You use a test kit, which has either test strips, or a little white pill that you drop into a graduated tube full of pool water. The degree of colour change will tell you the free chlorine level. Ideal is between 1 and 2 PPM. If a spa is present, 1 to 3 PPM.

Swimming pool water, with adequate chlorine disinfection levels and pH maintenance, is much less likely to contain infectious microorganisms and so represents a much safer environment for recreational swimming.

Swimming pools are a miracle of science and hygiene; thanks to the wonders of chlorine—a poisonous gas that's been coopted to keep pool water clean and sparkling—your local pool can operate safely without risk of spreading potentially dangerous water-borne pathogens.