They are intensely ionising but can be readily stopped by a few centimetres of air, a sheet of paper, or the human skin. They are only dangerous to people if they are released inside the body. Alpha-radioactive substances are safe if kept in any containers sealed to air. They can be stopped by a thin piece of wood or plastic and are generally less dangerous to people than gamma radiation.
Exposure produces an effect like sunburn, but which is slower to heal.
Beta-radioactive substances are also safe if kept in appropriate sealed containers. They are very penetrating, and need substantial thicknesses of heavy materials such as lead, steel or concrete to shield them. They are the main hazard to people in dealing with sealed radioactive materials. Doses can be detected by the small badges worn by workers handling any radioactive materials. These are mostly released by nuclear fission, and apart from a little cosmic radiation they are seldom encountered outside the core of a nuclear reactor.