Exploring the effects of short-term indoor CO 2 exposure on heart rate and cognition in university students: A pilot study

Abstract

Indoor carbon dioxide (CO₂) accumulation in university classrooms is associated with fluctuations in students’ cognitive performance. This study investigated the preliminary associations between short-term CO₂ exposure, heart rate (HR), and cognitive accuracy (ACC), leveraging a small-N intensive longitudinal design with 54 synchronized observation sets over 6 days. Results indicated that HR responded to CO₂ in two stages under the observed conditions: an initial sensitive response with relatively stable HR when CO₂ was below the 1000 ppm reference level, followed by a gradual adaptive decline as concentrations increased. Standardized mediation analysis confirmed a global indirect effect (β = −0.106, p < 0.05). Notably, segmented analysis revealed that this physiological-cognitive coupling was primarily driven by the intensified impact of HR on cognitive accuracy in Phase II (β = 0.389, p = 0.022), whereas the mechanism remained exploratory in Phase I. These preliminary findings suggest that the relationship between CO₂ and cognition may be mediated by autonomic regulation (reflected by HR). Under the observed classroom conditions, 1000 ppm may serve as a guideline-aligned environmental reference associated with physiological–cognitive shifts.

Practical applications

This pilot study suggests that heart rate (HR) mediates the association between indoor CO₂ and cognitive performance in university classrooms. Specifically, CO₂ levels exceeding 1000 ppm were associated with distinct physiological changes and reduced cognitive accuracy, highlighting this value as a critical reference for ventilation control. Practically, these results support implementing occupancy-sensitive ventilation strategies to limit CO₂ accumulation. Additionally, the observed CO₂-HR coupling indicates that aggregate HR trends derived from wearable devices could serve as non-invasive, supplementary indicators of indoor environmental conditions. These exploratory findings inform future human-centric approaches to indoor environmental quality (IEQ) management in educational settings.

Keywords

Indoor environment HR cognitive performance mediation mechanism

Get full access to this article

View all access options for this article.

References

Spengler

Sexton

. Indoor air pollution: a public health perspective. Science 1983; 221: 9–17. https://doi.org/10.1126/science.6857273

Gong

Liu

, et al. Development of a healthy assessment system for residential building epidemic prevention. Build Environ 2021; 202: 108038. https://doi.org/10.1016/j.buildenv.2021.108038

Becerra

Lizana

Gil

, et al. Identification of potential indoor air pollutants in schools. J Clean Prod 2020; 242: 118420. https://doi.org/10.1016/j.jclepro.2019.118420

Branco

PTBS

Sousa

SIV

Dudzińska

, et al. A review of relevant parameters for assessing indoor air quality in educational facilities. Environ Res 2024; 261: 119713. https://doi.org/10.1016/j.envres.2024.119713

Zhan

. Investigation and evaluation of winter indoor air quality of primary schools in severe cold weather areas of China. Energies 2019; 12: 1602. https://doi.org/10.3390/en12091602

Stabile

Buonanno

Frattolillo

, et al. The effect of the ventilation retrofit in a school on CO₂, airborne particles, and energy consumptions. Build Environ 2019; 156: 1–11. https://doi.org/10.1016/j.buildenv.2019.04.001

Korsavi

Montazami

Mumovic

. Indoor air quality (IAQ) in naturally-ventilated primary schools in the UK: occupant-related factors. Build Environ 2020; 180: 106992. https://doi.org/10.1016/j.buildenv.2020.106992

Chen

Y-C

Hsieh

P-I

Chen

J-K

, et al. Effect of indoor air quality on the association of long-term exposure to low-level air pollutants with cognition in older adults. Environ Res 2023; 233: 115483. https://doi.org/10.1016/j.envres.2023.115483

Gil-Baez

Barrios-Padura

Molina-Huelva

, et al. Natural ventilation systems in 21st-century for near zero energy school buildings. Energy 2017; 137: 1186–1200. https://doi.org/10.1016/j.energy.2017.05.188

10.

