Part per billion (ppb) is a dimension‑less unit that expresses the concentration of one part of a substance in one billion parts of a total mixture, typically written as a mass‑to‑mass, volume‑to‑volume, or mass‑to‑volume ratio; its symbol is “ppb.” This metric is widely used to quantify trace levels of contaminants, pollutants, or additives in air, water, soil, and food, allowing scientists, regulators, and industry professionals to assess compliance with safety standards and environmental guidelines. Because ppb represents extremely low concentrations, it is essential for monitoring hazardous chemicals such as heavy metals, pesticides, and volatile organic compounds, ensuring public health protection and supporting precise research in fields ranging from environmental science to pharmaceuticals.