Because at that level, it's a trace amount.

Copper sweetening introduces trace amount of copper into the resulting products, which tends to have detrimental effects as it leads to formation of gummy residues.

In inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopants, impurities called activators.

Metals such as lead and copper and zinc are found in a trace amount in all sediments.

Shallow electron traps are created by sulfur sensitization, introduction of a crystalline defect (edge dislocation), and incorporating a trace amount of non-silver salt as a dopant.