However, whereas walking is a low-impact activity because one foot is always in contact with the ground, in jogging and in running, impact is introduced. This is a high-impact exercise that places strain on the body, notably the joints of the knee.
Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations, such as x-ray photographs. Radiation can damage cells and make them cancerous.
Radiation occurs when energy is emitted by a source and then travels through a medium, such as air, until it is absorbed by matter. Radiation can be described as two basic types, ionizing and non-ionizing radiation. Non-ionizing radiation. Ionizing radiation. 1  Natural background radiation. 2  Cosmic radiation. 3  Terrestrial radiation. 4  Inhalation. 5  Ingestion. 6  Artificial sources of radiation.