Parts per million

Parts per million (ppm) is a measure of concentration that is used where low levels of concentration are significant. The ppm value is equivalent to the absolute fractional amount multiplied by one million (10⁶).

Illustrations of one part per million:

• one gram in a metric tonne (one ppm by mass)
• one millilitre in a cubic metre (one ppm by volume)

Examples of situations where parts per million are an appropriate measure include:

• relative abundances of rare elements[?] in the earth's crust[?]
• concentrations of pollutants in the environment

Similar, even smaller measures:

• Parts per billion[?] (ppb)
• Parts per trillion[?] (ppt)