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Almost all trace elements and compounds, even beneficial ones, can be poisonous if ingested or inhaled in large enough concentrations.  So what about carbon dioxide?  Do we have to worry about any deleterious health effects as its atmospheric concentration continues to climb?

Inhaling very high concentrations of atmospheric CO2 can induce a state of hypercapnia in people.  Characterized by an excessive amount of CO2 in the blood, which typically results in acidosis, this condition is accompanied by headache, nausea, visual disturbances, and is sometimes fatal.  Several studies have demonstrated, however, that these problems do not seriously impact human health until the air's CO2 concentration reaches approximately 15,000 ppm (Luft et al., 1974; Schaefer, 1982), which is approximately 40 times greater than its current concentration. 

Clearly, therefore, we do not have to worry about there being any direct adverse health effects associated with the ongoing rise in the air's CO2 content, even if it were to increase by a factor of ten, which is probably all that could be achieved by burning the entire supply of fossil fuels in the crust of the Earth.  In fact, the current CO2 concentration of the air in many homes and buildings is often two to three times greater than the CO2 concentration of outdoor air (Idso, 1997), which in large cities is itself often elevated by several tens of percent above the CO2 concentration of rural air (Idso et al. 1998, 2002).

References
Idso, C.D., Idso, S.B. and Balling Jr., R.C. 1998. The urban CO2 dome of Phoenix, Arizona. Physical Geography 19: 95-108.

Idso, S.B. 1997. The poor man's biosphere, including simple techniques for conducting CO2 enrichment and depletion experiments on aquatic and terrestrial plants. Environmental and Experimental Botany 38: 15-38.

Idso, S.B., Idso, C.D. and Balling Jr., R.C. 2002. Seasonal and diurnal variations of near-surface atmospheric CO2 concentrations within a residential sector of the urban CO2 dome of Phoenix, AZ, USA. Atmospheric Environment 36: 1655-1660.

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: In: Carbon Dioxide and Metabolic Regulations. G. Nahas and K.E. Schaefer (Eds.). Springer-Veriag, New York, NY, pp. 282-293.

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



** For additional peer-reviewed scientific references and an in-depth discussion of the science supporting our position, please visit Climate Change Reconsidered: The Report of the Nongovernmental Planel on Climate Change (www.climatechangereconsidered.org), or CO2 Science (www.co2science.org).