Man made radiation pose no less threat then the natural form of radiation. No body can ever forget the memories of Hiroshima and Nagasaki. Recently there was another nuclear accident, an major earthquake jolted the plant and causes Higher amount of radiation to be present in the environment. This is just one example of man made radiation hazard. There are lot other and more severe one. Even from the faulty apparatus of X-ray, diagnostician health can be in danger. These form of man made radiation are dangerous in nature as they emit highly ionizing ions as radiation which causes damages at cellular level to cause severe condition such as cancer. As the blog scrolls down more information can be retrieved about such hazard.
There are other sources of radiation: which include cosmic radiation and radiation from the crust of earth. Details on the source of radiaton can be found from the article.