parts per million
/ parts per MIL-yun /
One drop of food colouring in a large bathtub of water is roughly one part per million. The unit is built for the very dilute — traces so small that a percentage would be an awkward string of zeros.
Parts per million (ppm) is a ratio: one part of analyte for every million parts of total sample, by mass or by volume. For dilute water samples, where one litre weighs almost exactly one kilogram, 1 ppm conveniently equals one milligram of solute per litre.
ppm dominates environmental and clinical reporting because pollutant and contaminant limits live at this scale. Its weakness is hidden ambiguity: ppm can mean mass-per-mass, mass-per-volume, or volume-per-volume, so the basis must be stated or the number can be misread.
A drinking-water limit of 10 ppm nitrate means at most 10 milligrams of nitrate in every litre of water — a tiny amount, but enough to matter for infants' health.
For dilute water, 1 ppm ≈ 1 mg per litre.
Going further down, ppb (parts per billion) is a thousand times smaller still. Always confirm the basis (w/w, w/v, v/v); switching it can change the meaning by orders of magnitude.