TDIs are applied to chemical contaminants in food and drinking water. The presence of contaminants is unwanted and they have no useful function, differing from additives and residues where there is or was deliberate use resulting in their presence. TDIs are calculated on the basis of laboratory toxicity data with the application of uncertainty factors. A TDI is therefore an estimate of the amount of a substance (contaminant) in food or drinking water that can be ingested daily over a lifetime without a significant health risk.