A principal aim of this study was to determine if the adverse effect of low concentrations of CO 2 on the decision-making abilities of predominantly young college-age adults4 could be replicated in older astronaut-like individuals. Clearly, the dose-dependent, monotonic, reciprocal relationship between CO 2 concentration and performance on the SMS that was demonstrated in earlier studies4,5 was not replicated in this study, which included concentrations within the ranges used in those earlier studies (Fig. 1 and 2). Interestingly, the response from baseline to 1200 ppm, for most measures, exhibited a decrease in performance that was comparable to those observed in other studies (Fig. 3). However, this trend did not hold in this study population at higher concentrations.

Our findings at 2500 and 5000 ppm diverge from those anticipated by the findings of earlier studies that demonstrated substantial effects of CO 2 upon performance on the SMS at lower concentrations4,5 but the absence of an effect at 2500 ppm replicates the finding of Rodeheffer11 at that concentration. On the other hand we detect effects at 1200 ppm, as have other studies.4,5 Therefore, our findings, in part, both comport with and diverge from the finding of others.4,5,11 Several factors, discussed below, may contribute to our unusual and unexpected findings.

One potential variation among the studies that could affect differences in performance is the amount of sleep obtained by subjects preceding their exposures to CO 2 . Although the amount of sleep during the night preceding each of the exposures did not differ significantly among the targeted CO 2 concentrations, the amount of sleep by an individual was a significant covariate for the variable Initiative (p = 0.0332). The sleep status of the subjects was not reported in the studies of Satish.4 In a study26 in which the SMS was utilized it was observed that an improvement of 25% in sleep score was associated with a 2.8% increase in cognitive function scores. If decrements follow a reciprocal relationship to that shown for improvements then, because our subjects averaged only 6.3 h of sleep during the nights preceding their exposures (the nightly average of the general population is 6.8 h27), the difference between the large percent decrease in cognitive scores seen in the study of Satish,4 and the absence of similar effects in this study at the same concentrations of CO 2 , could be expected to be attributable to differences in the sleep status of the subjects of the two studies only if sleep scores among Satish’s4 subjects were well below those of this study.

Differences in characteristics of various subject populations may account for diverging outcomes in studies assessing effects of CO 2 upon decision-making. It may be that astronaut-like operations personnel and submariners, who are high-level performers, are more likely to have heightened situational awareness because of their stringent training. Therefore, these groups may develop faster adaptive patterns of responses and be more perceptive of their cognitive decline, and therefore may compensate more efficiently for self-perceived drops in performance than subjects drawn from the general population. Such distinctions could explain the differences in outcomes between college students4 and submariners11 to elevated CO 2 , but the decrements in performances of astronaut-like subjects that occurred when they were exposed to 1200 ppm CO 2 are inconsistent with this account.

There is abundant evidence that the default decision-making paradigms of young and/or novice individuals differ from those of older and/or experienced individuals.13,14,15,16,17 The former most often make use of expected utility or compensatory decision-making paradigms and the latter are more likely to employ heuristics or noncompensatory mechanisms.13,14,15,16,17 If the decision-making paradigms were different among different subject populations and the SMS provides a more sensitive measure of one paradigm than the other, such circumstances could produce the disparities in outcomes that have occurred among studies that have utilized the SMS to assess the effect of CO 2 upon complex decision making.4,5,11 Populations that are using similar decision-making paradigms may be more likely to share similarities in the subset of SMS measures most affected by CO 2 than those that are using different decision-making paradigms. Fig. 3 illustrates greater similarity in most affected measures between the studies of Satish4 and Allen5 and between our study and that of Rodeheffer11 when the effect is measured as a percentage deviation from baseline values. A post hoc analysis of variance with the data from Table 3 showed that the means of individual measures of the SMS are most often significantly different between the study of Satish4 and that of Rodeheffer11 and between Satish’s study4 and our study, whereas there were few measures with a significant difference between our study and that of Rodeheffer11 (Table 4). The subjects of Satish’s study4 were predominately college students whereas subjects of the studies of Rodeheffer11 (US submariners) and this study (astronaut-like subjects) were older and principally from operations-oriented disciplines. The performance scores reported by Allen’s study,5 which involved professional-grade employees (architects, designers, programmers, engineers, creative marketing professionals, managers), were normalized to a unique experimental condition and so could not be directly compared to those of other studies. Our subjects exhibited performance decrements at 1200 ppm comparable in magnitude to those observed in Satish4 and Allen5 at similar concentrations. This finding indicates that the SMS is also sensitive to CO 2 -induced decrements in the decision-making paradigm that may be shared by astronaut-like subjects and submariners, which likely differs from that of the subjects of the studies of Satish4 and Allen.5 Therefore, we conclude that it is unlikely that disparities in outcomes among the studies that have assessed effects of CO 2 on complex decision making with the SMS are due to differences in the sensitivity of the SMS to different decision-making paradigms used by the various subject populations. The disparities are more likely due to differing characteristics of the various subject populations and differences in the aggregation of unrecognized stressors, in addition to CO 2 .

