Ventilation with Outdoor Air

Ventilation with Outdoor Air in Schools

Ventilation rates and carbon dioxide concentrations in schools

Rates (amounts) of ventilation with outdoor air influence indoor air pollutant concentrations in schools. Indoor air concentrations of pollutants emitted from indoor sources increase as ventilation rates diminish. On the other hand, indoor air concentrations of some pollutants from outdoor air will increase as ventilation rates increase. Ventilation rates also affect the energy required for heating and cooling, with higher ventilation rates generally increasing energy requirements. Sometimes, increased ventilation can save energy, when conditions enable use of cool outdoor air to reduce the need for air conditioning. In schools without air conditioning, ventilation is commonly employed during periods of warm weather to limit indoor temperatures. Minimum ventilation rate standards have been established, seeking to strike a balance between effects of decreasing ventilation on air quality and energy. For example, a widely used minimum ventilation standard in the U.S. specifies a minimum ventilation rate for classrooms of approximately 7 liters per second (L/s) or 15 cubic feet per minute (cfm) per occupant at the default occupant density [1].

Carbon dioxide concentrations are often used as an easily measured proxy for ventilation rates. When an indoor space is unoccupied and there is air entering directly from outdoors, the indoor concentration of carbon dioxide approaches and eventually equals the outdoor concentration. When people enter the space, indoor concentrations increase over time because people are a source of carbon dioxide. If the number of occupants and the amount of ventilation is consistent over a sufficient period of time, the indoor carbon dioxide concentration will reach a steady value that depends on the amount of ventilation per person. Even though steady concentrations are not always reached, it is possible to use the “peak” or highest measured concentration to indicate if a ventilation standard is being met. Peak indoor carbon dioxide concentrations above approximately 1000 parts per million (ppm) indicate ventilation rates less than 7 L/s (15 cfm) per occupant.

For this IAQ Science Resource Bank, Fisk [2] performed a review of literature on ventilation rates and carbon dioxide concentrations in schools and their associations with the occupants’ performance, health, and absence. The review relied on articles published in refereed archival journals. Carbon dioxide and ventilation rate data were only used when these data represented periods of occupancy. The reported carbon dioxide data were often the maximum or peak values of carbon dioxide concentrations in each classroom - sometimes the highest average concentration in a 30 minute period was used to indicate a peak concentration. From the set of classrooms in each study, average, median, and/or maximum values of the measured peak carbon dioxide concentrations may have been reported. Alternately, or additionally, some studies have reported averages, medians, and/or maximums of the measured time average carbon dioxide concentrations in each classroom.

From this review [2], Figure 4 plots peak values of carbon dioxide from studies with measurements in 20 or more classrooms. The horizontal axis of this figure indicates data sources codes, with typically one code per study, provided in tabulation of study data [2]. For each study, Figure 4 shows, when available, the average, median, and maximum peak carbon dioxide concentrations measured in the set of classrooms. The average and median peak values of carbon dioxide concentration always exceed 1000 ppm and often exceed 2000 ppm. The maximum values of peak carbon dioxide concentration range from 3000 ppm to 6000 ppm. Figure 5 is similar but it shows time average, as opposed to peak, carbon dioxide concentrations during occupancy. Most of the averages and medians of time average carbon dioxide concentrations also exceed 1000 ppm, and maximum values of time average carbon dioxide concentrations range from 1400 ppm to 5200 ppm. The carbon dioxide data plotted in these two figures indicate a broad failure to provide the minimum amount of ventilation to classrooms that is commonly recommended in standards. Because carbon dioxide concentrations often exceed 1000 ppm by a wide margin, the data indicate that ventilation rates in classrooms are often far less than 7 L/s (15 cfm) per occupant.

Several of the identified published studies reported ventilation rates. Often the reported ventilation rates were calculated from measured carbon dioxide concentrations. These ventilation rate data were tabulated by Fisk [2]. Most of these studies reported average or median ventilation rates in the range of 3 to 5 L/s (6 to 11 cfm) per occupant, with one average as low as 1 L/s (2 cfm) per occupant.


1.         ASHRAE, ASHRAE Standard 62.1-2016  Ventilation for acceptable indoor air quality. 2016, American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc.: Atlanta, GA.

