As the 21st century progresses, rising atmospheric carbon dioxide (CO2) concentrations will cause urban and indoor levels of the gas to increase, and that may significantly reduce our basic decision-making ability and complex strategic thinking, according to a new CU Boulder-led study.
By the end of the century, people could be exposed to indoor CO2 levels up to 1400 parts per million–more than three times today’s outdoor levels, and well beyond what humans have ever experienced.
“It’s amazing how high CO2 levels get in enclosed spaces,” said Kris Karnauskas, CIRES Fellow, associate professor at CU Boulder and lead author of the new study published today in the AGU journal GeoHealth.
“It affects everybody–from little kids packed into classrooms to scientists, business people and decision makers to regular folks in their houses and apartments.”
Shelly Miller, professor in CU Boulder’s school of engineering and coauthor adds that “building ventilation typically modulates CO2 levels in buildings, but there are situations when there are too many people and not enough fresh air to dilute the CO2.”
CO2 can also build up in poorly ventilated spaces over longer periods of time, such as overnight while sleeping in bedrooms, she said.
Put simply, when we breathe air with high CO2 levels, the CO2 levels in our blood rise, reducing the amount of oxygen that reaches our brains. Studies show that this can increase sleepiness and anxiety, and impair cognitive function.
We all know the feeling: Sit too long in a stuffy, crowded lecture hall or conference room and many of us begin to feel drowsy or dull.
In general, CO2 concentrations are higher indoors than outdoors, the authors wrote. And outdoor CO2 in urban areas is higher than in pristine locations.
The CO2 concentrations in buildings are a result of both the gas that is otherwise in equilibrium with the outdoors, but also the CO2 generated by building occupants as they exhale.
Atmospheric CO2 levels have been rising since the Industrial Revolution, reaching a 414 ppm peak at NOAA’s Mauna Loa Observatory in Hawaii in 2019.
In the ongoing scenario in which people on Earth do not reduce greenhouse gas emissions, the Intergovernmental Panel on Climate Change predicts outdoor CO2 levels could climb to 930 ppm by 2100.
And urban areas typically have around 100 ppm CO2 higher than this background.
Karnauskas and his colleagues developed a comprehensive approach that considers predicted future outdoor CO2 concentrations and the impact of localized urban emissions, a model of the relationship between indoor and outdoor CO2 levels and the impact on human cognition.
They found that if the outdoor CO2 concentrations do rise to 930 ppm, that would nudge the indoor concentrations to a harmful level of 1400 ppm.
“At this level, some studies have demonstrated compelling evidence for significant cognitive impairment,” said Anna Schapiro, assistant professor of psychology at the University of Pennsylvania and a coauthor on the study.
“Though the literature contains some conflicting findings and much more research is needed, it appears that high level cognitive domains like decision-making and planning are especially susceptible to increasing CO2 concentrations.”
In fact, at 1400 ppm, CO2 concentrations may cut our basic decision-making ability by 25 percent, and complex strategic thinking by around 50 percent, the authors found.
The cognitive impacts of rising CO2 levels represent what scientists call a “direct” effect of the gas’ concentration, much like ocean acidification. In both cases, elevated CO2 itself–not the subsequent warming it also causes–is what triggers harm.
The team says there may be ways to adapt to higher indoor CO2 levels, but the best way to prevent levels from reaching harmful levels is to reduce fossil fuel emissions.
This would require globally adopted mitigation strategies such as those set forth by the Paris Agreement of the United Nations Framework Convention on Climate Change.
Karnauskas and his coauthors hope these findings will spark further research on ‘hidden’ impacts of climate change such as that on cognition.
“This is a complex problem, and our study is at the beginning. It’s not just a matter of predicting global (outdoor) CO2 levels,” he said. “It’s going from the global background emissions, to concentrations in the urban environment, to the indoor concentrations, and finally the resulting human impact.
We need even broader, interdisciplinary teams of researchers to explore this: investigating each step in our own silos will not be enough.”
Because humans produce and exhale carbon dioxide (CO2), concentrations of CO2 in occupied indoor spaces are higher than concentrations outdoors. As the ventilation rate (i.e., rate of outdoor air supply to the indoors) per person decreases, the magnitude of the indoor–outdoor difference in CO2 concentration increases.