Turanjanin

Vučićević

Jovanović

, et al. Indoor CO₂ measurements in Serbian schools and ventilation rate calculation. Energy 2014; 77: 290–296. https://doi.org/10.1016/j.energy.2014.10.028

11.

Al-Rashidi

Loveday

Al-Mutawa

. Impact of ventilation modes on carbon dioxide concentration levels in Kuwait classrooms. Energy Build 2012; 47: 540–549. https://doi.org/10.1016/j.enbuild.2011.12.030

12.

Conceição

EZE

Farinho

Lúcio

MJR

. Evaluation of indoor air quality in classrooms equipped with cross-flow ventilation. Int J Vent 2012; 11: 53–68.

13.

Arata

Sugiuchi

Ikaga

, et al. Economic benefits of the effects of office environment on perceived work efficiency and presenteeism. Build Environ 2023; 243: 110712. https://doi.org/10.1016/j.buildenv.2023.110712

14.

Gong

Jiang

, et al. Assessment of COVID-19 infection risks in an office building: field study investigation in shenzhen, China. J Environ Eng 2024; 150: 05024004. https://doi.org/10.1061/joeedu.eeeng-7513

15.

Fiordelisi

Piscitelli

Trimarco

, et al. The mechanisms of air pollution and particulate matter in cardiovascular diseases. Heart Fail Rev 2017; 22: 337–347. https://doi.org/10.1007/s10741-017-9606-7

16.

Daisey

Angell

Apte

. Indoor air quality, ventilation and health symptoms in schools: an analysis of existing information. Indoor Air 2003; 13: 53–64. https://doi.org/10.1034/j.1600-0668.2003.00153.x

17.

Kajtár

Herczeg

. Influence of carbon-dioxide concentration on human well-being and intensity of mental work. Quar J Hungar Meteorol Serv 2012; 116: 145–169.

18.

Zhang

Wargocki

Lian

. Human responses to carbon dioxide, a follow-up study at recommended exposure limits in non-industrial environments. Build Environ 2016; 100: 162–171. https://doi.org/10.1016/j.buildenv.2016.02.014

19.

Block

Calderón-Garcidueñas

. Air pollution: mechanisms of neuroinflammation and CNS disease. Trends Neurosci 2009; 32: 506–516. https://doi.org/10.1016/j.tins.2009.05.009

20.

Schikowski

Vossoughi

Vierkötter

, et al. Association of air pollution with cognitive functions and its modification by APOE gene variants in elderly women. Environ Res 2015; 142: 10–16. https://doi.org/10.1016/j.envres.2015.06.009

21.

Zhou

Huebner

Liu

, et al. Heart rate variability, electrodermal activity and cognition in adults: association with short-term indoor PM_2.5 exposure in a real-world intervention study. Environ Res 2024; 263: 120245. https://doi.org/10.1016/j.envres.2024.120245

22.

Chen

Wang

Tian

, et al. Evaluation of cognitive performance in high temperature with heart rate: a pilot study. Build Environ 2023; 228: 109801. https://doi.org/10.1016/j.buildenv.2022.109801

23.

Deng

Chen

. Development and validation of a smart HVAC control system for multi-occupant offices by using occupants’ physiological signals from wristband. Energy Build 2020; 214: 109872. https://doi.org/10.1016/j.enbuild.2020.109872

24.

Liu

Zhong

Wargocki

. Performance, acute health symptoms and physiological responses during exposure to high air temperature and carbon dioxide concentration. Build Environ 2017; 114: 96–105. https://doi.org/10.1016/j.buildenv.2016.12.020

25.

Kim

Hong

Kong

, et al. Building occupants’ psycho-physiological response to indoor climate and CO₂ concentration changes in office buildings. Build Environ 2020; 169: 106596. https://doi.org/10.1016/j.buildenv.2019.106596

26.

