Metallised nylon helium balloon resting on a ceiling. Helium is the first noble gas listed on the periodic table of elements. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements of Thorium and Uranium. Helium is the second lightest element after hydrogen making it ideal for filling balloons. There has been some environmental concern over metallised nylon balloons, as they do not biodegrade or shred as rubber balloons do. Release of these types of balloons into the atmosphere is considered harmful to the environment. This type of balloon can also conduct electricity on its surface and released foil balloons can become entangled in power lines and cause power outages