Miliaria: Heat Rash From Blocked Sweat Glands

By Sonia Gulati. May 7th 2016

Heat rash (also referred to as milaria) is a common condition that occurs when sweat accumulates under the skin as a result of a sweat duct obstruction. Heat rash is especially common in hot and humid climates, such as in the tropics or during the summer season. Although everyone is susceptible to developing heat rash, it is especially prevalent amongst children and infants due to their underdeveloped sweat glands. Heat rash can appear anywhere you have sweat glands, but is most often seen on the back and chest. It typically presents itself as superficial blisters or bumps that may sometimes burn or tingle.  Most types of heat rash do not require medical treatment and usually go away on their own.

Types of Heat Rash

Sweat utilizes ducts to travel from sweat glands (the site of its production), which are found in the deep layer of the skin (the dermis) to the surface of the skin. When these ducts become occluded a heat rash occurs. There are various types of heat rash that are classified based on the level of obstruction and any associated bacterial infection. There are four major types of heat rash are:

  • Milaria Crystaline: This is the mildest form of heat rash, which causes the appearance of multiple tiny clear blisters that resemble beads of sweat. There are no other symptoms associated with this type of heat rash.
  • Miliaria Rubra: This is the most common type of heat rash, in which the obstruction of the sweat glands causes sweat to leak into the deep layers of the skin, eliciting a local inflammatory reaction. This rash most commonly occurs on the chest or back and in skin folds, such as under your arms or beneath the breasts. This type of rash is associated with red bumps that are often very itchy.
  • Milaria profunda: This is most severe form of miliaria, and is often referred to as "wildfire" due to the rapid spread and severe burning sensations that it produces. This type of heat rash generally occurs as a result of a sweat gland obstruction that is located deep in the structure of the sweat gland, causing the gland's secretions to leak between the superficial and deep layers of the skin. The rash and associated symptoms occur quickly, within hours of an activity provoking sweating and disappear within hours of removing the stimulus for sweat. This type of heat rash is flesh-colored, does not usually cause itching, and is often associated with heat exhaustion.
  • Miliaria pustulosa: This type of heat rash is associated with a bacterial infection that occurs as a result of constant scratching.


Heat rash usually appears as tiny bumps found at the base of small hair follicles. In some areas, there may be red or pink patches of skin. More severe heat rash is often associated with greater degrees of irritation as well as large welts, hives, and raised red bumps. Some types of heat rash may also be accompanied by itching. Commonly affected areas are the: face, neck, under the breasts, under the scrotum, back, chest, armpits, groin and stomach.


Heat rash develops as a result of sweat being trapped beneath the skin due to a blockage of sweat ducts. This accumulation of sweat may cause inflammation and a rash. Although it is not known why sweat ducts become occluded, certain factors seem to play a role. These include:

  • Immature sweat ducts. This is a major risk factor for children and newborns.
  • Tropical climates.
  • Intense physical activity.
  • Certain fabrics. You may develop heat rash if you consistently wear clothing that doesn't allow sweat to evaporate.
  • Medications. Certain prescription medications and chemotherapeutic agents have been known to cause heat rash.
  • Overheating. For example, being in a very warm climate, bundling up too much in winter or sleeping under an electric blanket can lead to heat rash.
  • Heavy creams and ointments. These products can block the sweat ducts.


Heat rash can be diagnosed by visually examining the skin. There are no medical tests for heat rash.


Treatment for heat rash includes:

  • Reduce sweating. The best treatment for all forms of heat rash is to reduce sweating by staying cool and limiting physical activity. Once the skin is cool, the heat rash tends to clear quickly.
  • Applying calamine lotion. This may soothe any itching associated with a heat rash.
  • Applying anhydrous lanolin. This topical treatment may help prevent duct blockage and stop new bumps from forming
  • Topical steroids. This is only utilized in the most serious cases


Heat rash can be prevented by:

  • Avoiding excessive sweating
  • Avoid wearing nylon or polyester clothing
  • Staying in a cool environment (using a fan or air conditioning)
  • Taking frequent cool showers or baths with mild soap


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