Radiation can be cosmic or terrestrial. Cosmic radiation is from the sun and outer space. Terrestrial radiation comes from minerals within the earth. These are both sources of natural radiation. About 13% of radiation is man made. Medical uses such as X-rays and MRIs produce the vast majority of the man made radiation - about 12% of the total radiation on earth. Nuclear industries create less than 1% of earth's total radiation. Man made radiation is more harmful because it is more concentrated. Natural radiation is more spread out, and less concentrated, and is therefore less harmful.