Low-cost sensor-based investigation of CO2 and volatile organic compounds in classrooms: Exploring dynamics, ventilation effects and perceived air quality relations

Poor classroom air quality (CAQ) can have negative impacts on student health, well-being and performance. Although CO2 concentrations are the proxy most often applied when assessing classroom ventilation and air quality, volatile organic compounds (VOCs) are important contributors to classroom air pollution. However, little is known about the sources and dynamics of VOCs in classrooms and its relation to classroom occupancy, ventilation and perceived air quality. In this study, CAQ was investigated in 75 Danish classrooms throughout November 2021 using low-cost sensors for continuous measurements of occupancy, CO2- and total volatile organic compound (TVOC) concentrations in conjunction with thermal desorption gas chromatography mass spectrometry (TD-GC-MS) analysis of organic compounds. Measurements show significant increases in TVOC concentrations during school hours as well as a day-to-day increase in the morning TVOC levels from insufficient ventilation from preceding days. A weak (r = 0.21) but significant (p < 0.01) correlation was found between classroom CO2 and TVOC. However, unlike the CO2 concentrations, TVOC concentrations were affected by episodic emissions from classroom activities. Ventilation type was identified as a key factor influencing both CO2- and TVOC levels. Although mechanically ventilated classrooms experienced significantly lower CO2- and TVOC levels compared to naturally ventilated classrooms, all classrooms exceeded guideline values of 1000 ppm CO2 with 70% of the classrooms exceeding this more than half of the occupied time. In contrast to CO2, a positive correlation was identified between the median TVOC concentrations during occupancy and the reported perceived CAQ.