gamma rays
Gamma rays are the most dangerous type of radiation. They can travel farther and penetrate materials more deeply than can the charged particles emitted during alpha and beta decay. Gamma rays can be stopped only by several centimeters of lead or several meters of concrete. Its no surprise that they can penetrate and damage cells deep inside the body. You can learn more about the effects of gamma radiation on people at this URL: http://library. thinkquest. org/3471/radiation_effects_body. html .