Table 4 Significance differences of measures of the SMS from several studies in which performance was assessed during exposures to CO 2 Full size table

Because the decrements in performance on the SMS observed when 1200 ppm CO 2 was targeted were not observed at higher concentrations of CO 2 , the possibility that the effect observed could have arisen from circumstances that were unique to conditions during exposure at 1200 ppm was considered. Ventilation rates differed between the exposures at 600 ppm and those at the three higher concentrations of CO 2 . When 600 ppm was targeted, CO 2 produced metabolically by the subjects was prevented from accumulating by continuous operation of a blower that brought outside air into the third level of the chamber at 4.5 m3/min. CO 2 , when required, was introduced via the heating, ventilation, and air conditioning (HVAC) system, which at this targeted concentration was operated continuously at 5.4 m3/min. With all other targeted CO 2 concentrations, the fresh air blower was disengaged and the HVAC flow was operated continuously at 5.1 m3/min. Therefore, accumulation of volatile organic compounds (VOCs) and/or BEs emitted by the subjects would be expected to be lowest when 600 ppm CO 2 was targeted and higher during exposures to the other concentrations of CO 2 during which no outside air was brought into the exposure chamber. Because accumulation of VOCs or BEs have measurable effects on performance on the SMS,5,26,27,28,29 it would be expected that if these agents contributed to the depressed performance at 1200 ppm then their effects should also have been evident when the two higher concentrations of CO 2 were targeted unless these effects were alleviated by the higher concentrations of CO 2 . Increased CO 2 blood concentrations elicit a number of physiological responses triggered by a pH-induced stimulation of central and peripheral chemoreceptors, including increases in heart rate and minute ventilation, cerebral arterial vasodilation, and central nervous system (CNS) arousal.30,31,32,33 For these reasons, it is plausible that a slight to moderate increase in CO 2 levels increases CNS arousal and cognitive performance. However, the possibility of mitigation of effects of BEs by the higher levels of CO 2 seems disallowed by reports of adverse effects on performance on the SMS4,5 in subjects exposed to CO 2 at lower concentrations and ventilation rates sufficiently high to effectively purge BEs4,5 (Table 5).

Table 5 Exposure parameters Full size table

Findings converse to those discussed above4,5 have been reported8,9,10 from studies in which moderate accumulations of metabolically produced CO 2 and accompanying BEs, but not exposures to identical concentrations of pure CO 2 , caused decrements in cognitive performances.8,9,10 The finding by Zhang8,9,10 provide no support for the hypothesis that adverse effects of VOCs and BEs may be mitigated by CO 2 at our higher concentrations, unless the comparable levels of CO 2 in Zhang’s studies were accompanied by substantially greater levels of BEs than those in our study. The levels of BEs were not reported by Zhang but they could have been well in excess of the levels of BE accumulated in this study because our targeted concentrations of CO 2 were attained in a chamber volume that exceeded that of Zhang by a factor >2.5 (Table 5) and, unlike Zhang,8,9,10 exogenous CO 2 had to be added to achieve our high targeted concentrations.

