Definition - What does Parts Per Million mean?

Parts per million (ppm) is used to define concentrations of substances. It designates a ratio between the two substances. It means that there is x measurement of a specific substance for every million parts measured. This can be used to describe anything from the amount of a drug in the bloodstream to the amount of contaminants in the air. For example the Occupational Safety and Health Administration (OSHA) limits exposure to carbon monoxide in the workplace to 50 parts per million or 50/1,000,000. If air samples indicate a concentration above this permissible exposure limit (PEL), safety measures must be implemented.

Parts per million is most often used to express concentrations of mass or volume. However, the term may be used to describe the relationship between any two units of measure.

SureHire explains Parts Per Million

The definition of part in parts per million may vary based on what is being tested because it is a ratio in nature. However, it will be constant within that measuring situation. For example, parts per million (ppm) may be used in fluid testing to describe the ratio of a solute present in a solution. When considering mass, 1 ppm would be one milligram per kilogram. However, in chemistry formulations, a ppm designation assigned to a liquid is understood to mean 1 milligram of a solute present in 1 liter of a substance. In comparison, the Department of Transportation's cutoff concentrations for drug tests are expressed in nanograms (one billionth of a gram) per milliliter.

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