Radiation is energy in the form of high speed particles or electromagnetic waves.
Ionizing radiation has enough energy to change normal cellular functioning.
Ionizing radiation may cause cells to die or transform into a cancerous cell. They can travel a few inches through the air but can be stopped by something as thin as a sheet of paper. This means that cells can be protected or shielded from damage by alpha particles by clothing. Even your skin will protect you from damage from alpha particles. However, if alpha particles are inhaled or ingested or get into a cut on the skin, they can cause damage to cells.