Supporting Information

Purpose of Ventilation

"Ventilation," as defined here, is the flow of outdoor air into a building. Mechanical ventilation is provided in many buildings, including most U.S. commercial buildings, using fans and ductwork that are part of heating, ventilating, and air conditioning (HVAC) systems. Natural ventilation is provided in some buildings (such as most homes) by airflows through open windows, doors, and other openings in the building's envelope which are driven by wind and indoor-outdoor temperature differences. Most U.S. homes do not have mechanical ventilation systems other than bathroom or kitchen exhaust fans that, when operated, provide mechanical ventilation. New homes with low-leakage envelopes more frequently have mechanical ventilation systems.

Ventilation dilutes indoor-generated air pollutants and flushes those pollutants out of a building. Ventilation also brings outdoor air pollutants into a building, although outdoor air typically has lower pollutant levels than indoor air and some of these outdoor pollutants may be removed from the ventilation air using filters. The quantity of ventilation air can impact the size of a building's HVAC equipment, and heating and cooling energy costs. In humid climates, ventilation air can introduce significant amounts of moisture to the indoor environment if not conditioned properly.

Ventilation Rate

The ventilation airflow rate is the rate of flow of outdoor air into a building per unit of time, and is often expressed in units of cubic feet per minute (cfm) or liters per second (L/s). "Ventilation rates" are normally expressed as ventilation airflow rates divided by the number of people in the building (yielding cfm per person or L/s per person), by the indoor air volume (yielding air changes per hour or h-1), or by the indoor floor area (yielding cfm per square ft or L/s per square meter).

Ventilation Rates and Carbon Dioxide (CO2)

Since people produce and exhale CO2 as a consequence of their normal metabolic processes, the concentrations of carbon dioxide inside occupied spaces are higher than the concentrations of CO2 in the outdoor air. In general, a larger peak difference between indoor and outdoor CO2 concentration indicates a smaller ventilation rate per person. The ventilation rate per person can be estimated with reasonable accuracy from the difference between the maximum steady-state (equilibrium) indoor CO2 concentration and the outdoor CO2 concentration, if several critical assumptions are met, including: the occupied space has nearly constant occupancy and physical activity level for several hours, the ventilation rate is nearly constant, and the measured indoor CO2 concentration is representative of the average indoor or exhaust airstream concentration in the space [98]. For example, in an office space under these conditions, if the equilibrium indoor CO2 concentration is 650 parts per million (ppm) above the outdoor concentration, the ventilation rate is approximately 15 cfm (7.1 L/s) per person [98]. In many real buildings, occupancy and ventilation rates are not stable for sufficient periods and other critical assumptions may not be met to enable an accurate determination of ventilation rate from CO2 data. The American Society for Testing and Materials (ASTM) [98] states that this technique has been misused when the necessary assumptions have not been verified and results have been misinterpreted. Nevertheless, CO2 concentrations remain a rough and easily measured surrogate for ventilation rate. In addition, many studies have found that occupants of buildings with higher indoor CO2 concentrations have an increased prevalence of sick building syndrome symptoms. However, indoor CO2 concentrations may be poor indicators of health risks in buildings and spaces with strong pollutant emissions from the building or building furnishings, particularly when occupant densities are low.

Direct Impacts of Carbon Dioxide (CO2) on Perceived Air Quality, Health, And Work Performance

Indoor concentrations of CO2 in occupied buildings exceed outdoor concentrations because CO2 is a product of peoples' metabolism. Indoor concentrations of CO2 are indicators of the rates of ventilation of buildings with outdoor air, particularly with the rates of ventilation per person. When indoor CO2 concentrations increase and decrease, concentrations of many other indoor air pollutants emitted from indoor sources, particularly the bioeffluents, also change correspondingly. Increased indoor CO2 concentrations have often been associated (correlated) with decreases in ratings of air quality, with increases in acute health symptoms, and with reductions in aspects of human performance. Research prior to 2012, although often with conditions atypical of normal buildings, indicated that levels of CO2 itself, with other conditions constant, had no significant impacts on peoples' health or performance unless the CO2 concentrations far exceeded the levels found in buildings [99-105]. Therefore, the associations of acceptability of air quality, health symptoms, and aspects of performance with indoor CO2 levels has been attributed to the other indoor air pollutants with concentrations that increase or decrease in concert with the indoor concentrations of CO2.