Mitigation of CO 2 effects due to VOCs and BEs at the higher concentrations in this study may be refuted by the observation of performance decrements among office workers in locations described as afflicted with sick building syndromes. In these locations, high levels of VOCs and BEs are accompanied by elevated levels of CO 2. However, in these settings, the sources of VOCs are potentially much greater than those in exposure chambers of controlled studies, and other environmental factors may be influencing performance as well.28

Although it is possible that CO 2 at higher concentrations mitigates effects of BEs and/or VOCs in this study, in view of the disparate outcomes among this study and the various studies that have assessed the effects of CO 2 upon complex decision making4,5,11 or general cognitive performance,8,9,10 it seems most probable that differing characteristics of the various subject populations and differences in the aggregation of unrecognized stressors, in addition to CO 2 , were responsible for the varied, disparate, and conflicting outcomes among these studies.

A principal objective in utilizing Cognition was to investigate whether performance on this test battery, which was specifically designed for the high-performing astronaut population, is affected by short-term exposure to levels of CO 2 routinely occurring on the ISS. This ground-based study avoided other environmental stressors typically encountered on the ISS that could have confounded the effects of CO 2 on cognition (e.g., fatigue, stress, high workloads) and permitted a direct assessment of the effects of brief exposures to low concentrations of CO 2 on cognitive functions assessed by Cognition. A significant CO 2 main effect was only observed for accuracy on the VOLT and for the probability to achieve perfect accuracy on the DSST. However, there was no clearly discernable dose–response pattern for any of the individual measures of Cognition.

When the results obtained with all Cognition measures were taken in aggregate, a slight decrease in performance at 1200 ppm relative to 600 ppm was observed. Performance with higher, but still modest, CO 2 concentrations (2500 and 5000 ppm) were similar to performance at baseline (600 ppm). With effect sizes <0.2, the differences between CO 2 conditions were small. This “dose–response” of performance on Cognition to CO 2 recapitulates the dose–response obtained with the SMS test, which was administered during the same exposure sessions. It seems likely that the factors that were responsible for the dose–response pattern seen with the SMS, identified in the earlier discussion of results of the SMS, also produced the similar pattern in the aggregated scores of Cognition. The convergence of results obtained with Cognition and with the SMS provides confidence in results that differ significantly from those anticipated by the findings of Satish.4

The effects of short-term exposure to CO 2 concentrations of up to 5000 ppm on Cognition performance were small and with no dose–response function that would indicate decreasing performance levels with increasing CO 2 levels. Past studies on the effects of elevated CO 2 levels on cognitive performance investigated substantially higher CO 2 concentrations, and only some studies found effects on cognitive performance.12 As noted earlier, it is plausible that a slight to moderate increase in CO 2 levels increases CNS arousal and cognitive performance. Based on the paucity of literature, symptom reports related to increased levels of CO 2 , and the CNS arousing properties of CO 2 , both positive and negative associations between CO 2 levels and cognitive performance were plausible outcomes of our study.

The current findings suggest that performance on Cognition is not relevantly affected if astronaut-surrogate subjects are exposed to CO 2 concentrations of up to 5000 ppm for less than 3 h. On the other hand, it could be that none of the 10 Cognition tests was sensitive enough to detect subtle CO 2 -induced changes in cognitive performance, or that the 10 tests did not cover those cognitive domains that would be considerably affected by elevated CO 2 . This is unlikely, however, as Cognition covers a range of cognitive domains and has been shown to be sensitive to other stressors like sleep loss,34,35 recovery from anesthesia,36 and head-down tilt bed rest.35 It is thus more likely that any observed effects induced by short-term exposure to CO 2 concentrations of up to 5000 ppm were simply too subtle to induce relevant changes in performance on the measures of Cognition.

Interestingly, a recently published study on the effects of 12° head-down tilt with and without elevated levels of CO 2 also found the VOLT as the most sensitive test relative to 5000 ppm CO 2 levels.35 Therefore, it could be that the medial temporal cortex and the hippocampus are especially sensitive to changes in CO 2 concentration, with concomitant changes in memory performance.