As with hives, heat rash features raised, red bumps on the skin. While heat rash shares its redness, swelling and itchiness with hives, the raised bumps on the skin are usually smaller than hives and resemble small pimples. Also, heat rash is more common in babies, whereas hives can affect any age group.
Heat rash develops when sweat ducts are blocked in hot temperatures, and it commonly happens to infants because their sweat ducts aren't as developed as adults. It can occur when a baby is overdressed for weather, when heavy creams or clothing block the pores or during physical activity that causes sweating [source: Family Doctor].
Most cases of heat rash will disappear in a couple days if you air dry the skin, loosen and remove clothing and keep the temperature controlled whenever possible. If those things don't work, you can apply calamine lotion or hydrocortisone cream to calm the rash, but you'll want to avoid other types of lotions or ointments because they can act as an irritant on the sensitive skin [source: WebMD].