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.
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 . 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 . 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)  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 and Work Performance
The conventional wisdom has been that CO2 at concentrations below 5000 ppm have no direct impacts on people's perceptions, health or performance. Rather, the belief has been that higher CO2 concentrations are simply correlated with worsened health and decreased student performance because concentrations of many other indoor air pollutants tend increase as the indoor CO2 concentration increases. However, based on the results of recent studies, the conventional wisdom about no direct effects of CO2 must be questioned. Five studies, with results published in journals, have subjected subjects to a range of CO2 concentrations in chamber facilities while having subjects performs tests, with test results used to assess the subjects’ cognitive performance. Some of these studies also employed questionnaires on acute health symptoms, such as headache, and satisfaction with indoor air quality In some experiments within each study, CO2 concentrations were maintained low, for example 600 ppm, by maintaining a high ventilation rate per person. In other experiments, the researchers increased indoor air CO2 concentrations, to between 1000 and 5000 ppm by adding pure CO2 to the indoor air, while maintaining all other conditions unchanged. The subjects, were unaware of the CO2 concentrations. Three of these studies have reported statistically significant, and sometimes quite substantial, decreases in cognitive performance when indoor CO2 concentrations were higher. The findings are summarized in the following paragraphs
- In studies performed in Hungary , subjects’ performance in proof reading tests, but not other tests, were diminished with 4000 ppm and 3000 ppm CO2, relative to 600 ppm CO2. Subjects’ levels of satisfaction with indoor air quality also diminished as the CO2 concentration increased.
- In the first U.S. study , each subject completed tests of decision making performance with CO2 concentrations of 600, 1000, and 2500 ppm. Carbon dioxide was increased above the baseline level of 600 ppm by injecting ultrapure CO2. The subjects' performance on most measures of decision making performance was moderately and statistically significantly diminished at 1000 ppm CO2, relative to 600 ppm CO2. At 2500 ppm CO2, relative to 600 ppm, the subjects' performance on most measures of decision making performance was highly and statistically significantly diminished.
- In a second study from the U.S. , a test of decision making performance was again used to assess subjects’ levels of cognitive performance. On average, scores on the various measures of cognitive performance decreased by about 15% with 945 ppm CO2 relative to 550 ppm CO2. With 1400 ppm CO2 relative to 550 ppm CO2, on average scores decreased by 50%.
Two studies from Denmark found no statistically significant effects of higher CO2 concentrations on cognitive performance, with all other factors maintained constant. Performance in text typing, arithmetical calculations, proof reading, attention level, memory, and reaction time were employed to assess subjects’ cognitive performance levels.
- The first of these Danish studies  found no statistically significant effects of 1000 ppm and 3000 ppm CO2, relative to 500 ppm CO2 on cognitive performance, satisfaction with indoor air quality, or intensity of acute health symptoms. In the same study, exposure to 3000 ppm CO2 was associated with a small increase in end tidal CO2 (maximum CO2 concentration in exhaled breath) and affected heart rate, but had no statistically significant effects on blood pressure, oxygen saturation, respiration rate, nasal peak flow, forced expiration volume, and levels of markers in saliva that indicate levels of stress .
- In a follow-up study employing the same methods but with only ten subjects , the Danish researchers again found no significant effects on cognitive performance, satisfaction with indoor air quality, and intensity of acute health symptoms of 4900 ppm CO2 relative to 500 ppm CO2. They also found that 4900 ppm CO2 did not influence the physiological outcomes except for an increase in end tidal CO2.
The results of the five controlled studies cited above are inconsistent, with three of five studies finding statistically significant decreases in cognitive performance with increased CO2 concentrations. The type of tests used to assess cognitive performance might be a factor. The strong and consistent adverse effects of elevated CO2 were found the in the studies employing demanding tests of decision making performance. No effects of CO2, or limited effects, were found when more simple tests (e.g., addition, proof reading) were used to assess cognitive performance. With one exception , a decrease in satisfaction with indoor air quality as CO2 concentrations increased, the research has found no significant direct effects of elevated CO2 concentrations, up to 5000 ppm, on satisfaction with air quality or intensity of acute health symptoms.
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 [103, 104]. 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.  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 [106, 107]. 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 2 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. . This best-fit curve is reproduced below.
Figure 5. Best-fit composite weighted curve of relative performance versus ventilation rate from Seppänen et al. , 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.  did not enable a relationship to be established for lower ventilation rates.