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.
Ionizing radiation is invisible and not directly detectable by human senses, so radiation detection instruments such as Geiger counters are required. However, ionizing radiation may lead to secondary emission of visible light upon interaction with matter, such as in Cherenkov radiation and radioluminescence.osmic rays and the decay of radioactive isotopes are the primary sources of natural ionizing radiation on Earth referred to as background radiation. In space, natural thermal radiation emissions from matter at extremely high temperatures (e.g. plasma discharge or the corona of the Sun) may be ionizing.