The lowest value at which the atmospheric concentration of carbon dioxide could be stabilized by reduction of additions made by human activity (fossil fuel-burning, etc.) is estimated as 550 ppm. To achieve this, severe limitations are required on the latter activities. The most often quoted desirable/attainable stable concentration is 750 ppm. This concentration level is not related in any way to health considerations and is above the estimated dangerous level of 426 ppm. The value is also above the 600 ppm level, which results in the ‘stuffy room’ conditions described above. At the very least, 600 ppm of carbon dioxide in the atmosphere will be unpleasant and there will be no readily available means of reversing the changes giving rise to the above symptoms. Such a situation is unlikely to be tolerable for a lifetime by humans (and other mammals with the possible exception of seals) without deterioration in general health along with serious curtailing of physical activity presently taken as normal.
It is likely that when the concentration of carbon dioxide in the atmosphere reaches 426 ppm in less than two generations from the present date(2005), the health of at least some sections of the world population will deteriorate, including those of the developed nations. It is also obvious that if the extremes of conditions described above come to pass, then the biosphere and humankind are seriously threatened.http://www.alfaintek.com/assets/files/D_S_Robertson.pdf