When a substitution of a substance or a material is to be made, it is important to make a qualified assessment of the health properties of the substance or the material, including the relevant alternatives. A company addresses the Occupational Health Service because some workers suffer from headache and fatigue when bonding PVC materials. If the assessment of the substitution is based on a comparison of Threshold limit values (TLVs), the basis may be changed drastically when new TLVs are determined. Toxicology means the “science of toxic substances,” and toxicological data may be based on widely different studies and experiments. The toxicity assessment thus describes the damages to health which a product may inflict. Contrary to the previously mentioned tools, numerical values may seldom be compared directly with a toxicity assessment to decide if a substitution is successful or not.