In considering the changes in the atmospheric concentration of carbon dioxide wrought by man's activities, it has been the practice to refer to some presumed value of concentration in the late 19th century as the “pre-industrial” value. Implicit in many of these discussions has been the assumption that prior to the significant use of fossil fuels, the concentration was more or less constant about this value and that fossil fuels were the main reason for the recent departures from it. A value of about 290 parts per million by volume (ppmv) was usually selected as representative of the 19th century concentration. This value arose from a study by Callendar , who examined a number of direct chemical measurements made then. If it is assumed that the fraction of fossil fuel produced CO2 remaining in the air was the same from 1860 to 1958 as it evidently has been since 1958, when systematic measurements began at Mauna Loa Observatory, one calculates a value of about 295 ppmv for the late 19th century.