The EPA tolerance allows test results to be measured against the allowable level of residue in which units?

• A. Parts per million, billion or trillion (ppm, ppb, ppt)
• B. Percent by weight
• C. Moles per liter
• D. Pounds per square inch

Answer: (A)

Regulatory limits for pesticide residues are given in very small mass fractions, so the EPA uses units like parts per million, parts per billion, or parts per trillion. These units express the amount of residue relative to the total sample, making it easy to compare tiny amounts across different foods and contexts. For practical sense, 1 ppm is about 1 milligram of residue per kilogram of food (roughly 1 mg/kg), 1 ppb is about 1 microgram per kilogram, and 1 ppt is about 1 nanogram per kilogram. Percent by weight is a much coarser measure, and moles per liter depends on the substance’s molar mass and the sample’s volume, while pounds per square inch is a pressure unit with no relevance to residue amounts. Therefore, ppm, ppb, or ppt are the appropriate units for EPA tolerance measurements.