We need to be more specific: Radon is a gas that goes through alpha decay. It is this specific combination that makes it dangerous. The gaseous form enables eas

We need to be more specific: Radon is a gas that goes through alpha decay. It is this specific combination that makes it dangerous. The gaseous form enables easy ingestion by breathing and alpha particles are extremely damaging at short ranges - sub-mm in solid matter. The dead outer skin layer is normally able to perfectly absorb alpha particles which make most emitters effectively harmless. Ingested alpha emitters are different as the emitted alpha particle enters living cells immediately. At that point it is a game of chance whether something gets destroyed or altered that is important for cell reproduction. Also, ingested radioactive emitters are impossible to remove from the human body.