Heat rash develops when your sweat ducts become blocked and perspiration is trapped under your skin. Symptoms range from superficial blisters to deep, red lumps. Some forms of heat rash can be intensely itchy or prickly.

Most often, heat rash goes away on its own. Severe forms of heat rash may need medical care, but the best way to relieve symptoms is to cool your skin and prevent sweating.

Signs and symptoms

Adults usually develop heat rash in skin folds and wherever clothing causes friction. In infants, the rash is mainly found on the head, neck, shoulders, chest and back, but it can also occur in the armpits and groin.

There are three types of miliaria, which are classified according to where the sweat ducts are blocked. Signs and symptoms for each type vary considerably:

• Miliaria crystallina. The mildest form of heat rash, this affects the sweat ducts in the stratum corneum, the topmost layer of skin. Miliaria crystallina is marked by tiny, clear, superficial blisters and bumps (papules) that break easily but aren't itchy or painful. It usually clears on its own in a few days but can come back if hot, humid weather persists. And though it's most common in newborns, adults can develop it, too, especially if they've recently moved from a temperate climate to the tropics.
• Miliaria rubra. Occurring deeper in the outer layer of skin (epidermis), miliaria rubra causes red bumps that are intensely itchy or prickly, giving rise to its common name, prickly heat. There is often little or no sweating in the affected areas (anhidrosis). Adults can develop miliaria rubra shortly after they're exposed to hot weather, but the rash more often appears after several months of exposure. Infants usually develop this type of heat rash between the first and third weeks of life.
• Miliaria profunda. A less common form of heat rash, miliaria profunda occurs mainly in adults who have had repeat bouts of miliaria rubra. It affects the dermis, a deeper layer of skin, and appears soon after exercise or any activity that causes sweating. The lesions are firm and flesh-colored, much like goose bumps. Though it's not uncomfortable, miliaria profunda can cause a widespread lack of perspiration, leading to symptoms of heat exhaustion, such as dizziness, nausea and a rapid pulse.

Complications

Heat rash usually heals without problems, but complications sometimes occur, such as:

• Infection. Occasionally, heat rash becomes infected with bacteria, causing inflamed and itchy pustules. This is especially common in children in diapers but shouldn't be confused with diaper rash, which results from irritation to tender skin, not from blocked pores.
• Heat exhaustion. In hot weather, people with miliaria profunda are at risk of heat exhaustion, which can cause low blood pressure, nausea, headache and a rapid pulse. Untreated heat exhaustion can lead to heatstroke, a life-threatening condition. 

Treatment

Mild heat rash doesn't require any other treatment, but more severe forms may need topical therapies to relieve discomfort and prevent complications. Topical treatments may include:

• Calamine lotion to soothe itching
• Anhydrous lanolin, which may help prevent duct blockage and stop new lesions from forming
• Topical steroids in the most serious cases

Some anecdotal evidence supports the use of oral vitamin C supplements to help relieve heat rash.