Erythema ab igne (EAI), which literally means "redness from fire" in Latin, is a disorder of hyperpigmentation caused by long-term exposure to heat. This exposure produces cutaneous hyperthermia, which in turn results in histopathologic changes similar to those seen in sun-damaged skin. Prolonged use of hot water bottles, heating pads, electric blankets, or sitting near a wood stove or fireplace can trigger the formation of this disorder. EAI has also reportedly been caused by sitting for long periods with a laptop computer on the lap.

Although burns do not occur from the heat exposure, the skin develops mottled, reticulated (net-like), pink, reddish, or violaceous patches that eventually become brown from melanin deposition. There may be pruritus or mild burning paresthesias. Rarely, vesicles or bullae have been described in affected areas. The resultant pigmentation changes can be permanent. Currently, there is no effective treatment for them. The disorder is more common in women. One retrospective study found an approximately 2:1 female predominance, with the anterior legs being the most common site affected.

Chronic exposure to infrared radiation in the form of heat may predispose to the development of malignancy, with squamous cell carcinoma (SCC), or rarely Merkel cell carcinoma, having been observed in isolated cases of EAI.