Consequently, peak indoor CO2 concentrations, or the peak elevations of the indoor concentrations above those in outdoor air, have often been used as rough indicators for outdoor-air ventilation rate per occupant (Persily and Dols 1990).
The need to reduce energy consumption provides an incentive for low rates of ventilation, leading to higher indoor CO2 concentrations.
Although typical outdoor CO2 concentrations are approximately 380 ppm, outdoor levels in urban areas as high as 500 ppm have been reported (Persily 1997). Concentrations of CO2 inside buildings range from outdoor levels up to several thousand parts per million (Persily and Gorfain 2008).
Prior research has documented direct health effects of CO2 on humans, but only at concentrations much higher than those found in normal indoor settings. CO2 concentrations > 20,000 ppm cause deepened breathing; 40,000 ppm increases respiration markedly; 100,000 ppm causes visual disturbances and tremors and has been associated with loss of consciousness; and 250,000 ppm CO2 (a 25% concentration) can cause death (Lipsett et al. 1994).
Maximum recommended occupational exposure limits for an 8-hr workday are 5,000 ppm as a time-weighted average, for the Occupational Safety and Health Administration (OSHA 2012) and the American Conference of Government Industrial Hygienists (ACGIH 2011).
Epidemiologic and intervention research has shown that higher levels of CO2 within the range found in normal indoor settings are associated with perceptions of poor air quality, increased prevalence of acute health symptoms (e.g., headache, mucosal irritation), slower work performance, and increased absence (Erdmann and Apte 2004; Federspiel et al. 2004; Milton et al. 2000; Seppanen et al. 1999; Shendell et al. 2004; Wargocki et al. 2000).
It is widely believed that these associations exist only because the higher indoor CO2 concentrations at lower outdoor air ventilation rates are correlated with higher levels of other indoor-generated pollutants that directly cause the adverse effects (Mudarri 1997; Persily 1997).
Thus CO2 in the range of concentrations found in buildings (i.e., up to 5,000 ppm) has been assumed to have no direct impacts on occupants’ perceptions, health, or work performance.
Researchers in Hungary have questioned this assumption (Kajtar et al. 2003, 2006). The authors reported that controlled human exposures to CO2 between 2,000 ppm and 5,000 ppm, with ventilation rates unchanged, had subtle adverse impacts on proofreading of text in some trials, but the brief reports in conference proceedings provided limited details.
This stimulated our group to test effects of variation in CO2 alone, in a controlled environment, on potentially more sensitive high-level cognitive functioning.
We investigated a hypothesis that higher concentrations of CO2, within the range found in buildings and without changes in ventilation rate, have detrimental effects on occupants’ decision-making performance.
Synthesis and interpretation of findings. Performance for six of nine decision-making measures decreased moderately but significantly at 1,000 ppm relative to the baseline of 600 ppm, and seven decreased substantially at 2,500 ppm.
For an eighth scale, “information search,” no significant differences were seen across conditions. In contrast to other scales, an inverse pattern was seen for “focused activity,” with the highest level of focus obtained at 2,500 ppm and the lowest at 600 ppm.
Thus, most decision-making variables showed a decline with higher concentrations of CO2, but measures of focused activity improved. Focused activity is important for overall productivity, but high levels of focus under nonemergency conditions may indicate “overconcentration.”
Prior research with the SMS has shown repeatedly that individuals who experience difficulty in functioning [e.g., persons with mild-to-moderate head injuries (Satish et al. 2008), persons under the influence of alcohol (Streufert et al. 1993), and persons suffering from allergic rhinitis (Satish et al. 2004)] tend to become highly focused on smaller details at the expense of the big picture.
High levels of predictive validity for the SMS (r > 0.60 with real-world success as judged by peers and as demonstrated by income, job level, promotions, and level in organizations), as well as high levels of test–retest reliability across the four simulation scenarios (r = 0.72–0.94) have repeatedly been demonstrated (Breuer and Streufert 1995; Streufert et al. 1988).
Additional validity is demonstrated by the deterioration of various performance indicators with 0.05% blood alcohol intoxication and seriously diminished functioning with intoxication at the 0.10 level (Satish and Streufert 2002)
. Baseline scores at 600 ppm CO2 for the participants in this study, mostly current science and engineering students from a top U.S. university, were all average or above.
