Cosmic ray attenuation is dependent on the density of the material of the overburden, so the m.w.e. is defined as the product of depth and density (also known as an interaction depth). Because the density of water is 1 g/cm, 1 m (100 cm) of water gives an interaction depth of 1 hectogram per square centimetre (100 g/cm). Some publications use hg/cm instead of m.w.e., although the two units are equivalent.