 
Chlorine is a chemical element with the symbol Cl and atomic number 17. It's a toxic, very poisonous corrosive gas that's yellow-green in colour at room temperature, and has a pungent odour similar to bleach. Chlorine is not flammable, but can react explosively with other substances.The density of chlorine gas is approximately 2.5 times greater than air, which causes it to remain near the ground where there is little air movement. It was the first gas used as chemical warfare in World War I. 
Chlorine has a variety of uses. It kills bacteria, so it can be used as a a disinfectant. It disinfects water and is part of the sanitation process for sewage and industrial waste. During the production of paper and cloth, chlorine is used as a bleaching agent. It is also used in cleaning products, including household bleach, which is chlorine dissolved in water. Chlorine is used in solvents, pesticides, polymers, synthetic rubbers, and refrigerants.About 20% of chlorine produced is used to make PVC, a versatile plastic used in window frames, car interiors, electrical wiring insulation, and water pipes. Another major use for chlorine is in organic chemistry. It is used as an oxidizing agent. 85% of pharmaceuticals use chlorine or its compounds at some stage in their manufacture. The chloride ion is essential to life. It is mostly present in cell fluid as a negative ion to balance the positive potassium ions. It is also present in blood to balance the positive sodium ions. We get most of the chloride we need in our bodies from salt. Typical daily salt intake is about 6 grams, but we could manage with half this amount. In the past chlorine was commonly used to make chloroform (an anaesthetic) and carbon tetrachloride (a dry-cleaning solvent). However, both of these chemicals are now strictly controlled as they can cause liver damage. Because chlorine is a gas at room temperature, exposure to it occurs via inhalation. Symptoms of acute exposure include coughing, eye and nose irritation, and a burning sensation in the chest. The health effects of chlorine are primarily due to its corrosive properties. The strong oxidizing effects of chlorine cause hydrogen to split from water in moist tissue, resulting in the release of oxygen and hydrogen chloride, which produce corrosive tissue damage. The oxidation of chlorine may also form hypochlorous acid, which will penetrate cells and react with proteins to destroy cell structure. There is no antidote for chlorine poisoning. 
Chlorine is added to swimming pools to kill bacteria. Interestingly, pool water with a small amount of chlorine added has nospecific smell.
The smell that comes from a pool is actually chloramine, a chemical compound that forms when chlorine in the pool combines with organic substances from swimmers, such as urine. The typical pool smell is because of people who have peed in the water!
See the proof here, if you have a strong stomach!
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