Although the modest reductions in multiple aspects of decision making seen at 1,000 ppm may not be critical to individuals, at a societal level or for employers an exposure that reduces performance even slightly could be economically significant.
The substantial reductions in decision-making performance with 2.5-hr exposures to 2,500 ppm CO2 indicate, per the available norms for the SMS test, impairment that is of importance even for individuals.
These findings provide initial evidence for considering CO2 as an indoor pollutant, not just a proxy for other pollutants that directly affect people.
CO2 concentrations in practice. The real-world significance of our findings, if confirmed, would depend on the extent to which CO2 concentrations are ≥ 1,000 and ≥ 2,500 ppm in current or future buildings.
There is strong evidence that in schools, CO2 concentrations are frequently near or above the levels associated in this study with significant reductions in decision-making performance.
In surveys of elementary school classrooms in California and Texas, average CO2 concentrations were > 1,000 ppm, a substantial proportion exceeded 2,000 ppm, and in 21% of Texas classrooms peak CO2 concentration exceeded 3,000 ppm (Corsi et al. 2002; Whitmore et al. 2003).
Given these concentrations, we must consider the possibility that some students in high-CO2 classrooms are disadvantaged in learning or test taking.
We do not know whether exposures that cause decrements in decision making in the SMS test will inhibit learning by students; however, we cannot rule out impacts on learning.
We were not able to identify CO2 measurements for spaces in which students take tests related to admission to universities or graduate schools, or from tests related to professional accreditations, but these testing environments often have a high occupant density, and thus might have elevated CO2 levels.
In general office spaces within the United States, CO2 concentrations tend to be much lower than in schools. In a representative survey of 100 U.S. offices (Persily and Gorfain 2008), only 5% of the measured peak indoor CO2 concentrations exceeded 1,000 ppm, assuming an outdoor concentration of 400 ppm.
One very small study suggests that meeting rooms in offices, where important decisions are sometimes made, can have elevated CO2 concentrations—for example, up to 1,900 ppm during 30- to 90-min meetings (Fisk et al. 2010).
In some vehicles (aircraft, ships, submarines, cars, buses, and trucks), because of their airtight construction or high occupant density, high CO2 concentrations may be expected.
In eight studies within commercial aircraft, mean CO2 concentrations in the passenger cabins were generally > 1,000 ppm and ranged as high as 1,756 ppm, and maximum concentrations were as high as 4,200 ppm (Committee on Air Quality in Passenger Cabins of Commercial Aircraft 2002).
We did not identify data on CO2 concentrations in automobiles and trucks. One small study (Knibbs et al. 2008) reported low ventilation rates in vehicles with ventilation systems in the closed or recirculated-air positions.
From those results, and using an assumption of one occupant and a 0.0052 L/sec CO2 emission rate per occupant (Persily and Gorfain 2008), we estimated steady-state CO2 concentrations in an automobile and pickup truck of 3,700 ppm and 1,250 ppm, respectively, above outdoor concentrations. These numbers would increase in proportion to the number of occupants. It is not known whether the findings of the present study apply to the decision making of vehicle drivers, although such effects are conceivable.
There is evidence that people wearing masks for respiratory protection may inhale air with highly elevated CO2 concentrations. In a recent study, dead-space CO2 concentrations within a respirator (i.e., N95 mask) were approximately 30,000 ppm (Roberge et al. 2010), suggesting potentially high CO2 concentration in inhaled air.
The inhaled concentration would be lower than that within the mask, diluted by approximately 500 mL per breath inhaled through the mask. Although the study did not report the actual inhaled-air CO2 concentrations, partial pressures of CO2 in blood did not differ with wearing the mask. Caretti (1999) reported that respirator wear with low-level activity did not adversely alter cognitive performance or mood.
Findings by others. The Hungarian studies briefly reported by Kajtar et al. (2003, 2006) were the only prior studies on cognitive effects of moderate CO2 elevations that we identified. In these studies, the ventilation rate in an experimental chamber was kept constant at a level producing a chamber CO2 concentration of 600 ppm from the occupant-generated CO2; in some experiments, however, the chamber CO2 concentration was increased above 600 ppm, to as high as 5,000 ppm, by injecting 99.995% pure CO2 from a gas cylinder into the chamber.
