What is the first clinical sign of excessive exposure to radiation?

The first clinical sign of excessive exposure to radiation is erythema. Erythema is characterized by redness of the skin, which occurs due to increased blood flow as a response to injury or damage, such as that caused by radiation. This initial reaction is indicative of cellular damage occurring in the skin tissues, and it can result from the effects of ionizing radiation on the dermal layer.

In the context of radiation exposure, erythema serves as an early warning sign that indicates the skin is being affected before more severe symptoms manifest. Following this initial sign, more severe health effects can become evident as exposure increases or persists. Other symptoms, such as hair loss, fatigue, and nausea, typically occur at later stages of radiation exposure. Hair loss is often associated with significant doses of radiation affecting rapidly dividing cells like those in hair follicles, while fatigue and nausea represent systemic effects that develop due to more serious biological impacts or damage to internal organs and systems.