With its 2 protons and 2 neutrons, alpha is a large particle with a positive charge. As it travels along it will, by attraction, remove tens of thousands of electrons. It will stop after passing through one of the following materials:

	the epidermis (the outer layer of the skin),
	a sheet of paper,
	1 mm of water,
	2 cm of air.

(These examples give a rough guide only because the thicknesses penetrated depend on the energy of the particles).