ABSTRACT
Parts per million (ppm) is also used frequently. It has to be used for recording in the same units (e.g. ppm by weight means one gram in a million grams). Ppm by volume is one litre in a million litres. Historically, ppm was used for concentrations in water (e.g. one gram in a million grams of water). Because one litre of water weighs a thousand grams, a concentration in ppm is numerically the same as mg/l: 5ppm is the same concentration in water as 5mg/l. This does not work in air as one litre of air weighs a lot less than one kg.
