<s>[INST] My 15 year old son has a smooth hot rash that has just appeared on his leg. A few hours ago the same looking rash (smaller) appeared on his upper arms and has now disappeared. He has no other symptoms. His rash is smooth hot and slightly itchy however does not feel the desperate need to scratch the area. I have applied a cold pack and it seeems to have helped. [/INST] Hello. Thank you for asking at Chat Doctor. A smooth, hot, slightly itching rash, not lasting for long duration, improving by cold packs, otherwise fine - can be due to many things. I would suggest you to look for - sweating, type of clothing (cotton/woolen/other), fitness of clothing (tight/loose), whether rash appear after bath/exercise, whether it appears after ingestion of specific food/ Chat Doctor. I feel you may be able to identify the cause from above clues. As general measures, I would suggest your son to wear loose clothes, avoid sweating & apply cold packs when rash appears. If it is not relieved, an antihistamine  </s>