To chlorinate means to add chlorine to water, usually to kill harmful germs and bacteria. Swimming pools are chlorinated to help make them safer for swimmers. Without chlorine, pools would quickly fill with algae and microorganisms that could make people sick.

Cities also chlorinate drinking water at treatment plants before it flows through pipes to homes and businesses. This process, developed in the early 1900s, was a major breakthrough in public health. Before chlorination, contaminated water spread diseases like cholera and typhoid fever. Today, chlorinated water helps protect millions of people from disease.

You can sometimes smell or taste chlorine in tap water or pool water. That sharp, distinctive smell means chlorine is present. Pool owners test chlorine levels regularly because too little won't protect swimmers, while too much can irritate eyes and skin. The process of adding chlorine is called chlorination, and water that has been treated this way is called chlorinated.