How is parts per million (PPM) expressed?

Parts per million (PPM) is a unit of measurement that expresses the concentration of a substance in a given solution or environment. It conveys how much of one substance is present in a million parts of another substance, typically used in the context of solutions in water, air pollution levels, or various concentrations in environmental science.

When discussing PPM, it often refers to the number of units of a particular substance (like a pesticide or pollutant) per one million units of the total solution or mixture. This measurement is essential for understanding the appropriate levels of chemicals and ensuring they are within safe or effective limits, particularly in fields like aquatic pest control, where regulation and safety are critical.

The other options discuss unrelated concepts or measurements, making them incorrect in the context of how PPM is specifically defined and used.