To reduce the dose of ionizing radiation absorbed from electromagnetic radiation, it is important to minimize the time exposed, be as far as possible from the source and take shelter behind an adequate shield. These three factors of time, distance and shielding can be applied individually or in combination to minimize exposure.
Radiation is emitted at a given amount per unit of time. This is often expressed as the number of atomic disintegrations per second, or the Curie (Ci). One Ci is equal to 37 billion disintegrations per second, which produces a steady stream of radiation from the source. Limiting the amount of exposure time reduces the number of Curies that take place in close proximity, which reduces the overall dose received.
Distance is another important factor in reducing radiation exposure. Radiation is emitted in a three-dimensional shell around the source, and its intensity rapidly drops as distance from the source increases. The minimum safe distance varies with the intensity and type of radiation, but as a general rule, the farther the source, the less the risk.
Shielding is the third factor in limiting exposure to harmful radiation. Some types of radiation, such as alpha particles, can easily be blocked by thin materials such as paper. Longer wavelength radiation, such as gamma rays, can pass through many feet of concrete before stopping. In general, the denser the material, the better shield it makes. This is why lead is often used as a shielding material.