Every cloud has a silver lining: Positive effects of Covid-19?

I read an article today in my local paper Dagblad De Limburger about a school district that: “… in the morning when the cleaning service is present and in the evening when they are back at work, all windows are open against each other. There can also be ventilated in between…at the request of teachers, CO2 meters were also purchased to measure the amount of carbon dioxide in the classrooms …” Dit zette mij aan het denken omdat ik herinnerde een aantal artikelen over CO2 en leren.

Normal outside air contains carbon dioxide (CO2) at a level around 400 ppm (parts per million) and an accepted standard in a classroom is 1000 ppm which is not hard to maintain if it’s empty. But if the classroom is full of children, the level rises dramatically because we breathe carbon dioxide out with every breath. A classroom study in California (2013) found CO2 levels can reach 2200 ppm, more than 2X the recommended level, and 3X the level normally in an office setting. A study in Texas (2002) found CO2 levels over 3000 ppm in 21% of the classrooms tested; levels not conducive to efficient learning!

If the windows in a class room are closed all day or if there is poor ventilation in the room, the level of CO2 in the room is increased. That research was about CO2 concentrations and the adverse effect on lessons later in the day. In lessons at the start of the day, and therefore where the classroom could air all night, the learning and learning results were better than in lessons later in the day. Those results were attributed to the increase in CO2 during the day. In search of that article (which I didn’t find) I came across the following research results from Pawel Wargocki, José Alí Porras-Salazar, Sergio Contrertas-Espinoza, and William P. Bahnfleth (2020):

…reducing CO2 concentration from 2,100 ppm to 900 ppm would improve the performance of psychological tests and school tasks by 12% with respect to the speed at which the tasks are performed and by 2% with respect to errors made. For other learning outcomes and short-term sick leave, only the relationships published in the original studies were available. They were therefore used to make predictions. These relationships show that reducing the CO2 concentration from 2,300 ppm to 900 ppm would improve performance on the tests used to assess progress in learning by 5% and that reducing CO2 from 4,100 ppm to 1,000 ppm would increase daily attendance by 2.5%. These results suggest that increasing the ventilation rate in classrooms in the range from 2 L/s-person to 10 L/s-person can bring significant benefits in terms of learning performance and pupil attendance; no data are available for higher rates. The results provide a strong incentive for improving classroom air quality and can be used in cost-benefit analyses.

Petersen, Jensen, Pedersen, en Rasmussen (2016) found that:

Analysis of the total sample suggested the number of correct answers was improved significantly in four of four performance test, addition (6.3%), number comparison (4.8%), grammatical reasoning (3.2%), and reading and comprehension (7.4%), when the outdoor air supply rate was increased from an average of 1.7 (1.4-2.0) to 6.6 l/s per person. The increased outdoor air supply rate did not have any significant effect on the number of errors in any of the performance tests. Results from questionnaires regarding pupil perception of the indoor environment, reported Sick Building Syndrome symptoms, and motivation suggested that the study classroom air was perceived more still and pupil were experiencing less pain in the eyes in the recirculation condition compared to the fresh air condition.

Finally, though I could go on for pages, research at Harvard University by Allen and colleagues (2016) found “statistically significant declines” in cognitive function scores when CO2 concentrations were increased to 950 ppm, which is “common in indoor spaces “. The study found even larger declines when CO2 was raised to 1,400 ppm.

In other words, every cloud – no matter how dark – can also have a silver lining.


Allen, J. G., MacNaughton, P., Satish, U., Santanam, S., Vallarino, J. & Spengler, J. D. (2016). Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic compound exposures in office workers: A controlled exposure study of green and conventional office environments. Environmental Health Perspectives 124, 6.

Corsi, R. L., Torres, V. M., Sanders, M., & Kinney, K. A. (20020). Carbon dioxide levels and dynamics in elementary schools: Results of the TESIAS study. In Indoor Air 2002, the 9th International Conference on Indoor Air Quality and Climate, (pp. 74-79). Monterey, Calif. Espoo, Finland: ISIAQ.

Mendell, M. J., & Heath, G. A. (2005). Do indoor pollutants and thermal conditions in schools influence student performance? A critical review of the literature. Indoor Air Journal; 15, 27–32.

Petersen, S., Jensen, K. L., Pedersen, A. L., & Rasmussen, H. S. (2016). The effect of increased classroom ventilation rate indicated by reduced CO2 concentration on the performance of schoolwork by children. Indoor air, 26, 366–379.

Wargocki, P.; Porras-Salazar, J.A.; Contreras-Espinoza, S., & Bahnfleth, W. (2020). The relationships between classroom air quality and children’s performance in school. Building Environment, 173, 106749.