The Kma is estimated on the basis of the saturated vapour pressure of the substance Ps, and the activity coefficient of the substance in the matrix γ.
The Kma follows from the relation between y0, the air concentration directly above the material surface and C0, the concentration of the substance in the material. This relation is assumed to be linear, the Kma being the coefficient of proportionality, i.e.:
y0 = C0/Kma
The Kma is related to the saturated vapour pressure ps of the compound as:
with ρ the density of the material.
The method is based on the research described in Eichler et al. 2018. 'Equilibrium Relationship between SVOCs in PVC Products and the Air in Contact with the Product', Environmental Science and Technology, 52: 2918-25.
The authors of this work found an empirical value for the activity coefficient of 5.12 for phthalates in PVC products, to relate C0 (given as a weight fraction) to the vapor pressure. This value is used as a default.