Indoor air contaminants, symptoms and effects of mechanical ventilation in school buildings

Research output: ThesisDoctoral ThesisCollection of Articles


According to a recent national survey, significant indoor air quality (IAQ) problems occur in almost 20% of municipal school buildings in Finland. The causes of the problems are often difficult to detect, and this leads to ambiguous and challenging situations. Mechanical supply and exhaust ventilation plays a major role in maintaining the IAQ of highly occupied buildings, such as schools, but poorly functioning systems result in occupant discomfort and health problems. This study aimed to identify the differences in IAQ between schools with and without reported complaints. In addition, the effects of ventilation system interventions were monitored. Perceived and measured IAQ were studied in 40 classrooms in six mechanically ventilated comprehensive schools in the Helsinki metropolitan area. Schools were categorized as 'problematic' (n = 4) and 'comparison' (n = 2) schools, based on the known history of occupants' IAQ complaints. Occupant perceptions were recorded with a commonly used indoor air survey and a novel online questionnaire. Altogether, 14 parameters which contribute to the IAQ, such as gaseous, particulate matter (PM) and biological pollutants, were measured. As a new approach, indoor airborne fungi were identified from the exhaust air filter dust of the air handling units of the schools. In addition, the application of a moderate (5–7 Pa) positive differential pressure (dPa) and adjustments to a hybrid ventilation system were studied. A total of 105 teachers and 1268 students responded to the questionnaires. The responses to the two questionnaires were in agreement, and also corroborated the categorization of the schools as problematic or comparison schools. The online questionnaire showed significant differences between classrooms. Median relative humidity in the schools was 15–27%. Gaseous pollutant, PM and endotoxin levels were low, and there were no notable differences between the two types of schools. The sampling of airborne cultivable microorganisms and cultivation of settled and filter dust showed potential in identifying schools with a significant amount of occupant complaints. The total concentration of the airborne microbes exceeded the outdoor concentration in 58% of the samples in the four problematic schools, and in 8% of the samples in the two comparison ones. Representative species of the genus Trichoderma, including mycoparasitic, biocide-resistant species such as T. atroviride and T. trixiae, and potentially pathogenic, biocide-sensitive species as T. citrinoviride and T. longibrachiatum, were found only in the problematic schools. After ventilation interventions, a change in the indoor mycobiota composition was observed, and minor but indicative changes in symptoms were reported. Importantly, the positive dPa did not promote the transfer of moisture through the structure and the moisture excess was kept low. Further, even a drying effect was observed during the measured moisture excess of 0.02 g/m3.
Translated title of the contributionSisäilman epäpuhtaudet, koetut oireet ja koneellisen ilmanvaihdon vaikutukset koulurakennuksissa
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Salonen, Heidi, Supervising Professor
  • Kurnitski, Jarek, Thesis Advisor
  • Mikkola, Raimo, Thesis Advisor
Print ISBNs978-952-60-3993-0
Electronic ISBNs978-952-60-3994-7
Publication statusPublished - 2020
MoE publication typeG5 Doctoral dissertation (article)


  • indoor air quality
  • school building
  • ventilation
  • positive pressure
  • indoor climate survey
  • indoor mycobiota


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