Actinides are a transition group between actinium and lawrencium (inclusive). The group includes actinium, thorium , protactinium , uranium , neptunium , plutonium , americium , curium , californium etc. The members of the group are called actinides after the first in the series and they are all radioactive. Of the group, only thorium, uranium, plutonium, americium and californium are significant as the others are only produced artificially and have a relatively short Half-Life. The radiologically significant members all have much longer half-life's (e.g. plutonium-239 is in excess of 24,000 years) and are all Alpha Emitters. This means they have the potential to cause a significant Internal Radiation hazard through Inhalation and Ingestion.