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Question:
Almost all trace elements, even beneficial ones, can be poisonous if present in large enough concentrations.  What about trace gases? Do we have to worry about any direct deleterious effects of atmospheric CO2 as its concentration continues to climb?

Answer:
Very high concentrations of atmospheric CO2 can produce a state of hypercapnia (Nahas et al., 1968; Brackett et al., 1969; van Ypersele de Strihou, 1974), or an excessive amount of CO2 in the blood, which typically results in acidosis (Poyart and Nahas, 1968; Turino et al., 1974), a serious and sometimes fatal condition characterized in humans by headache, nausea and visual disturbances.  Several studies of these phenomena, however, indicate that they do not seriously impact human health until the CO2 content of the air reaches approximately 15,000 ppm (Luft et al., 1974; Schaefer, 1982), which is more than 40 times greater than the current concentration of atmospheric CO2.  So, no, we do not have to worry about any direct adverse health consequences of the ongoing rise in the air's CO2 content.

References
Nahas, G., Poyart, C. and Triner, L.  1968.  Acid base equilibrium changes and metabolic alterations.  Annals of the New York Academy of Science.  150: 562-576.

Brackett, N.C., Jr., Wingo, C.F., Muren, O. and Solano, J.T.  1969.  Acid-base response to chronic hypercapnia in man.  New England Journal of Medicine  280: 124-130.

Van Ypersele de Strihou, C.  1974.  Acid-base equilibrium in chronic hypercapnia.  In: Carbon Dioxide and Metabolic Regulations.  G. Nahas and K.E. Schaefer (Eds.).  Springer-Verlag, New York, NY, pp. 266

Poyart, C.F. and Nahas, G.  1968.  inhibition of activated lipolysis by acidosis.  Molecular Pharmacol4: 389-401.

Turino, G.M., Goldring, R.M. and Heinemann, H.O. 1974.  The extracellular bicarbonate concentration and the regulation of ventilation in chronic hypercapnia in man.  In: Carbon Dioxide and Metabolic Regulations.  G. Nahas and K.E. Schaefer (Eds.).  Springer-Verlag, New York, NY, pp. 273 -28 1.

Luft, U.C., Finkelstein, S. and Elliot, J.C.  1974.  Respiratory gas exchange, acid-base balance, and electrolytes during and after maximal work breathing 15 mm Hg PICO2.  In: Carbon Dioxide and Metabolic Regulations.  G. Nahas and K.E. Schaefer (Eds.).  Springer-Veriag, New York, NY, pp. 273-281.

Schaefer, K.E.  1982.  Effects of increased ambient CO2 levels on human and animal health.  Experientia 38: 1163-1168.