A very short trajectory !
Because of their great ability to ionise, the trajectory of alpha particles through matter is very short. In a dense medium such as water, even those energetic alpha particles emitted by polonium 212 can barely travel 90 microns - less than a tenth of a millimetre. A sheet of paper or the thickness of an item of clothing is more than enough to render them harmless. The inset shows that inside a ‘cloud chamber’ saturated with water vapor, the range of alpha particles does not exceed a few centimetres. It is very easy to protect against alpha when the source is external.