The range of alpha particles in air depends on their energy and the density of the air. For alpha particles with an energy of about 5 MeV, the range in air at room temperature and pressure is about 4 cm. In other materials, the range is shorter. For example, the range in water is about 2 cm and the range in lead is about 0.05 cm.

The range of alpha particles is also affected by the presence of other particles in the air. For example, the range is shorter in humid air than in dry air. This is because water vapor molecules absorb alpha particles.