Since 2012, eight studies, described in Table 2, have investigated whether increases in moderate CO2 concentrations, with other conditions maintained constant, influence perceptions of indoor air quality, health, or cognitive performance. All of these studies have been performed with subjects in special research facilities enabling CO2 concentrations to be modified by adding pure CO2 to the indoor air while maintaining other conditions constant. By providing high ventilation rates, these studies have maintained low concentrations of bioeffluents. All of the studies maintained subjects blinded (unaware) with respect to the CO2 concentrations. All studies used healthy adults, mostly college-age adults, as subjects. All but one of these studies [106] have employed strong study designs by measuring changes in perceptual, health, or performance outcomes within each subject. In contrast, Rodeheffer et al. [106] employed three different groups of subjects, each group exposed to a different level of CO2 in the indoor air. With only 12 subjects in each group, differences among subjects may have affected study results. Also, the study of Rodeheffer et al. [106] included some changes in ventilation rates, thus indoor CO2 concentrations were not the only changing conditions. The subjects of this study were submarine-qualified Navy personnel. Since high CO2 levels, e.g., 5000 ppm, are common in submarines, the Navy personnel have most likely experienced highly elevated CO2 for extended periods of time.

With respect to subjects' cognitive performance, there are substantial inconsistencies among the results of the eight studies. Four studies [13, 107-109] found aspects of cognitive performance to be diminished by a statistically significant, and sometimes very substantial, amount when CO2 concentrations were increased. Concentrations of CO2 as low as approximately 1000 ppm, relative to 500 to 600 ppm, significantly reduced performance [13, 108]. Three of these four studies [13, 108, 109] employed demanding tests of cognitive performance, either a 90 minute test of decision making via a test system called the strategic management simulation (SMS) or a 180 minute test of pilots' performance in flight simulations. The fourth study [107] found performance decreased in a proof reading task but not in other tasks, when CO2 levels were increased. In is notable, however, that Kajtar and Hertczeg [107] reported reductions in proof reading performance in only one of two series of experiments and that the reductions in performance occurred with relatively high CO2 concentrations ( 3000 ppm or higher). Four additional studies [14, 106, 110, 111] found that CO2 levels had no statistically significant effects on performance. Three of these studies [14, 110, 111] used tests of task performance (e.g., arithmetic tasks, text typing, proof reading, memory) as well as tests of reaction time and attention to assess performance and in one study, [110], CO2 levels as high as 5000 ppm did not influence performance. One of these studies, [111] was conducted with high indoor air temperatures (35 oC). The fourth study [106], the one with a weaker study design, found CO2 levels as high as 15000 ppm to not affect performance in a test of decision making (the SMS test).

Five of eight studies [14, 106, 107, 110, 111] investigated whether subjects' perceptions of indoor air quality, e.g., acceptability of the indoor air, was influenced by CO2 concentrations. Only one study [107] found perceived air quality to be affected by the level of CO2. Subjects reported air quality as less acceptable with 3000, 4000, and 5000 ppm CO2 relative to 600 ppm CO2.

Five studies, reported in six papers, [14, 106, 107, 110-112] 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. Three studies that included questionnaires on acute health symptoms [14, 110, 111] found that CO2 level had no statistically significant effect on symptoms. Kajtar and Herczeg [107] reported that subjects were significantly more tired with 5000 ppm CO2 relative to 600 ppm CO2. Kajtar and Herczeg [107] also found blood pressure, respiration rate and volume, and mental effort (based on heart period variability) were increased with higher CO2 concentrations. On the other hand, other studies [110-112] generally found no statistically significant effects of CO2 levels on a broad range of physiological outcomes except for increases in the concentrations of CO2 in exhaled air. In one study, heart rate decreased less during the exposure session with 3000 ppm CO2 vs. 500 ppm CO2 and in another study [111] levels of alpha-amylase, markers of mental stress, were higher with 3000 ppm CO2 compared to 380 ppm CO2.

