If there was a nuclear explosion and years later I come out of a fallout shelter like in the popular game series Fallout, what elements would react to the radiation in a way that could harm humans and what happens to humans when they interact with these substances?
One of the best examples would be Bikini Atoll.
Today, the key residual fallout radionuclides of potential radiological concern include cesium-137 and strontium-90 and, to a lesser extent, plutonium isotopes and americium-241.
The most important pathway for human exposure to residual fallout contmaination is ingestion of cesium-137 contained in locally grown food crop products such as coconuts, Pandanus, and breadfruit
According to the CDC's Radioisotope Brief: Cesium-137
External exposure to large amounts of Cs-137 can cause burns, acute radiation sickness, and even death. Exposure to Cs-137 can increase the risk for cancer because of exposure to high-energy gamma radiation. Internal exposure to Cs-137, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, exposing these tissues to the beta particles and gamma radiation and increasing cancer risk.