Attenuation is measured in decibels (dB) on a logarithmic scale. 10 dB of electromagnetic shielding will reduce the energy of the incident wave by a factor of 10. 20 dB will reduce it by a factor of 100; 30 dB by 1000, and so on.
It is important to note that the shielding effectiveness of all materials will differ depending on the wavelength of the radiation being blocked. A measurement of shielding effectiveness is only useful if the wavelength at which that measurement was taken is known.
MG Chemicals conductive paints have the shielding effectiveness per wavelength graphed on their technical data sheet as well as the category data sheet.