Feeling hot, hot, hot? Heat rash, also known as prickly heat or miliaria, most commonly affects babies, especially when they're overdressed on a sweltering day. (Unlike grown-ups and older kids, infants can't exactly peel off a layer or voice their discomfort -- at least not in any articulate fashion.) With heat rash, the sweat ducts become blocked and swollen, and sweat accumulates under the skin, causing tiny dots, pimples or blisters to appear on the face, head, neck, shoulders and under the armpits [source: Mayo Clinic]. This is why infants, whose sweat ducts aren't fully developed, are at risk.
But adults can develop heat rash too, especially if they live in a tropical climate, perspire abundantly and don't wear breathable clothing [source: Mayo Clinic]. Hospital patients and others confined to their beds for long periods of time can sometimes get heat rash on their backs [source: DermNet NZ]. Greasy or oily face creams and lotions can also trigger flare-ups by blocking the sweat glands [source: Braff].
Whether you're a babe in arms or a hiker in Death Valley, heat rash is easily treatable. If possible, step into an air-conditioned room, slip into something lighter and cool off in front of a fan [source: Mayo Clinic]. Cold water compresses and calamine lotion can often soothe the prickly feeling that accompanies heat rash [source: DermNet NZ].
If the spots and blisters don't disappear after a few days, give your dermatologist a call. He or she may prescribe a topical steroid or even an oral antibiotic [source: Braff].