What does the term PPM refer to when measuring substances?

The term PPM stands for "parts per million," which is a measure that describes the concentration of a substance in a solution or mixture. This metric is commonly used in various fields, such as chemistry, environmental science, and food safety, to indicate the amount of a substance present in one million parts of the total solution.

Using PPM allows for precise communication of very small concentrations, which is essential when dealing with pollutants, nutrients, or additives where even a minute quantity can have significant effects. For instance, in water quality testing, measuring contaminants in PPM helps determine whether the water meets safety standards.

The other terms, though they may sound plausible, do not represent a commonly recognized standard for measuring substances. This distinction highlights why parts per million is the correct and universally adopted terminology for indicating such concentrations.