2.1.3 ppm, ppb, etc
ppm stands for "parts per million" and is thus a unit of concentration. As such, it is dimensionless, just like "percent," %.

1 ppm =
1 mg of a substance (dry) per 1 liter of liquid
1 ml of a liquid per 1 million ml (or 1000 liters)
1 flamingo among 1 million flamingos
1 ounce of vermouth in 1 million ounces of gin

1% corresponds to 1 part per 100 parts, or 10,000 ppm.

graphic
ppb stands for "parts per billion." Everything stated above applies analogously (multiplied by 1,000).
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