Radiation exposure by natural sources is the largest source of radiation to humans. Man-made radiation and other sources such as x-rays and nuclear medicine, pale in comparison.
Determining which type of radiation is most harmful is a variable process. Alpha particles cannot penetrate a piece of ordinary paper, yet beta particles are stopped by a thin sheet of aluminum, and it takes at least an inch of lead to stop gamma rays. Although alpha and beta particles cannot even penetrate through a person’s skin, gamma rays can get in to the body through skin and cause damage. Therefore, with regards to external radiation, gamma rays are most harmful.
However, to every rule there is at least one exception and alpha particles are the exception with regards to internal radiation. An isotope that is an alpha emitter doesn’t have to penetrate the skin to enter the body, it can enter through ingestion or inhalation. Some alpha emitters, when inhaled, can pass into the blood and irradiate the whole body. In these situations, the alphas are most harmful due to the particles’ rapid rate of deceleration causing the particles to deposit all their energy in a short distance and concentrating their effect in a localized area. It is because of this effect that smokers receive a much higher annual dose of radiation than non-smokers. Radioactive polonium (from the radon decay chain) gets deposited from the air onto tobacco leaves when they are dried. When the smoker inhales the smoke, they are also inhaling polonium and other isotopes in the decay chain, all of which are alpha and beta emitters. This radiation is harmful once it gets into the lungs.