parts per billion

The "parts-per" notations are used to denote low concentrations of chemical elements. Also known as mixing ratios, they are often used to denote the relative abundance of trace elements in the Earth's crust, trace elements in forensics or other analyses, dissolved minerals in water, or pollutants in the environment.
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Unit commonly used to express contamination ratios, as in establishing the maximum permissible amount of a contaminant in water, land, or air.

Number of parts of a chemical found in one billion parts of a particular gas, liquid, or solid mixture. See concentration.