Why can early fallout be an immediate biological hazard?

Early fallout is considered an immediate and extreme biological hazard because it consists of intense, concentrated fission products that settle on the ground within the first 24 hours following a surface or near-surface nuclear explosion.1

The primary reasons for this hazard include:

• High Intensity: The total radioactivity of these fission products is extremely high immediately following the detonation, delivering radiation doses so intense that survival may depend on remaining in shielded underground shelters for weeks.21
• Ionizing Radiation: As these materials decay, they emit ionizing radiation—such as gamma rays and high-energy particles—which causes molecular damage to living tissue. This can lead to acute radiation syndrome, characterized by severe symptoms like nausea, vomiting, internal hemorrhaging, and damage to rapidly proliferating cells such as those in the bone marrow and gastrointestinal tract.34
• Internal Exposure: If radioactive particles are inhaled or ingested, they can accumulate in critical organs, where they deliver continuous, localized radiation doses that increase the risk of long-term injury and cancer.24