How many milligrams per kilogram (mg/kg) is equivalent to six parts per million?

To understand why the equivalent of six parts per million (ppm) is six milligrams per kilogram (mg/kg), it's important to recognize the relationship between these two units of measurement.

Parts per million is a way of expressing very dilute concentrations of substances. Specifically, one part per million means that there is one part of a substance in one million parts of the solution. In terms of mass, one ppm can be directly translated to milligrams per kilogram since one million parts can be considered the same as one million milligrams contained in one kilogram of the solution.

Thus, if you have six parts per million, it translates directly to six milligrams of the substance in one kilogram of the solution. This direct equivalence means that whenever you see a value in parts per million, you can interpret it directly as milligrams per kilogram, making six ppm equal to six mg/kg.

Given this understanding, the answer to the question is indeed six milligrams per kilogram.