When we think of allergies, we usually think of pollen, pet hair, nuts or penicillin. These affect millions of people worldwide. The world of allergies is huge, so what is the rarest allergy?

Aquagenic Urticaria: The allergy to water.

Aquagenic urticaria is an extremely rare condition in which a person develops hives, rashes, or an intense itching when their skin comes into contact with water, no matter the temperature or source. Drinking water, however, does not cause internal symptoms, the allergy is only triggered at skin level.

There have been fewer than 100 documented cases, which makes it one of the rarest allergies that have ever been identified.

Symptoms of aquagenic urticaria include itchy, red welts after water exposure, burning or prickling sensation, hives - these can last anywhere from 30 minutes to several hours.

Aquagenic urticaria can therefore cause the affected person to have a lot of anxiety around completing daily activities, such as washing.

The exact cause of aquagenic urticaria is still unknown. It’s not a typical IgE-medicated allergy, such as peanuts or latex. Scientists currently believe that the reaction occurs when water interacts with a natural substance on the skin, which creates a compound that causes a release of histamine.

Because aquagenic urticaria is so rare, there is currently no single effective treatment, therefore management is key. Management can include daily antihistamines, barrier creams, phototherapy and lifestyle changes which reduce water exposure.