In hot, humid weather, you may experience a skin irritation known as a heat rash. Despite its name, the heat is only an indirect cause of the rash. Heat rash is an outbreak of blisters or red lumps in the skin resulting from excessive sweating. The bumps are created when sweat ducts get blocked and trap perspiration under your skin.
Heat rash usually clears up on its own, but you can relieve its swelling, itching and prickly feeling by cooling your skin and moving to a place where you won't continue to sweat. You can try to prevent heat rash by keeping cool on hot, humid days. Even with prevention, though, some people are more prone to heat rash than others, including newborns and those taking certain medications.
Heat rash can also defy weather conditions. Any time your body overheats or lacks sufficient exposure to sweat normally, heat rash may be a risk. This includes bundling up a lot in the winter, using heavy ointments or creams, or being confined to a bed for long periods.