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 . 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. There is evidence that the level of thermal comfort is a better predictor of work performance that indoor air temperature, but this evidence is not sufficient for firm conclusions.
Table 5 provides key summary information from studies of the impact of temperature or thermal comfort on work performance that were not available when Figure 1 and Equation 1 were developed. The results of these studies are discussed in the section on Temperature and Office Work Performance.
Table 6 provides summary information from studies that compared performance when temperatures drifted up or down by 8 to 11 °F to performance when the temperature was steady at the midpoint value. The results of these studies are also discussed in the section on Temperature and Office Work Performance.
|Study||Subjects||Study Design||Temperatures||Results [Notes]|
|||19 office employees||Field study, number of key strokes monitored, temperatures varied naturally||75.4-83.1 °F||Number of correct key strokes increased with temperature. [Unknown if number of correct keystrokes is a useful measure of performance. Unclear how well study controlled for other factors that may affect keystrokes]|
|||35 college students||Laboratory study, temperature varied between sessions, test of working memory (as described includes attention and response speed), test of text memory (recall of facts from previously read text)||70 °F, 77 °F,
|Performance in "working memory" test was significantly reduced at 84 °F relative to 70 °F and 77 °F. Performance in "text memory" test was not significantly affected by temperature. The authors noted the following limitations of the text memory test: "The three texts did vary slightly in difficulty and this may have confounded the results, despite the counterbalancing procedures. The task may also have been too short to capture the effects of temperature as it only lasted for 5 minutes."|
|||14 males age 21 to 23||Laboratory study, temperature and amount of clothing varied between sessions, test of performance in multiplication task.||77.9 °F, 83.3 °F, 88.7 °F (amount of clothing was also varied)||Based on application of model to data from this study and a prior study, relative performance increased with thermal satisfaction, but paper does not state whether the relationship is statistically significant. [Subjects were paid a bonus for higher performance. Highly motivated subjects are often considered less affected by environmental conditions of limited duration.]|
|||30 female college students||Laboratory study, temperature varied between sessions, test of performance in addition task, in some sessions subjects received financial bonus for higher performance||71.6 °F and 77 °F||When subjects were financially motivated to perform better, performance in the addition test was better at 71.6 °F relative to 77 °F. When not financially motivated, performance was not significantly affected by temperature. [Many other authors have suggested that work performance is less (not more) affected by temperature when workers are highly motivated.]|
|[51, 52]||21 college students, age 18-20||Laboratory study, temperature varied between sessions, 13 neurobehavioral test of performance||62.6 °F, 69.8 °F, and 82.4 °F||Overall slight increase in accuracy and slight decrease in response time at 62.6 °F and 82.4 °F, relative to 69.8 °F. [Subjects were paid a bonus for higher performance. Highly motivated subjects are often considered less affected by environmental conditions of limited duration.]|
|[2, 53]||12 college students age 21-25||Laboratory study, temperature varied between sessions, neurobehavioral tests of spatial orientation, memory, attention, etc.) and office tasks (typing, addition)||71.6 °F vs. 86 °F||For all tasks except text typing, performance was decreased at 86 °F by 1.4% to 25%, but not all decreases were statistically significant. [Also, Lan et al.  performed an analyses of the joint results of three studies and derived a relationships of performance with both thermal comfort and temperature. The relationship with temperature was similar to that in ]|
|||7 adult male software programmers||Field study, work performance based on number of key strokes and computerized test of reaction speed||Generally, the temperature varied from 73.4 to 82.4 °F||Key strokes and reaction speed increased with temperature. Because of high air speeds and clothing levels, subjects were thermally neutral at 83.5 °F. Thus, performance increased as thermal comfort improved. Unknown if number of correct keystrokes is a useful measure of performance.]|
|||40 college students||Laboratory study, temperature varied between sessions, computer tests of addition, positioning, text typing, computer performance assessment battery of pattern comparison, spatial rotation, memory with distraction, etc.||77.9 °F, 82.4 °F, 91.4 °F||For most performance measures, temperatures did not significantly affect performance. Measures of fatigue increased as temperature increased above 77.9 °F|
|||96 adults age 20-23||Laboratory study, temperature varied between sessions, computerized test of arousal and alertness||68 °F, 73.4 °F, 78.8 °F||In arousal test, accuracy improved and speed increased with increased temperature. [Subjects adjusted clothing during sessions to maintain thermal comfort.]|
|||32 adult females age < 25||Laboratory study, temperature and clothing varied between sessions, tests of simulated office work: typing, addition, creative thinking, proof-reading and multiplication.||74.3 °F, 75.2 °F, 84.0 °F, 84.2 °F (amount of clothing was also varied)||No statistically significant impact of temperatures on performance.|
|||18 adults||Laboratory study, temperature varied between sessions, in two of three hours per session subjects used personal thermal conditioning system with local fans and heaters to maintain comfort, tests of typing, math, and logical thinking performance||64.4 °F, 68 °F, 76.1 °F, 82.4 °F, 86 °F||No statistically significant impact of temperatures on performance during the periods without personal thermal conditioning (PTC) systems. With temperature of 82.4 °F logical thinking was improved by PTC system. With temperature of 64.4 °F, math score was improved by the PTC system. No significant impacts of PTC on performance for other test conditions. [The short one hour time elapsed under each condition is a study limitation.]|
|Study||Subjects||Study Design||Temperatures||Results [Notes]|
|||23 to 29 college students||Laboratory study. Results from sessions with intentional increases or decreases in temperatures compared to results from sessions with temperature maintained steady at the midpoint value. Simulated office work used to evaluate performance in addition, reading and comprehension, and proofreading.||
Experiment 1 generally compared a steady 75.9 °F to ramps of 71.6 to 79.5 °F.
Experiment 2 compared steady 70.7 °F to ramps of:
64.9 to 76.6 °F
|There were no consistent and statistically significant impacts of ramps of temperature on work performance. [Occupant clothing was fixed at a light value in Experiment 1 and a moderate value in Experiment 2.]|
|||25 college students||Laboratory study. Results from sessions with intentional increases or decreases in temperatures compared to results from sessions with temperature maintained steady at the midpoint value. Simulated office work used to evaluate performance in addition, reading and comprehension, proofreading, and typing.||
Simulated summer experiment compared steady 75.9 °F to ramps of 71.6 to 80.2 °F
Simulated winter experiment compared steady 70.5 °F to ramps of:
66.2 to 74.8 °F
|There were no consistent and statistically significant impacts of ramps of temperature on work performance. [Occupant were allowed to adjust their clothing levels to maintain comfort.]|
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.
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 . 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 7 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 because concentrations of many other indoor air pollutants tend increase as the indoor CO2 concentration increases. However, based on the results of chamber studies from Hungary [60, 61] and a subsequent study in the U.S. , the conventional wisdom about no direct effects of CO2 must be questioned. In the studies performed in Hungary, 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. In the 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. All other conditions were maintained constant, including the outdoor air ventilation rate and air temperature. 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. Given the small size of these studies, replications 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 [63, 64]. 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. . 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.  did not enable a relationship to be established for lower ventilation rates.
(cfm per person)
15 cfm per person
20 cfm per person
30 cfm per person
|Relative Performance||Relative Performance||Relative Performance|