Many medical conditions can cause skin rashes on the extremities. A red rash confined to one arm usually indicates a local process as opposed to a systemic condition. Allergies, infections and contact with irritating chemicals may cause skin reactions, leading to a red arm rash. A careful medical history reviewing contact with possible skin irritants or allergens and an examination of the rash characteristics usually provide sufficient information to make a diagnosis and initiate treatment.
Allergic Dermatitis
Allergic dermatitis is a form of eczema. Within a few hours of contact with a substance to which the patient is allergic, a red and itchy rash develops at the contact site. Costume jewelry commonly contains nickel, a common metal allergen, reports the American Academy of Dermatology on the patient information website EczemaNet. Wearing a bracelet, watch or cuff containing nickel may trigger allergic dermatitis of the arm in susceptible patients.
Antibiotic ointments and perfumes also commonly cause dermatitis if applied to the arm of an allergic patient. 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 the formation of 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, explains the New Zealand Dermatological Society on the health information website DermNet NZ. 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
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 patient scratches the involved area, it becomes itchier, explains the University of Michigan Health System. The itch-scratch-itch cycle continues, causing a thickened, inflamed red patch that may become infected. Neurodermatitis most commonly affects the ankles, lower legs, forearms, wrists and neck.
Cellulitis
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 streptococci, Haemophilus influenzae, Pseudomonas aeruginosa and group B streptococci, reports "The Merck Manual for Healthcare Professionals."
