Prior to the Industrial Revolution of the late 19th and early 20th centuries, the carbon dioxide level was about 280 ppm.
Today global atmospheric carbon dioxide concentrations are at 395 parts per million (ppm).
Atmospheric carbon-dioxide levels have risen 38% in the industrial era, from 280 to 385 parts per million (ppm).
abbreviation of parts per million, attested by 1913.
parts per million