An alpha particle is a fast traveling package comprising a helium nucleus with two protons and two neutrons. Particles with alpha are charged +2 and interact intensely with matter. Alpha particles formed in alpha decay can only pass a few inches in the air and can be stopped quickly with a sheet of paper.
Like helium nuclei, alpha particles have a net spin of zero. Because of their processing processes, alpha particles typically have about 5 MeV kinetic energy and a speed nearby at about 5 percent light rate due to their alpha-radioactive decay mechanism. They are strongly ionizing sources of particulate radiation that have poor penetration density (as a consequence of radioactive alpha decay). A few centimeters of air or the skin will hinder them.
When isotopes with alpha particles are ingested, they are far more dangerous than their half-life or decay rate indicates, since alpha radiation has a high relative biological effectiveness which causes biological harm. The typical alpha radiation is about 20 times more harmful and up to 1000 times more harmful than the comparable beta or gamma emission operation of radioisotopes, in studies of inhaled alpha emitters.