The radioactive dust that settles on the environment around us is an example of potential external exposure. Radionuclides that emit alpha and beta particles would pose a lower external exposure threat because they do not travel very far in the atmosphere and are not as penetrating as more energetic radiation. Shielding, one of the three principles of radiation protection, prevents some external exposure because alpha particles are blocked by the dead skin cells that sit on the surface of our bodies. Gamma rays, however, travel much farther in the atmosphere, and are higher energy rays that can only be blocked by heavy shielding, like a concrete wall or a lead apron. These rays pose a higher external exposure risk.