Urticaria – also known as hives welts or nettle rash – is a raised, itchy rash that appears on the skin.
The rash can be on just one part of the body or be spread across large areas.
The affected area may change in appearance within 24 hours, and the rash will usually settle within a few days. If it clears completely within six weeks, it's known as acute urticaria.
Sometimes the rash can persist or come and go for more than six weeks, often over many years, although this is less common. Doctors refer to this as chronic urticaria.
Hives can change size rapidly and move around, disappearing in one place and reappearing in other places, often in a matter of hours.
Urticaria occurs when a trigger causes high levels of histamine and other chemical messengers to be released in the skin.
These substances cause the blood vessels in the affected area of skin to open up (often resulting in redness or pinkness) and become leaky. This extra fluid in the tissues causes swelling and itchiness.
Histamine is released for a wide range of reasons, including:
- an allergic reaction to substances such as latex
- cold or heat exposure
- the effect of certain chemicals found in some types of food and medications, such as non-steroidal anti-inflammatory drugs (NSAIDS)
- Pain medications
- Insects or parasites
- Heat or cold
- Alcohol, food or food additives
- Pressure on the skin, as from a tight waistband