The major results of this body of research is depicted in Figure 5. In this figure, each horizontal line represents results from a single study or portion of a study. The circles indicate the CO2 concentrations employed in the study. A filled in circle indicates a statistically significant worsening in the outcome at the indicated CO2 concentration relative to the CO2 concentration denoted by the left-most circle. In summary, there is very limited evidence that CO2 levels below 5,000 ppm influence perceived air quality, acute health symptoms, or physiological outcomes other than the concentrations of CO2 in exhaled air. With respect to acute health symptoms and perceived air quality the study results, with one exception [107], are consistent and find no effects at CO2 below 5,000 ppm. The results of research on the effects of moderate CO2 levels on human performance vary among studies. Some high quality studies find effects of higher CO2 concentrations on cognitive performance while other high quality studies find no effects on this outcome. There is substantial evidence that performance on challenging tests of decision making and challenging flight simulations is worsened by CO2 concentrations as low as 1000 ppm. Since none of these studies reported other outcomes, like perceptions, health symptoms or physiological responses, the mechanisms underlying the reductions in performance are unknown. The reasons for discrepancy in the observed results are unknown. Liu et al. [111] and Zhang et al. [14]  hypothesized that the level of stress associated with the performance test might explain the discrepancy of 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 alpha-amylase concentrations, suggesting higher mental stress, when CO2 concentrations were increased.

Table 2. Overview and results of studies of the direct effects of moderate CO2 concentrations on human perceptions, health, and performance.

Reference(s) Subjects CO2 Concentrations Exposure period Outcomes Strengths & Weaknesses Statistically Significant Findings



Series 1: 600, 1500, 2500, 5000 ppm

2.3 h

Detection of typographic errors, Perceived air quality, Reported thermal comfort, Tiredness, Freshness, Concentration, Blood pressure, Heart rate and heart rate variance

Strengths: Within person assessment, Subjects blinded

Weaknesses: Few subjects; Detailed results of statistical tests not reported; Order of exposure was not balanced (varied) among subjects; Temperature levels not reported

Air quality less acceptable and subjects more tired at 5000 ppm vs. 600 ppm; Decrease in pulse rate over session duration was less at 5000 ppm vs. 600 ppm; Increase of diastolic blood pressure at 5000 ppm vs. 600 ppm; Higher respiratory frequency and volume at 5000 ppm vs. 600 ppm; Higher mental effort at 5000 ppm vs 600 ppm based on heart period variability



Series 2: 600, 1500, 3000, 4000 ppm

2.3 h @ 1500 and 4000 ppm, 3.5 h @ 600 and 3000 ppm

See above

See above

Air quality rated less acceptable at 3000 and 4000 ppm vs. 600 ppm; Subjects more tired at 3000 ppm vs. 600 ppm; Subjects found fewer typographic errors at 4000 ppm vs. 600 ppm; Subjects read fewer rows of text at 3000 ppm vs. 600 ppm



600, 1000, 2500 ppm

2.5 h

Nine measures of cognitive performance (decision making) based on strategic management simulation (SMS) test

Strengths: Within-person assessment; Subjects blinded; Order of exposure balanced; Design controlled for day of exposure. time of day; and day of week; Excellent temperature control

For six of nine measures of decision making, scores were reduced by 12% to 23% at 1000 ppm vs. 600 ppm; For 7 of nine measures of decision making, scores were reduced 44% to 94% at 2500 ppm vs. 600 ppm; For one measures of decision making, score increased by 20% at 2500 ppm vs. 600 ppm.



Approximately 550, 945, 1400 ppm

8 hr

Nine measures of cognitive performance (decision making) based on strategic management simulation (SMS) test

Strengths: Within-person assessment; Subjects and analysts blinded; Mid-week testing avoided Monday/Friday effects; Excellent temperature control, One exposure level was repeated.

For seven of nine measures of decision making, there was a progressive decrease in scores as CO2 increased; On average, scores were 15% lower (range -17% to 34%) at 945 ppm vs. 550 ppm and 50% lower (range 12% to 78%) at 1400 ppm vs. 550 ppm.



