Radioactive decay produces three main types of radiation, each with different properties:

• Alpha particles — two protons and two neutrons ejected together. Heavy and slow, stopped by a sheet of paper. Dangerous only if inhaled or swallowed.
• Beta particles — high-speed electrons or positrons. They get through skin but are stopped by aluminum foil. Used in medical tracers.
• Gamma rays — pure electromagnetic energy with no mass. They pass through most materials and need thick lead or concrete to block them. Used to sterilize medical equipment.

The key point: radiation danger depends on penetrating power and on whether the source sits inside or outside your body.