The primary health effect of acute radiation syndrome (ARS) is bone marrow suppression because ionizing radiation primarily targets rapidly dividing cells. The hematopoietic system, particularly the bone marrow, contains highly proliferative precursor cells essential for producing blood components. Radiation causes destruction of these stem cells, leading to pancytopenia, increased risk of infection, hemorrhage, and anemia. This effect manifests early in ARS and is critical in patient prognosis and management, requiring prompt supportive care. Other organs are affected at higher doses or later stages, but bone marrow suppression is the hallmark of ARS.

Reference: Harrison's Principles of Internal Medicine, Kasper et al., 20th Edition.