Red dots under the skin on the arm

A flat patch after sun exposure, heat, a new product, or rubbing may reflect irritation or Sunburn. Fine clustered dots after sweating can occur with Heat Rash. A localised area that becomes hot, painful, swollen, and increasingly red needs assessment for Cellulitis. Contact Dermatitis may follow an adhesive, plant, cosmetic, or cleaning product. A rough or scaly persistent patch can occur with Psoriasis. Note any injury, injection, new medicine, or bleeding elsewhere.

Photograph the arm and mark the edge of spreading redness with a pen so change is easier to judge. Keep the area cool, protect it from sun and friction, and use gentle cleansing. Avoid squeezing, vigorous rubbing, and applying multiple medicated creams at once. Arrange medical review if the dots persist, recur, become raised, or are joined by pain, warmth, swelling, drainage, or fever.

Seek urgent care for rapidly expanding redness, severe pain, marked warmth or swelling, red streaking, pus, fever, chills, facial swelling, breathing difficulty, faintness, or widespread unexplained bruising. Prompt review is also needed for sudden dots that do not fade with pressure.

Are these dots in the skin surface or beneath it? Do they blanch, and is there warmth or swelling? Could sun, friction, a product, medicine, or infection explain the location?