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Douglas Harper's Etymology Dictionary
ppm

abbreviation of parts per million, attested by 1913.

Wiktionary
ppm

init. 1 (qualifier: common) Parts per million. (10000 ppm = 1%) 2 Pulse Position Modulation 3 Process Production Model 4 PowerPerformance Management 5 pages per minutes 6 Prediction by Partial Match

Wikipedia
PPM

PPM is an initialism that may refer to:

Usage examples of "ppm".

There's no dispute that it has risen from the region of 280 parts per million (ppm) at the end of the nineteenth century to 350 ppm by the close of the twentieth, an increase of 25 percent, attributed mainly to the burning of fossil fuels.

The concentration 100 million years ago is estimated to have been 3,000–5,000 ppm against today's paltry 350.

Iklind wasn't breathing at all, and no wonder—the hydrogen sulfide concentration in here had peaked at over 1,000 ppm, according to the monitor above the open access hatch.

Now the monitor indicated the concentration was below 200 ppm, still dangerous but not instantly lethal.

Iklind wasn't breathing at all, and no wonder—the hydrogen sulfide concentration in here had peaked at over 1,000 ppm, according to the monitor above the open access hatch.

The text predicts carbon dioxide levels of 370 ppm in the year 2000 and a surface-temperature increase of .