Common causes of a rash
Rashes can be caused by a wide variety of things, and it can be difficult to identify the exact cause straight away. Two different rashes, with completely different causes, may look exactly the same on initial examination.
The main causes of rashes can be roughly categorised as follows:
- Contact rashes
- Heat rashes/sun-induced rashes
- Allergic rashes
- Infectious skin diseases
- Infectious diseases of the body
- Chronic skin diseases
Contact rashes are a localised allergic reaction or irritation of the skin, and are caused by direct contact with a wide variety of substances. This is called contact dermatitis and causes local rash and inflammation only in the area where contact has been made. Common causes for contact dermatitis include:
- Rubber, latex and elastic products
- Detergents, perfumes and cosmetics
- Chemicals in the environment or in clothing
- Certain plants such as poison ivy
- Metals such as zinc, nickel and silver
Prickly heat is another form of contact rash that is caused by wearing tight fitting clothing, or too many layers, which does not allow the skin to breathe. Sunburn or excessive use of artificial sunlamps or sunbeds can also cause a heat rash. A heat rash that occurs on flexural skin and aggravated by friction is called intertrigo.
Heat rashes / sun-induced rashes
- Prickly heat (otherwise known as miliaria) develops when sweat glands in the skin get blocked. It may be aggravated by wearing tight fitting clothing, or too many layers, as this does not allow the skin to breathe.
- Sunburn or excessive use of artificial sunlamps or sunbeds can also cause a type of heat rash.
- Polymorphic light eruption (PLE), is an immune response to increasing ultraviolet light (UV light) exposure. It typically comes on in the Spring or while on holiday in sunnier climates.
Allergic rashes (urticaria or urticarial rashes) are caused by excess production of histamines in the body in response to something in your environment or something that you have eaten. Sometimes these type of rashes occur without any known trigger. These rashes are often called hives, and can appear anywhere on the body and do not require direct contact with the skin. You may come out in an allergic rash in response to a wide variety of stimuli, including:
- Medications or vaccinations
- Foods, especially shellfish and nuts
- Insect or nettle stings
- Infectious skin diseases
Infectious skin diseases
Several infectious skin diseases also cause a rash. These can be caused by a number of infections with bacteria or viruses, and infestations of mites within the skin. They can easily be passed on by direct contact, including:
- Molluscum contagiosum
Some infectious skin diseases, such as scabies, are caused by mites that bore into the skin surface causing irritation and a localised rash.
Most infectious skin diseases are common and relatively harmless. Many of them usually only occur in children.
Infectious diseases of the body
There are several diseases of the whole body that include a skin rash as one of their symptoms. These include:
- Chicken pox
Disease based rashes generally appear suddenly, without any obvious trigger, and are usually accompanied by other symptoms such as a fever, lethargy and flu-like symptoms. Rashes caused by infections can often be unbearably itchy.
Chronic skin diseases
There are two main chronic skin diseases that cause rashes – eczema and psoriasis. Inflamed rashes and scaly patches can appear anywhere on the body but most often occur around the knees and elbows or in patches on the face. The symptoms of chronic skin diseases tend to come and go in flare ups, often in response to external stimuli.