IAQ | IMPACT ON HEALTH AND PRODUCTIVITY and increased CO2 did not modify responses attributable to increased temperature. The fourth study18 found CO2 levels as high as 15,000ppm did not affect performance in the SMS test. Overall, among the ten studies, three 13, 16, 17 provide strong evidence of reductions in cognitive performance with increased levels of CO2. Two additional studies 12, 20 provide limited evidence of cognitive performance decreases with increased CO2 levels, but also include evidence of CO2 not affecting performance. The results of one additional study, 22 suggest a possible effect of increased CO2.The mechanism by which increased CO2 concentrations may affect cognitive performance was not clearly identified in any of the studies. A possible explanation is provided by another study,22 albeit one that increased CO2 levels by restricting ventilation rate per person, so concentrations of other bioeffluents increased when the CO2 levels were higher. In this study, as subjects were exposed to levels of CO2 increasing from approximately 400ppm to 3,000ppm, their forced expiratory volume in one second and forced vital capacity decreased. Using models and previously published data, the authors predicted that with exposure to higher levels of CO2 there would be increases in arterial CO2 partial pressure and corresponding increases in the bicarbonate content of the blood, with a reduction in blood pH. The increased blood bicarbonate and reduction in blood pH was suggested as the explanation for a change in brain functioning when occupants are exposed to higher levels of CO2. Two papers15,19 hypothesised that the level of stress associated with the cognitive performance test might explain the discrepancies among findings. Higher CO2 levels were associated with diminished performance primarily from studies with very demanding, likely stressful, tests of performance. In support of their hypothesis, they found a tendency for subjects to have higher salivary -amylase concentrations, suggesting higher mental stress, when CO2 concentrations were increased. Further support for this hypothesis comes from findings that pilots performance in flight simulations was reduced when their heart rate variability indicated a high level of stress.21 Also, two studies12,15 report some increases in blood pressure with exposure to higher CO2 levels, suggesting higher levels of stress. The authors of two papers hypothesized that the discrepancies among research findings when subjects took stressful cognitive performance tests was a consequence of the different types of subjects. In one paper,20 the authors suggested that the astronaut-like operations personnel and submariners might have been better able to compensate for effects of elevated CO2 due to their prior training. The authors of the other paper18 hypothesized that their subjects (submariners) might have been unaffected by CO2 as a consequence of their prior regular occupational exposure to CO2 at 2,500 ppm or higher concentrations. Five studies12, 14, 18, 19, 23 investigated whether subjects perceptions of IAQ acceptability of indoor air was influenced by CO2 concentrations. One study12 found subjects reported air quality as less acceptable with 3,000ppm, 4,000ppm, and 5,000ppm CO2 relative to 600ppm. Six studies, reported in eight papers,12, 14, 15, 18, 19, 21-23 investigated whether the level of CO2 influenced health symptoms reported on questionnaires or health-related physiological outcomes, such as blood pressure, pulse, respiration rate, markers of stress, and exhaled concentrations of CO2. Four studies that included questionnaires on acute health symptoms,14, 19, 22,23 including fatigue, found that CO2 levels had no SS effect on symptoms. One study12 reported that subjects were significantly more tired with 5,000ppm CO2 relative to 600ppm CO2. This study12 also found that blood pressure, respiration rate and volume, and mental effort based on heart-period variability increased with higher CO2 concentrations. In contrast, other studies15, 19,23 generally found no statistically significant effects of CO2 With respect to acute health symptoms and perceived air quality, all but one study found no effects at CO2 below 5,000ppm levels on a broad range of physiological outcomes, except for increases in the concentrations of CO2 in exhaled air, called end-tidal CO2, and two instances of increases in heart rate. In one study,15 heart rate decreased less during the exposure session with 3,000ppm CO2 versus 500 ppm CO2 while another study22 reported a SS increase in heart rate with exposure to 2,680 ppm CO2 relative to 700 ppm CO2. Another study19 found that levels of -amylase, markers of mental stress, were higher with 3,000ppm CO2 compared with 380ppm CO2. Other research26 has shown that exposure of mice to 2,000 and 4.000 ppm CO2 for two hours triggers an inflammatory response and vascular injury with generation of microparticles by immune system cells. Also, in human immune system cells, microparticle generation resulted from increased CO2 exposures.27 The main findings of the research are listed below: There is very limited evidence that CO2 levels below 5,000ppm influence perceived air quality, acute health symptoms, or physiological outcomes other than endtidal CO2 and heart rate. The studies using mice and human immune cells (in vitro) indicate that higher CO2 levels trigger inflammatory responses, but these findings have not yet been demonstrated in people With respect to acute health symptoms and perceived air quality, the study results with one exception12 are consistent and find no effects at CO2 below 5,000ppm The results of research on the effects of moderate CO2 levels on human cognitive performance are not consistent. Some studies find effects of higher CO2 concentrations on cognitive performance, while other studies find no effects on this outcome There is substantial, but still inconsistent, evidence that performance on challenging tests of decision-making and challenging flight simulations is worsened by CO2 concentrations as low as 1,000ppm. The mechanisms underlying the reductions in performance are unknown. Further research is needed to address the discrepancies among the current findings. Additionally, research to date has not investigated the effects of CO2 on children, the elderly, and people with health problems. The effects of long-term, continuous or periodic exposures to elevated CO2 levels has also not been investigated. Finally, the extent to which CO2 mediates the influence of other factors on health or performance requires more research. CJ This article also appears in the September ASHRAE Journal. W ILLIAM FISK, Lawrence Berkeley National Laboratory, Berkeley, CA, USA; PAWEL WARGOCKI, Technical University of Denmark, Lyngby; XIAOJING ZHANG, Beijing University of Technology, China 68 September 2019 www.cibsejournal.com CIBSE Sep19 pp66-69 CO2 Research v2.indd 68 23/08/2019 18:17