500 and 4900 ppm

2.5 h

Perceived air quality, Thermal sensation, Noise level, Acute symptoms, Typing performance, Addition, Cue utilization, Heart rate, Blood Pressure, Respiration rate, End-tidal CO2, Oxygen saturation, Salivary alpha-amylase and salivary cortisol as markers of stress

Strengths: Within-person assessment; Subjects blinded; Order of exposure balanced; Excellent temperature control, Subjects exposed twice to each of the two CO2 conditions

Weaknesses: Few subjects (substantially counteracted by repeat of exposures)

End-tidal (final exhaled) CO2 was higher with 4900 ppm vs. 500 ppm. There were no statistically significant effects of indoor CO2 level on other outcomes.



435, 1083, 3004 ppm

4.25 h

Thermal sensation, Perceived air quality, Acute symptoms, Typing performance, Arithmetic calculations, Proof reading, Attention, Memory capacity, Grammatical reasoning, Reaction time

Strengths: Within-person assessment; Subjects blinded; Order of exposure balanced; Excellent temperature control

No statistically significant effects of indoor CO2 levels on thermal sensation, perceived air quality, acute health symptoms, or any measure of performance


See above

See above

See above

Heart rate, Blood pressure, End-tidal CO2, Oxygen saturation, Respiration rate, Nasal peak flow, Forced expiration (spirometry) parameters, Salivary alpha-amylase and salivary cortisol as markers of stress

See above

End-tidal (final exhaled) CO2 was higher with 3004 ppm vs. 435 ppm in indoor air. During the exposure sessions, heart rate decreased less with 3004 ppm vs. 435 ppm. There were no statistically significant effects of indoor CO2 level on other physiological outcomes.



380 ppm and 3025 ppm, both with air temperature of 35 oC

3 hr

Skin temperature, Heart rate (indicating respiratory ventilation rate), Blood pressure, Blood oxygen saturation, End-tidal CO2, Salivary alpha-amylase and salivary cortisol as markers of stress, Perceived comfort and several health symptoms, Acceptability of air quality, Cognitive tests (mental redirection, grammatical reasoning, digit span memory, visual learning memory, number calculation, Stroop (reaction time) test indicating attention, visual reaction time, d2 test indicating attention and concentration, Tsai_Partington test indicating level of arousal)

Strengths: Within-person assessment; Subjects blinded; Order of exposure balanced; Excellent temperature control

Weaknesses: Few subjects

There were no statistically significant changes in any outcome with increased indoor CO2 concentration except alpha-amylase levels increased*, suggesting mental stress when the CO2 level was 3000 ppm.



700, 1500, 2500 ppm

3 h

Pilots' performance levels in simulated flights based on: 1) output of flight simulator and 2) rating of a Federal Aviation Administration-designated flight examiner

Strengths: Within-person assessment; Order of exposure randomized, Pilots and examiner were blinded, Temperature constant within 1 oC

Weakness: Examiner, who assessed performance, was exposed to the same changes in CO2 levels as the pilots

Pilots' performance levels, as indicated by the flight simulator, were not affected by the CO2 level. The more widely used and comprehensive assessments of pilots' performance levels, as rated by the examiner, were significantly poorer when the CO2 level was elevated and the effects increased with duration of exposure. With 2500 ppm CO2 as the reference condition, the odds of a passing rating were 1.52 (1.02 – 2.25) at 1500 ppm CO2 and 1.69 (1.11 – 2.55) at 700 ppm CO2


36 (12 at each CO2 level)

600, 2500, 15000 ppm

2.5 h

Nine measures of cognitive performance (decision making) based on strategic management simulation (SMS) test; Thermal sensation, Perceived air quality, Perceived alertness, Perceived physical discomfort

Strengths: Subjects blinded

Weaknesses: Not a within-person assessment and only 12 subjects at each CO2 level; Fresh air ventilation rate was not maintained constant (details not provided), Temperature, which has affected performance in many studies, was ~ 2 oC higher in sessions with elevated CO2

There were no statistically significant effects of CO2 exposure level on any outcome

*statistical significance of increase in alpha-amylase based on personal communication, not reported in paper

Figure 5. Graphical presentation of key results of research results summarized in Table 2.

