Laser penetration into skin

In the range of visible light and near infrared light (400 to 1200 nm), lasers that emit long wavelengths of light penetrate more deeply into the skin than lasers that emit shorter wavelengths of light.
Mid-infrared (eg, Er:YAG) and far-infrared (eg, CO₂) lasers do not follow this rule. Heavy absorption by water in the skin greatly limits the penetration of these lasers.