In two series of studies, participants blinded to CO2 concentrations performed proofreading significantly more poorly in some but not all sessions with CO2 concentrations of 4,000 ppm relative to 600 ppm. Similar, marginally significant differences were seen at 3,000 versus 600 ppm.
(Differences were seen only in proportion of errors found, not in speed of reading.) The studies by Kajtar et al. (2003, 2006) were small (e.g., 10 participants) and found only a few significant associations out of many trials; these results may have been attributable to chance, but they did suggest that CO2 concentrations found in buildings may directly influence human performance.
Our research, which was motivated by the Hungarian studies, involved lower concentrations of CO2, a larger study population, and different methods to assess human performance.
Prior studies on CO2 exposures, mostly at higher levels, have focused on physiologic effects. CO2 is the key regulator of respiration and arousal of behavioral states in humans (Kaye et al. 2004). The initial effects of inhaling CO2 at higher concentrations are increased partial pressure of CO2 in arterial blood (PaCO2) and decreased blood pH.
However, PaCO2 is tightly regulated in healthy humans through reflex control of breathing, despite normal variation within and between individuals (Bloch-Salisbury et al. 2000). Inhaled CO2 at concentrations of tens of thousands of parts per million has been associated with changes in respiration, cerebral blood flow, cardiac output, and anxiety (Brian 1998; Kaye et al. 2004; Lipsett et al. 1994; Roberge et al. 2010; Woods et al. 1988).
Little research has documented physiological impacts of moderately elevated CO2 concentrations, except one small study that reported changes in respiration, circulation, and cerebral electrical activity at 1,000 ppm CO2 (Goromosov 1968).
We do not have hypotheses to explain why inhaling moderately elevated CO2, with the expected resulting increases in respiration, heart rate, and cardiac output to stabilize PaCO2, would affect decision-making performance.
Bloch-Salisbury et al. (2000) have summarized prior knowledge on effects of elevated PaCO2. PaCO2 has a direct linear relationship with cerebral blood flow in a broad range above and below normal levels, through dilation and constriction of arterioles.
Moderately elevated (or reduced) PaCO2 has dramatic effects on central nervous system and cortical function.
Bloch-Salisbury et al. (2000) reported that experimental changes in PaCO2 in humans within the normal range (in 2-hr sessions involving special procedures to hold respiration constant and thus eliminate the normal reflex control of PaCO2 through altered breathing), showed no effects on cognitive function or alertness but caused significant changes in electroencephalogram power spectra.
Limitations. This study successfully controlled the known environmental confounding factors of temperature and ventilation rate. Although exposures to CO2 in prior sessions may theoretically have affected performance in subsequent sessions, such carryover effects should not invalidate study results because of the balanced order of exposures.
Suggestion effects were unlikely, because participants and the researcher explaining the SMS to them were blinded to specific conditions of each session. Although we conclude that the causality of the observed effects is clear, the ability to generalize from this group of college/university students to others is uncertain.
Effects of CO2 between 600 and 1,000 ppm and between 1,000 and 2,500 ppm, and effects for longer and shorter periods of time are also uncertain.
The strength of the effects seen at 2,500 ppm CO2 is so large for some metrics as to almost defy credibility, although it is possible that such effects occur without recognition in daily life. Replication of these study findings, including use of other measures of complex cognitive functioning and measures of physiologic response such as respiration and heart rate, is needed before definitive conclusions are drawn.
Implications for minimum ventilation standards. The findings of this study, if replicated, would have implications for the standards that specify minimum ventilation rates in buildings, and would also indicate the need to adhere more consistently to the existing standards.
Many of the elevated CO2 concentrations observed in practice are a consequence of a failure to supply the amount of outdoor air specified in current standards; however, even the minimum ventilation rates in the leading professional standard [American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 2010] correspond to CO2 concentrations > 1,000 ppm in densely occupied spaces.
There is current interest in reducing ventilation rates and the rates required by standards, to save energy and reduce energy-related costs. Yet large reductions in ventilation rates could lead to increased CO2 concentrations that may adversely affect decision-making performance, even if air-cleaning systems or low-emission materials were used to control other indoor pollutants.
It seems unlikely that recommended minimum ventilation rates in future standards would be low enough to cause CO2 levels > 2,500 ppm, a level at which decrements in decision-making performance in our findings were large, but standards with rates that result in 1,500 ppm of indoor CO2 are conceivable.
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