The Effects of Elevated Carbon Dioxide Levels in Schools

Carbon dioxide (CO2) is a colorless and odorless gas that exists in the atmosphere at a concentration of approximately 0.04% or 400 parts per million (ppm). Humans continuously produce and exhale CO2 as they metabolize carbohydrates and lipids to produce energy in the process of respiration.(Zhang, Wargocki, & Lian, 2015). CO2 concentrations indoors typically exceed outdoor concentrations depending on occupancy, size of the room or space, and ventilation.   An increasing difference between the outdoor and indoor CO2 concentrations can signify a reduced ventilation rate or supply of fresh air. (Persily, 2015). Historically there has been a focus on creating tighter buildings and saving energy through reducing ventilation rates while in some instances inadvertently creating a suboptimal indoor environment for occupant health and productivity. The American Society of Refrigeration and Air Conditioning Engineers (ASHRAE) recommends a specific airflow rate per person and that the difference between outdoor and indoor CO2 concentrations not exceed 700 ppm.  The ASHRAE criteria is based on ensuring the control of bioeffluents (e.g. body odors)(ANSI/ASHRAE, 2016) and not on preventing health effects. Also, the ASHRAE criteria is not enforceable. Thus, CO2 concentrations are measured in indoor environmental quality (IEQ) studies as a proxy for ventilation rates as well as an indicator of general air quality. CO2 concentrations in office buildings typically range from 350-2500 ppm. (Seppänen, Fisk, & Mendell, 1999)

A number of studies conducted in a simulated (controlled) office environment have revealed that CO2 present at commonly found levels indoors (ranging approximately from about 600 to 5000 ppm) can impair cognitive function.  (Allen et al., 2015) (Satish et al., 2012) (Kajtár & Herczeg, 2012) Exposure assessment studies conducted at schools have found elevated carbon dioxide (CO2) levels, in some instances as high as 4000 ppm and 6000 ppm. (Vehvilainen et al., 2016) (Haverinen-Shaughnessy, Moschandreas, & Shaughnessy, 2011)  Studies measuring CO2 levels and student academic performance have varied in design but have utilized CO2 as a surrogate for ventilation rates or indoor environmental quality (IEQ) to determine the effect of ventilation rates on academic performance.   Thus, such studies may only be suggestive of the direct effects of CO2 concentrations on student health and cognition.   Additional studies are needed, in a classroom setting, that control for several confounders to shed light on the potential causal link between elevated CO2 levels and student academic performance and health. 

Twardella et al. (Twardella et al., 2012) assessed the effect of IEQ as indicated by the median CO2 level in the classroom, on the concentration performance (CP), total characters processed (TN) and total number of errors (TE) of 417 nine and ten year old students.   Two test conditions were assessed, poor (median CO2 = 2115 ppm) and improved IAQ (median CO2 = 1045 ppm) were established by mechanical ventilation on two days in one week each in every classroom. Results showed that TE was increased significantly by 1.65 (95% confidence interval 0.42–2.87) in poor compared to improved air quality.  Sidorin et al. (Sidorin, 2015) tested the ability of seventh grade students to solve 5 letter anagram word puzzles requiring mental concentration under low and elevated CO2 concentration conditions (below 1000 ppm and above 2000 ppm, respectively). Two exams were administered each under low and elevated CO2 conditions. For exam 1, ten students were tested in each group while 19 and 25 students were tested in exam 2 in the low and elevated CO2 groups, respectively.  Students were matched in two different groups by academic level.  Students in the elevated CO2 group had almost twice as many errors than the low exposure group.   

Wargocki and Wyon adjusted ventilation rates in classroom settings to investigate the effects on student performance on several tests (subtraction, multiplication, number comparison, logical thinking, acoustic proofreading, reading and comprehension). An improvement in school performance was found by increasing classroom ventilation from about 3.0 to 8.5 L/s (6.4 to 18 cfm) per person as evidenced by the increased speed at which the tests were completed without increased errors.  (Pawel Wargocki & Wyon, 2007) As mentioned earlier, these studies utilized CO2 as an indicator of air quality and rate of ventilation thus it is only suggestive of the direct effects of CO2 on student academic performance since other contaminants can be present when the ventilation rates are low and the classroom is not being exhausted sufficiently.

