how to properly test your water will allow you to identify the chlorine residual and demand in pool water. More frequent testing is needed if there is heavy bather use.

Listed below are some helpful definitions that will assist you in understanding the terms and tasks of applying chlorine-based sanitizers.

  • Free available chlorine (FAC). The portion of the total chlorine remaining in chlorinated water that has not reacted with contaminants—and is “free” to go to work to kill bacteria and other contaminants. Make sure your test kit can measure FAC; many only test for total chlorine.
  • Combined available chlorine (CAC) or chloramines. The portion of chlorine in the water that has reacted and combined with ammonia, nitrogen-containing contaminants and other organics such as perspiration, urine and other swimmer waste. Some chloramines can cause eye irritation and chlorine odors.
  • Total chlorine. The sum of both the free available and combined chlorines.
  •  Forms of chlorine commonly used in commercial pools. Pools are treated with chlorine gas, sodium hypochlorite (liquid bleach), calcium hypochlorite (granular or tablet), lithium hypochlorite or chlorinated isocyanurates. When any of these compounds contact water, they release hypochlorous acid (HOCl), the active sanitizing agent. Chlorinated isocyanurates, a family of chemical compounds such as sodium dichloroisocyanurate and trichloroisocyanurate, also add cyanuric acid or stabilizer. A stabilizer, which can also be added separately, helps reduce excess loss of chlorine in water due to the ultraviolet rays of the sun.
  • Parts per million (ppm). Measurement that indicates the parts of a substance, such as chlorine, by weight in relation to one million parts by volume of pool water. A rule of thumb to follow to maintain good water quality in pools is to keep FAC levels between 2.0 and 4.0 ppm. (see NSPI recommendation chart)
  • Shock treatment. The practice of adding significant amounts of an oxidizing chemical to water to destroy ammonia, nitrogen-containing and organic contaminants. Adding chlorine as a shock treatment can also control algae and bacteria, but read the label to make sure that your product can do this.