A chlorine allergy is an extremely rare condition which is usually wrongly suspected. Exposure to higher than normal levels of chlorine in swimming pools or other water sources can cause symptoms like rashes or itching, but this is usually simply irritation from a highly corrosive substance.
Chlorine is a corrosive element. In its natural state it is a toxic gas which is strong enough to have been used as a weapon in war. During World War I, Germany used the chemical in its gaseous form to incapacitate and even kill their enemies; it has even been used by insurgents as recently as the Iraq War. In everyday life, however, it is rare to come into contact with more than a miniscule amount of chlorine.
While it is a highly corrosive substance on its own, it is the corrosive properties of chlorine which, when used in a solution, give it the ability to act as a disinfectant in swimming pools and drinking water supplies. The amount of chlorine in such places is very limited, usually amounting to between one and four parts per million, enough to serve its purpose, but usually sparse enough to avoid causing any physical reaction.
A chlorine allergy is quite rare, but over time some people develop a reaction which can seem like a chlorine allergy. If too much chlorine has been added to a swimming pool, it can cause some symptoms of irritation to develop, including rashes and eye dryness or itchiness. However, this does not amount to an actual chlorine allergy. This is generally a reaction to coming into contact with more chlorine than the safe level that body usually experiences.
While many people experience irritation in swimming pools or when otherwise exposed to chlorine, it is almost always not a chlorine allergy. The chemical itself is naturally corrosive and it can cause reactions which seem allergic in nature when the real problem is often that a pool has too much chlorine. Though it is rare, anyone with questions or concerns about a possible chlorine allergy or other reaction to chlorine should talk to their doctor.