Alpha particles are created when heavy and unstable atoms try to become more stable. Elements like uranium and radium have too many protons and neutrons inside their nuclei. To fix this unstable overcrowding, the atom violently spits out an alpha particle.
This specific type of radioactive breakdown is officially called alpha decay. Because an alpha particle has two protons, it carries a positive two electric charge. It also has a mass of four atomic mass units, which is quite heavy.
This heavy weight and double charge cause alpha particles to crash into things easily. When they hit normal atoms, they steal electrons and create newly charged ions. These constant crashes slow the alpha particle down and stop it very quickly.
In normal air, an alpha particle only travels a few centimeters before stopping entirely. You can easily block them with a simple sheet of paper or your clothing. Even the dead outer layer of your skin is enough to stop them completely.
This means alpha particles are fairly harmless when the radiation source remains outside your body. However, they become extremely dangerous if you accidentally inhale or swallow radioactive dust. Once inside the body, they crash directly into living cells and soft tissues. These internal crashes can severely damage human DNA and lead to serious health problems.