Definition - What does Alpha Particle mean?
An Alpha particle is a positively charged radiation particle that contains two neutrons and two protons that are bound together into a particle which is similar to a helium nucleus. Alpha Particles are usually produced during the process called alpha decay, but can be produced in other ways too. They have net spin of zero. They generally have a kinetic energy of around 5MeV and velocity in vicinity of 5% of the speed of light because of the mechanism of production in the standard radioactive decay. They are highly ionizing and have a low penetration depth.
Petropedia explains Alpha Particle
Alpha Particles are very dangerous than even their decay rate or half-life could suggest if their emitted isotopes are ingested. They are harmful because of the high rate of relative biological effectiveness of alpha radiation that causes biological damage after the alpha emitting radioisotopes enter the living cells.
The ingested alpha emitting radioisotopes are about 20 times more dangerous and sometimes 1000 times more harmful than the similar activity of gamma or beta emitting radioisotopes. These are not dangerous if its source is external to the body so they do not penetrate deep enough to reach the living tissue. But if the source is internal, then ionization can damage the living tissue.