Many medical conditions can cause skin rashes on the arms and legs. But a red rash confined to one arm usually indicates a local process as opposed to a widespread medical condition. Allergies, infections and contact with irritating chemicals often cause skin reactions, leading to a red arm rash. A careful medical history reviewing contact with possible skin irritants or allergy-provoking substances along with an examination of the rash characteristics usually provide sufficient information to make a diagnosis.
Allergic dermatitis is a form of eczema. Within a few hours of contact with a substance to which the person is allergic, a red and itchy rash develops at the contact site. Costume jewelry often contains nickel, a common allergy-provoking metal. Wearing a bracelet, watch or cuff containing nickel may trigger allergic dermatitis of the arm in susceptible people. Antibiotic ointments and perfumes also commonly cause dermatitis if applied to the arm of an allergic person. Development of a rash after contact with poison sumac, ivy or oak represents another form of allergic dermatitis, which may affect the arm or any other site that comes into contact with the plant.
Irritant Contact Dermatitis
Strong chemicals may cause an irritating, nonallergic reaction in the skin, leading to a red rash at the site of contact. The skin typically swells and appears waxy. This condition, known as irritant contact dermatitis, reflects physical damage to the skin. Examples of chemicals that may splash on the arms and cause irritant contact dermatitis include strong detergents, paint thinner and other solvents, drain cleaner and certain glues.
Neurodermatitis is a form of eczema characterized by irritation of the nerve endings associated with the itch sensation. An intensely itchy area of skin may develop at the site of an insect bite, scar or contact dermatitis. As the person scratches the involved area, the itchiness intensifies. The itch-scratch-itch cycle continues, causing a thickened, inflamed red patch that may become infected. Neurodermatitis most commonly affects the forearms, wrists, ankles, lower legs and neck.
Infection of the deep layer of the skin, or cellulitis, may cause a red rash on the arm. The infection often begins with a small cut, insect bite or scratch. Bacteria invade the skin through the wound and rapidly spread outward from the site of entry. The affected area becomes red, swollen, painful and warm to the touch. Expansion of the skin redness signals spread of the infection. The species of bacteria most frequently associated with cellulitis include Staphylococcus aureus, group A and B streptococci, Haemophilus influenzae and Pseudomonas aeruginosa.
Bug Bites and Infestations
A number of different types of bugs can bite or infest the skin of the arm. Mosquitoes are a common example, though it would be unusual for there to be a number of bites isolated to one arm. Scabies is another possibility. This condition occurs when the human itch mite burrows into the skin and lays eggs, triggering a red, intensely itchy, pimple-like rash. Skin creases are favored sites, such as those at the elbow, wrist and armpit. Other possibilities include bedbug, spider and tick bites, among others.
See your doctor for any persistent red rash that does not improve within a few days. Seek immediate medical care for a red rash that is rapidly spreading, especially if accompanied by a fever, pain, numbness, skin discoloration or a central ulcer. Most isolated rashes affecting the arms can be easily treated with over-the-counter or prescription medicines.