Haverinen-Shaughnessy et al. examined the association of classroom ventilation rates (as determined by CO2 concentrations) and academic achievement (using standardized tests as a metric of performance) of 5th graders. A linear association was identified between classroom ventilation rates and students academic achievement within the range of 0.9–7.1 l/s per person. For every unit (1 l/s per person) increase in the ventilation rate within that range, the proportion of students passing standardized test (i.e., scoring satisfactory or above) is expected to increase by 2.9% (95%CI 0.9–4.8%) for math and 2.7% (0.5–4.9%) for reading. The researchers pooled schools from two districts to increase the total number of schools (100) and improve the strength of the study.  Continuous CO2 monitoring results from at least one full day from a classroom from each school were correlated with the classroom results from math and reading standardized exams. (Haverinen-Shaughnessy et al., 2011) Since CO2 concentrations depend on many factors (activity of students, number of students, HVAC system, heating or cooling season, operable windows, etc.) and are highly variable and the one day of monitoring may not be representative of the true CO2 levels throughout the school year. 

Gaihre et al. showed that time weighted average (TWA) CO2 concentrations were inversely associated with school attendance but not academic attainments. An increase of 100 ppm CO2 was associated with a reduced annual attendance of 0.2% (0.04, 0.4). (Gaihre, Semple, Miller, Fielding, & Turner, 2014) Elevated classroom CO2 concentration can have an educational cost to children due to school absences and an economic cost to parents due to missing days at work. (Kats, 2006)  Other factors that may affect student performance include socioeconomic status (SES), ethnicity, temperature and humidity, other air contaminants (e.g. mold, VOCs), lighting, noise, classroom size, and teacher effectiveness. (Daisey, Angell, & Apte, 2003)  The effect of these factors need to be considered when evaluating the association of CO2 concentrations and student academic performance and health.

Successful interventions to decrease CO2 levels in schools have been implemented. Norbak et al. utilized a CO2 controlled ventilation system in computer classrooms (with CO2 sensors), to demonstrate that reducing elevated levels of CO2, significantly reduced headache (p=.003) and tiredness (p=.007) and improved satisfaction with the air quality.(Norbäck, Nordström, & Zhao, 2013) Wargocki et al. installed CO2 sensors in classrooms with natural ventilation to provide a visual indicator when air quality was deteriorating. Green diodes signified CO2 levels below 1000 ppm, yellow denoted that it was in the range from 1000 to 1600 ppm, and red diodes indicated that the levels exceeded 1600 ppm. Yellow and red indicators prompted teachers to open windows to reduce CO2 levels. Opening windows maintained CO2  concentrations below 1000 ppm and increased energy use for heating while reduced the cooling requirement in summertime. (P. Wargocki & Da Silva, 2015) Rosbach et al. demonstrated that classrooms with natural ventilation can be retrofitted with a portable mechanical ventilation system to supply more outside air and reduce CO2 levels. The systems were CO2 controlled, using a real-time, self-calibrating CO2 sensor to regulate the amount of outdoor air supplied, and achieve a target steady-state CO2 concentration in the classroom.  A mean decrease of 491 ppm was achieved from a baseline mean of 1335 ppm (range: 763–2000 ppm). 

To address the issue of potentially elevated CO2 levels in a school, its important to establish baseline concentrations. Since CO2 levels fluctuate throughout the day, collecting continuous measurements (datalogging) throughout the day provides a more useful set of baseline data.  Evaluating the baseline data along with other information (e.g. on-site inspection, ventilation performance) should inform any corrective actions and improvements to the school IEQ. Having a school designated as “green” or LEED certified does not necessarily mean that there are no IEQ issues or the CO2 concentrations are being maintained as low as possible.

More focus should be placed on school IEQ specifically CO2 concentrations. The Environmental Protection Agency has a developed a website and guidelines (Tools for Schools) to help schools prevent and address IEQ issues. The site includes guidance and inspection checklists as well as a mobile app.  I believe this is a step in the right direction however these are simply guidelines and additional emphasis is needed on creating standards and regulations for schools to uniformly follow including increased ventilation rates to maintain CO2 levels as low as possible. Current technology can be implemented that reduces energy use while allowing for greater air flow and fresh air in schools.

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