Question

when are parts per million and parts per billion used as measurements?
when measuring concentrations that exceed 10%
when measuring concentrations that are larger than 100%
when measuring concentrations that are smaller than 1%
when measuring concentrations that are exactly 50%

Explanation:

Parts per million (ppm) and parts per billion (ppb) are used for very dilute concentrations. Concentrations larger than 10% or 100% or exactly 50% are better expressed as percentages. Concentrations smaller than 1% (e.g., trace amounts of a substance in a solution) are appropriately measured with ppm or ppb as they represent very small proportions.

Answer:

when measuring concentrations that are smaller than 1% (the third option)