A rash on the upper arms can appear due to various reasons. A rash usually indicates that you are allergic to a certain type of food product. If the rash occurred recently, then you might want to cut down or rather eliminate the food product or diet that you may have begun recently. If the rash disappears, then indeed it's a food allergy.
Rashes, more often than not, are a result of the reaction of the skin to humid or hot weather. Sensitive skin usually has a hard time dealing with such rashes. If that's the case, switch to a cool cool diet. Eat plenty of cucumbers and watermelons. Also, make it a point to drink several glasses of water regularly. Water removes the toxins out of your body. If you can, bathe twice a day in cold water.
Finally, avoid any tight fitting clothing around the upper arm. Chances are the tight clothing might be irritating the skin in that area. Wear loose clothing for a while – at least till the rash disappears. Make it a point to opt for natural cotton fibers. They are the best attire to decide upon if you are in a place that suffers from a hot and humid climate.
answered by M W