Erythema ab igne (Latin for 'redness from fire') EAI, also known as hot water bottle rash,[2] is a skin condition caused by long-term exposure to heat (infrared radiation).

[3] Prolonged thermal radiation exposure to the skin can lead to the development of reticulated erythema, hyperpigmentation, scaling, and telangiectasias in the affected area.

It has been proposed that thermal radiation exposure can induce damage to superficial blood vessels that subsequently leads to epidermal vascular dilation.

[4] Red blood cell extravasation and deposition of hemosiderin that follows clinically appear as hyperpigmentation, which can occur in a reticular distribution.

It has also been proposed that the distribution of affected blood vessels—predominantly in the superficial subcutaneous plexus (found in the papillary dermis)—results in the net-like pattern of erythema ab igne skin lesions.