Ventilation and Energy Use

Under many weather conditions, the outdoor air supplied to a building must be heated, or cooled and dehumidified. Consequently, higher ventilation rates generally increase a building's energy use and energy costs. The required capacity and cost of heating and cooling equipment may also increase with a higher ventilation rate. The magnitude of the increases in energy usage will vary with climate, building type, and the building design, particularly the design of the building's heating, ventilating, and air conditioning system. The most detailed analyses of annual energy impacts pertain to increasing ventilation rates from 5 to 20 cfm (2.4 to 9.4 L/s)per occupant in offices and from 5 to 15 cfm (2.4 to 7.1 L/s) per occupant in schools [113, 114]. Total heating and cooling costs were predicted to increase by approximately 0% to 20%. Percentage increases in energy costs were largest in more severe climates and in buildings with a high occupant density, for example in schools. Additional estimates of the energy impacts of ventilation are provided by Benne et al. [115] for a range of commercial building types and climate zones in the United States. For the full stock of existing commercial buildings, eliminating mechanical ventilation (but maintaining air infiltration) was projected to reduce total energy use by 6.5%. Above average energy impacts are projected for buildings in more serve climate zones, in health care buildings, and in buildings with a high occupant density. The previous section, "Implications for Good Ventilation Practices", lists approaches for increasing time average ventilation rates with little or no increase in energy use, or even with energy savings.

Sick Building Syndrome Symptoms

Sick Building Syndrome (SBS) symptoms are acute symptoms, such as irritation of eyes, nose, and throat, headache, fatigue, cough, and tight chest, that occur at work and improve when away from work. These symptoms can have multiple causes, thus, they do not indicate a specific type of disease or a specific type of pollutant exposure. SBS symptoms have been widely reported by occupants of offices and schools, and in a few studies by occupants of homes. Some occupants in every office building will report some SBS symptoms, but indoor environmental factors that are known or suspected to lead to increased SBS symptoms include a lower ventilation rate (throughout the normal ventilation rate range encountered in buildings), strong indoor pollutant sources, air conditioning, and higher indoor temperatures [116, 117]. The fraction of occupants experiencing SBS symptoms is often called the symptom prevalence or symptom prevalence rate.

Estimation of Relative Performance With Changes in Ventilation Rates

Figure 6 shows curves of relative performance versus ventilation rate for reference ventilation rates of 15, 20, and 30 cfm (7.1, 9.4, 1.2 L/s) per person. These curves were derived using an equation representing the best-fit composite weighted curve shown in Figure 2 of Seppänen et al. [11]. This best-fit curve is reproduced below.

Graph showing the best-fit composite weighted curve of relative performance versus ventilation rate

Figure 6. Best-fit composite weighted curve of relative performance versus ventilation rate from Seppänen et al. [11], after conversion of the ventilation rate scale to English units.

For convenient calculations, the following table provides values of relative performance with three reference ventilation rates. The numbers in this table were derived using equations 1 and 2. The lowest value of ventilation rate in these tables is 13.8 cfm (6.5 L/s) per person because the original data analyzed by Seppänen et al. [11] did not enable a relationship to be established for lower ventilation rates.

Table 3. Relative performance for three reference ventilation rates, from equations 1 and 2.

Ventilation rate
(cfm per person)
Reference = 15 cfm
per person
Reference = 20 cfm
per person
Reference = 30 cfm
per person
Relative Performance Relative Performance Relative Performance
13.8 0.998 0.992 0.984
15 1.000 0.994 0.986
16 1.001 0.995 0.987
17 1.003 0.997 0.988
18 1.004 0.998 0.990
19 1.005 0.999 0.991
20 1.006 1.000 0.992
22 1.008 1.002 0.994
24 1.010 1.004 0.996
26 1.012 1.005 0.997
28 1.013 1.007 0.999
30 1.014 1.008 1.000
32 1.016 1.010 1.001
34 1.017 1.011 1.002
36 1.018 1.012 1.004
38 1.019 1.013 1.005
40 1.020 1.014 1.006
42 1.021 1.015 1.007
44 1.022 1.016 1.007
46 1.023 1.017 1.008
48 1.024 1.017 1.009
50 1.024 1.018 1.010