In EPA residue tolerances, which unit is not typically used to express residue levels?

• A. Parts per million
• B. Parts per billion
• C. Parts per trillion
• D. Parts per thousand

Answer: (D)

The key idea is that regulatory residue levels are expressed using scales that reflect very small amounts, so precise, tiny fractions are needed. In EPA residue tolerances, units like parts per million, parts per billion, and parts per trillion are used because they describe 10^-6, 10^-9, and 10^-12 fractions, respectively. Parts per thousand would describe 0.1% increments, which is far too coarse to capture the very small residue levels typically involved in regulatory safety limits. That coarser scale could miss important differences in safety margins, whereas ppm, ppb, and ppt provide the precision regulators rely on. For perspective, 1 ppm is one part in a million, 1 ppb is one in a billion, and 1 ppt is one in a trillion, illustrating how finely these tolerances are expressed.