Hi there. There can be multiple causes for "red arm rashes", and it is difficult to give specific advice without more information or a photo. The most common causes for red rashes on the arms would include eczema, allergies, hives or infections.
All of these can be effectively and safely treated when seen by a trained dermatologist. Do see a doctor near you for a quick and effective diagnosis and treatment.
Best regards
Dr Stephanie Ho, Dermatologist
