Effect of low-concentration CO2 on stereoacuity and energy expenditure

Low-concentration CO2 commonly exists in our living environment in such locations as airplanes and cars. Its effect on visual performance and energy metabolism (through O2 uptake) has not been well studied, although several studies have shown that high-level CO2 (6-10%) significantly impaired visual performance.

We felt that breathing CO2 might also interfere with the energy expenditure pathway, partly due to the reduced O2 fraction. Thus, we studied the effect of low-concentration CO2 (2.5%) on human visual stereoacuity and energy expenditure.

Computer-controlled Random Dot Stereograms and psychometric function were used for stereoacuity tests. A whole-room indirect calorimeter allowed for the accurate measurement of energy expenditure, and also served as a good testing environment.

Of 10 subjects tested, 8 had a slight increase in resting energy expenditure after exposure to 2.5% CO2. The increase ranged from 0.4-6.1% (p = 0.087, paired t-test). In stereoacuity tests, the psychometric function curves of all three subjects tested shifted to the right when breathing 2.5% CO2 and returned to baseline 2 h after breathing fresh air. Stereoacuity value, which is the reciprocal of stereoscopic threshold, was significantly lower (p < 0.01, 2-tailed z score test) after CO2 exposure for all subjects tested.

We conclude that CO2 levels as low as 2.5% can cause temporary reduction in human stereoacuity, and also cause a persistent but small increase in energy expenditure.