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The levels of CO2 in the atmosphere, even at the projected elevated levels (even in the worst case scenario people have projected) still amounts to such minute amounts that it is not likely to have any direct biological impact upon aerobic respiration.There has been some speculation as to whether it will increase the efficiency of photosynthesis (since more CO2 will mean more carbon available for photosynthesis to process), but so far the evidence for this is quite unclear (maybe because the increase in CO2 we see is the leftover after photosynthesis has done its work, and is not actually a measure of the amount of CO2 actually available to drive the photosynthetic processes).
The concentration of carbon dioxide must be over about 2% (20 000 ppm) before most people are aware of its presence unless the odor of an associated material (auto exhaust or fermenting yeast, for instance) is present at lower concentrations. Above 2%, carbon dioxide may cause a feeling of heaviness in the chest and/or more frequent and deeper respirations. If exposure continues at that level for several hours, minimal "acidosis" (an acid condition of the blood) may occur but more frequently is absent.