ASHRAE . ANSI/ASHRAE Standard 55-2004: Thermal Environmental Conditions for Human Occupancy. 2004. https://www.ashrae.org/File%20Library/Technical%20Resources/Standards%20and%20Guidelines/Standards%20Addenda/55_2004_a_b_final.pdf

27.

ISO. ISO 7730: 2005 . Ergonomics of the thermal environment. Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. London: BSI, 2005.

28.

Yamagami

Nomura

Kometani

, et al. Early detection of symptom exacerbation in patients with SARS-CoV-2 infection using the fitbit charge 3 (DEXTERITY): pilot evaluation. JMIR Formative Res 2021; 5: e30819. https://doi.org/10.2196/30819

29.

Zhang

Sharp

Scott

, et al. Characterization of post-viral infection behaviors among patients with long COVID: prospective, observational, longitudinal cohort analyses of fitbit data and patient-reported outcomes. JMIR Formative Res 2025; 9: e77644. https://doi.org/10.2196/77644

30.

Ceugniez

Devanne

Hermand

. Reliability and accuracy of the fitbit charge 4 photoplethysmography heart rate sensor in ecological conditions: validation study. JMIR mHealth uHealth 2025; 13: e54871. https://doi.org/10.2196/54871

31.

Gagnon

Khau

Lavoie-Hudon

, et al. Comparing a fitbit wearable to an electrocardiogram gold standard as a measure of heart rate under psychological stress: a validation study. JMIR Formative Research 2022; 6: e37885. https://doi.org/10.2196/37885

32.

Green

. The heart rate method for estimating metabolic rate: review and recommendations. Comp Biochem Physiol Mol Integr Physiol 2011; 158: 287–304. https://doi.org/10.1016/j.cbpa.2010.09.011

33.

ISO . ISO 8996: 2004 ergonomics of the thermal environment—determination of metabolic rate. London: BSI, 2004.

34.

Choi

J-H

Loftness

Lee

D-W

. Investigation of the possibility of the use of heart rate as a human factor for thermal sensation models. Build Environ 2012; 50: 165–175. https://doi.org/10.1016/j.buildenv.2011.10.009

35.

Shaffer

Ginsberg

. An overview of heart rate variability metrics and norms. Front Public Health 2017; 5: 258. https://doi.org/10.3389/fpubh.2017.00258

36.

Georgiou

Larentzakis

Khamis

, et al. Can wearable devices accurately measure heart rate variability? A systematic review. Folia Med 2018; 60(1): 7–20. https://doi.org/10.2478/folmed-2018-0012

37.

Rossi

Pedreschi

Clifton

, et al. Error estimation of ultra-short heart rate variability parameters: effect of missing data caused by motion artifacts. Sensors 2020; 20: 7122. https://doi.org/10.3390/s20247122

38.

Brink

Loomans

MGLC

Mobach

, et al. A systematic approach to quantify the influence of indoor environmental parameters on students’ perceptions, responses, and short-term academic performance. Indoor Air 2022; 32(10): e13116. https://doi.org/10.1111/ina.13116

39.

Lan

Lian

. Use of neurobehavioral tests to evaluate the effects of indoor environment quality on productivity. Build Environ 2009; 44: 2208–2217. https://doi.org/10.1016/j.buildenv.2009.02.001

40.

Zhang

Wargocki

Lian

, et al. Effects of exposure to carbon dioxide and bioeffluents on perceived air quality, self-assessed acute health symptoms, and cognitive performance. Indoor Air 2017; 27: 47–64. https://doi.org/10.1111/ina.12284

41.

Scarpina

Tagini

. The stroop color and word test. Front Psychol 2017; 8: 557. https://doi.org/10.3389/fpsyg.2017.00557

42.

Abbasi

Gong

Korivand

. Transforming stroop task cognitive assessments with multimodal inverse reinforcement learning. Smart Health 2025; 36: 100567. https://doi.org/10.1016/j.smhl.2025.100567

43.

