2022 Yes https://www.sciencedirect.com/science/article/pii/S0048969721073472?via%3Dihub Hu, Dawei Europe Exposure Study Students Elementary (K-5) Middle/Jr High (6-8) High School (9-12) Volatile Organic Compounds (VOCs) Indoor air quality Formaldehyde School classrooms Seasons Indoor average formaldehyde concentrations at those three educational institutions were observed to be in the range 10.96-17.95 ?g/m3, not exceeding the World Health Organization (WHO) guideline value of 100 ?g/m3. As expected, ventilation was found playing an important role in the control of indoor formaldehyde concentration. After opening windows for 30 min, formaldehyde level decreased by ~25% and 38% in the classroom at the elementary school and the high school, respectively. In addition to the primary sources, the objective of this study was also to determine potential secondary sources of indoor formaldehyde in these schools by measuring the other volatile organic compounds (VOCs) present in the classrooms by a Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS). The measurements suggest that the release of residue from tobacco smokers is one of the major sources of indoor HCHO at the high school, which increases HCHO by ~55% averagely within 1 h. Moreover, the control experiments conducted in the university suggests that VOCs such as that released from cleaning products like terpenes, can contribute to the increase of indoor formaldehyde levels through chemical reactions with ozone. This study confirms simple recommendations to reduce the indoors HCHO concentration in schools: use ventilation systems, limit the emissions like cigarette smoke or cleaning products. It also points out that the secondary sources of formaldehyde must be also considered in the classroom.