# How linearly additive are the x-ray mass attenuation coefficients for molecules?

Having a look at https://www.nist.gov/pml/x-ray-mass-attenuation-coefficients, the introduction states:

1. For compounds and mixtures, values for $μ/ρ$ can be obtained by simple additivity, i.e., combining values for the elements according to their proportions by weight. To the extent that values for $μ_{en}/ρ$ are affected by the radiative losses (bremsstrahlung production, annihilation in flight, etc.) suffered during the course of slowing down in the medium by the electrons and positrons that have been set in motion, simple additivity is no longer adequate. The 1982 compilation ignored such matrix effects (they tend to be small at photon energies below 20 MeV)

So how valid is this for typical x-rays in the medical range, i.e. 40-150 keV?