The half life or half life period is defined as the time required for the radio-activity of an isotope to reduce to half of its original value. Radioactive material is dangerous because of its half life periods. Half life for radio active materials such as uranium, plutonium lasts for thousands of years.When the radio activity is exposed to the environment, the element will exist in the environment for thousands of years before it decays results in making the environment ( land, water, habitation ) radioactive for thousands of decades.