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Why can ionizing radiation burns take days or weeks to fully appear?

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Cross-section diagram of human skin showing burn depth by degree, with arrows indicating that first-degree burns affect only the epidermis, second-degree burns reach the dermis, third-degree burns extend into subcutaneous tissue, and fourth-degree burns penetrate to muscle. — burn This diagram shows how burn injuries are classified by severity. First-degree burns damage the outer skin, known as the epidermis. First-degree burns are the least severe form of burn injury. Second-degree burns damage the outer skin and some of the underlying skin, known as the dermis, which also includes some connective tissue. Third-degree burns damage all the layers of skin, including subcutaneous tissue. Fourth-degree burns, which are the most severe, destroy every layer of skin and also damage muscle, tendons, and bones.

Ionizing radiation burns do not always appear immediately because the injury is the result of complex molecular and biochemical changes rather than an instantaneous thermal effect. While initial interactions between radiation and tissue occur almost immediately, the expression of the resulting biological damage often requires time to manifest.1

For skin, the initial reaction is often a transitory reddening that may appear within hours of exposure. However, more severe and prolonged damage, such as deeper reddening, blistering, or ulceration, typically develops two to four weeks later. This delay occurs because radiation-induced injury is often linked to the disruption of cell division and the accumulation of irreparable damage to critical targets like DNA. Cells are most vulnerable to dying when they attempt to divide for the first time following irradiation. Consequently, the outward signs of tissue damage often only become apparent as affected cell populations fail to replicate or undergo programmed death over the days and weeks following the initial exposure.23