As of 2015, all modern airships, party balloons and many weather balloons are inflated using helium. Helium is a lighter-than-air gas that is relatively inert, especially in comparison to hydrogen, another gas that is sometimes used for inflating lighter-than-air balloons or craft.
Helium is the second lightest element in the universe, after hydrogen. It is so light that it can float in mid-air. Both hydrogen and helium have been used for inflating balloons for entertainment, transportation and research; helium is the element of choice for these applications, due to safety reasons.
Prior to the 1930s, hydrogen was commonly used in airships, but after several infamous disasters, helium replaced hydrogen as the element of choice. American military airships switched to helium in the wake of a fire aboard the Army airship Roma in 1922. By the late 1930s, all civilian airships had followed suit.
Helium is also frequently used in weather balloons. Some weather balloons continue to use hydrogen rather than helium, due to cost, but safety concerns still make helium-inflated weather balloons commonplace.
The ubiquity of helium in children's party balloons has raised some controversy, as helium is a very rare element and is not a renewable resource. British scientists have spoken out against the practice.