Indoor Air Quality Scientific Findings Resource Bank
http://eetd.lbl.gov/ied/sfrb/

Additional Information

Thermal Comfort, Temperature, and Work Performance

Research has demonstrated that the temperatures required to assure that occupants are thermally comfortable vary with season, building type, and other factors [41]. In part, this is true because people modify their clothing levels to adapt to different expected temperatures. Also, indoor air velocities and air humidity values, which vary among buildings, affect thermal comfort. In addition, peoples' expectations regarding temperature can vary depending on prior experiences. For example, occupants of naturally ventilated buildings without air conditioning are thermally satisfied over a broader range of temperatures than occupants of sealed air-conditioned buildings. At the present time, it is not known whether the relationship of air temperature with work performance is also modified by these same factors.

Purpose of Ventilation

"Ventilation," as defined here, is the flow of outdoor air to 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). "Ventilation rates" are normally expressed as ventilation airflow rates divided by the number of people in the building (yielding cfm per person), by the indoor air volume [(yielding air changes per hour (ach or h^-1)], or by the indoor floor area (yielding cfm per square ft.)

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 [42]. 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 per person [42]. 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 [42]. 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 because concentrations of many other indoor air pollutants tend increase as the indoor CO₂ concentration increases. However, based on the results of recent chamber studies from Hungary [43, 44], it may be questioned whether the conventional wisdom about no direct effects of CO₂ is correct. In these studies, subjects worked and reported perceived air quality while in a chamber. The ventilation rate of the chamber was maintained constant at a rate that resulted in a chamber CO₂ concentration of 600 ppm from the occupant-generated CO₂; however, the concentration of CO₂ in the chamber air was increased above 600 ppm, to as high as 5000 ppm in some experiments, by injecting pure CO₂ from a gas cylinder into the chamber. The subjects, who were unaware of the CO₂ concentrations, reported successively poorer indoor air quality as the CO₂ concentration increased. In addition, in a second 70 minute-work session, the subjects' performance in a proofreading test was poorer with a CO₂ concentration of 4000 ppm, than with a CO₂ concentration of 600 ppm. In the third 70-minute work session, the subjects' proof-reading performance was significantly degraded at 3000 ppm CO₂ compared to 600 ppm CO₂. Given the small size of these studies, a replication of these findings is necessary before drawing any final conclusions about the direct effects of CO₂ on people.

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 per occupant in offices and from 5 to 15 cfm per occupant in schools [45, 46]. 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. The previous section, "Providing Adequate Building Ventilation", lists approaches for increasing time-average ventilation rates with little or no increase in energy use, or even with energy savings.

Self-estimated Performance and Objectively Measured Performance

Many studies have investigated how indoor air quality or perceived indoor air quality is related to people's subjective estimates of their own work performance, i.e., self-estimated performance. At present, however, the validity of these estimates, i.e., the degree to which they relate to actual performance, is not known. Consequently, this review has considered only studies with objective measures of work performance such as work speed and accuracy measured by the researchers as opposed to estimated by the workers.

Estimation of Relative Performance With Changes in Ventilation Rates

Figure 3 shows curves of relative performance versus ventilation rate for reference ventilation rates of 15, 20, and 30 cfm per person. These curves were derived using an equation representing the best-fit composite weighted curve shown in Figure 2 of Seppänen et al. [13]. This best-fit curve is reproduced below.

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 2 and 3. The lowest value of ventilation rate in these tables is 13.8 cfm per person because the original data analyzed by Seppänen et al. [13] did not enable a relationship to be established for lower ventilation rates.

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

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