2.         Fisk , W.J., The ventilation problem in schools: literature review. Indoor Air, 2017. 27: p. 1039-1051.

Associations of ventilation rates with health and performance

The review of Fisk [1] compiled and summarized the published information on the associations of ventilation rates or carbon dioxide concentrations in schools with student performance, health symptoms or signs, and absence rates. When drawing conclusions, the review considered measures of study quality such as study size and the extent to which studies were able to control for potential confounding by factors other than ventilation rates or carbon dioxide concentrations that may also affect student performance, health, or absence.

In the reviewed research on associations of ventilation rates or carbon dioxide concentrations with student performance, five studies used students’ scores on standard tests of academic achievement to indicate student performance and six studies employed special tests added by the researchers. These special tests typically used speed and accuracy in number addition, multiplication, proofreading, logical thinking, and similar activities to indicate performance. Based on a review of the 11 studies, Fisk [1] concluded that the available research provided “compelling evidence of an association of improved student performance with increased classroom ventilation rates.” Overall, eight out of 11 studies reported statistically significant improvements in at least some measures of student performance with increased ventilation rates or lower carbon dioxide concentrations, and a ninth study reported a statistically significant improvement in performance when applying a less stringent than typical criterion for statistical significance. Five of the studies were intervention studies that increased ventilation rates and measured changes in performance within students. The intervention studies are less subject to error due to confounding by factors other than ventilation rate or carbon dioxide concentration. All of the intervention studies reported statistically significant improvements in some aspects of performance with increased ventilation rates. The reported improvements in performance with increased ventilation rates were typically a few percent, but ranged as high as 15%.

The review [1] identified 11 studies of the associations of school ventilation rates with either health symptoms determined via questionnaires or measured signs of health such as indicators of inflammation in nasal passages or nasal patency which indicates openness of the nose. Most of these studies have focused on measures of respiratory health such as nasal symptoms or wheeze. Eight out of 11 studies reported statistically significant improvements in at least some health symptom or sign of health with increased ventilation rates, although all studies also found some health symptoms or signs to not be associated with ventilation rate. Fisk [1] concluded that the available research “suggests improvements in measures of respiratory health with increased ventilation rates, but the evidence of improvement in health is not as compelling as the evidence of improvements in student performance.”

The review [1] also identified five studies of the association of ventilation rates or carbon dioxide concentrations in schools with total absence or illness absence of students. Four out of five studies found statistically significant decreases in absence rates with more ventilation or lower carbon dioxide concentrations. The strongest study, which followed 162 classrooms for two years [2], found a 1.6% decrease in absence for each 1 L/s (2 cfm) per person increase in ventilation rate. Because ventilation rates in most classrooms can be increased by at least a few L/s (several cfm) per person, the study indicates a potential to reduce absence by several percent. Overall, the available research indicates that increased ventilation rates in classrooms are associated with reduced student absence. For elementary and middle school students, reduced student absence has been shown to be associated with higher grade point averages and higher scores in academic achievement tests [3].

1.         Fisk , W.J., The ventilation problem in schools: literature review. Indoor Air, 2017. 27: p. 1039-1051.

2.         Mendell, M.J., et al., Association of classroom ventilation with reduced illness absence: a prospective study in California elementary schools. Indoor Air, 2013. 23(6): p. 515-528.

3.         Gottfried, M.A., Evaluating the relationship between student attendance and achievement in urban elementary and middle schools an instrumental variables approach. American Educational Research Journal, 2010. 47(2): p. 434-465.


Corrective Measures

There are very few published studies of methods to prevent and remediate the widespread problem of low ventilation rates in schools; thus, the following comments rely substantially on judgment. It is clear from the carbon dioxide data in naturally ventilated schools that schools cannot consistently rely on opening of windows sufficiently to provide the recommended minimum ventilation rates. Carbon dioxide sensors that provide a visual warning signal when carbon dioxide concentrations are elevated may help to prompt windows use [1]. Mechanical ventilation with outside air is recommended and the ventilation control systems need to assure that sufficient ventilation is provided even when the need for heating and cooling is minimal. Mechanical ventilation systems should not be noisy because noisy systems are often turned off. More information on best ventilation practices is available in the section of this website on ventilation.

1.         Wargocki, P. and N.A. Da Silva, Use of visual CO2 feedback as a retrofit solution for improving classroom air quality. Indoor Air, 2015. 25(1): p. 105-14.