Bolger

Laurenceau

J-P

. Intensive longitudinal methods: an introduction to diary and experience sampling research. New York: The Guilford Press, 2013.

44.

Smith

Little

. Small is beautiful: in defense of the small-N design. Psychon Bull Rev 2018; 25: 2083–2101. https://doi.org/10.3758/s13423-018-1451-8

45.

Efron

Tibshirani

. An introduction to the bootstrap. New York: Chapman & Hall, 1993.

46.

Maxwell

Kelley

Rausch

. Sample size planning for statistical power and accuracy in parameter estimation. Annu Rev Psychol 2008; 59: 537–563. https://doi.org/10.1146/annurev.psych.59.103006.093735

47.

Preacher

Hayes

. Asymptotic and resampling strategies for assessing and comparing indirect effects in multiple mediator models. Behav Res Methods 2008; 40: 879–891. https://doi.org/10.3758/brm.40.3.879

48.

McNeish

Stapleton

. The effect of small sample size on two-level model estimates: a review and illustration. Educ Psychol Rev 2016; 28: 295–314. https://doi.org/10.1007/s10648-014-9287-x

49.

Hayes

. Introduction to mediation, moderation, and conditional process analysis: a regression-based approach. 2nd ed. New York, NY: The Guilford Press, 2017.

50.

WMA. Declaration of Helsinki . Ethical principles for medical research involving human subjects. Jahrb Wiss Ethik 2009; 14: 233–238.

51.

Xiao

Chen

, et al. Impact of fine particulate matter on liver injury: evidence from human, mice and cells. J Hazard Mater 2024; 469: 133958. https://doi.org/10.1016/j.jhazmat.2024.133958

52.

Daellenbach

Uzu

Jiang

, et al. Sources of particulate-matter air pollution and its oxidative potential in Europe. Nature 2020; 587: 414–419. https://doi.org/10.1038/s41586-020-2902-8

53.

Azuma

Kagi

Yanagi

, et al. Effects of low-level inhalation exposure to carbon dioxide in indoor environments: a short review on human health and psychomotor performance. Environ Int 2018; 121: 51–56. https://doi.org/10.1016/j.envint.2018.08.059

54.

Licina

Yildirim

. Occupant satisfaction with indoor environmental quality, sick building syndrome (SBS) symptoms and self-reported productivity before and after relocation into WELL-certified office buildings. Build Environ 2021; 204: 108183. https://doi.org/10.1016/j.buildenv.2021.108183

55.

ASHRAE.

ANSI/ASHRAE Standard 62.1-2022: Ventilation for Acceptable Indoor Air Quality. 2022.

56.

Pulimeno

Piscitelli

Colazzo

, et al. Indoor air quality at school and students’ performance: recommendations of the UNESCO chair on health education and sustainable development & the Italian society of environmental medicine (SIMA). Health Promot Perspect 2020; 10: 169–174. https://doi.org/10.34172/hpp.2020.29

57.

Meiss

Jimeno-Merino

Poza-Casado

, et al. Indoor air quality in naturally ventilated classrooms. Lessons learned from a case study in a COVID-19 scenario. Sustainability 2021; 13: 8446. https://doi.org/10.3390/su13158446

58.

Almeida

RMSF

Pinto

Pinho

, et al. Natural ventilation and indoor air quality in educational buildings: experimental assessment and improvement strategies. Energy Effic 2017; 10: 839–854. https://doi.org/10.1007/s12053-016-9485-0

59.

Wang

You

, et al. The effects of manual airing strategies and architectural factors on the indoor air quality in college classrooms: a case study. Air Qual Atmos Health 2022; 15: 1–13. https://doi.org/10.1007/s11869-021-01074-y

60.

Kulick

Elkind

MSV

Boehme

, et al. Long-term exposure to ambient air pollution, APOE-ε4 status, and cognitive decline in a cohort of older adults in northern manhattan. Environ Int 2020; 136: 105440. https://doi.org/10.1016/j.envint.2019.105440