# Effects of Night Ventilation on Indoor Air Quality in Educational Buildings--A Field Study
## 1 Introduction  
Ventilation plays an important role in supporting wellbeing indoors [1, 2, 3]. Research literature has shown that poor ventilation is usual in schools, worsening the health symptoms of individuals [4, 5, 6, 3]. Furthermore, poor indoor air quality may have a significant influence on learning [7, 8, 9]. However, taking care of health and wellbeing has caused additional ventilation use which rises the energy consumption of buildings. The European standard 15251:2007 recommends using pre-started or continuous minimum ventilation in unoccupied hours to remove material emissions before space usage [10]. In Nordiccountries, a usual method has been to stop mechanical ventilation after the building usage and pre-start the ventilation around 2 h before the occupied periods. An alternative ventilation method has been to use night ventilation at 30-50% partial power that can remove material emissions. A third method has been to use intermittent ventilation during unoccupied periods, and in this way, achieve a minimum night-time ventilation level in spaces on average. In earlier studies, [PERSON] et al. [11] discussed that increased TVOC concentration in the mornings may occur due to off-gassing of building materials because the ventilation is not used at night. [PERSON] and [PERSON] [12] concluded that a required purging rate of pollutants is closely related to the air distribution methods, prevailing buoyancy forces, as well as room size. In schools, a usual method has been to flush indoor air through the windows or doors after lectures [13; 14; 15]. [PERSON] and [PERSON] [16] discussed that classroom occupants can be exposed to harmful concentrations because emissions may accumulate without proper ventilation during the unoccupied periods. Earlier research shows that night ventilation has been typically used to enhance indoor thermal conditions [17; 18; 19; 20; 21; 22]. Even though many standards and guidelines have been provided for health-based ventilation [10; 23; 24], different night ventilation methods have been relatively rarely studied regarding organic pollutant levels in the mornings at the beginning of space use. Many hypotheses have been proposed on how indoor environments deteriorate during unoccupied hours without continuous ventilation. The first hypothesis states that by stopping the ventilation, harmful concentrations can increase in indoor air during unoccupied hours, and at the same time, may affect the daytime concentrations. In this sense, the flushing period is required also after the space usage. The second hypothesis is the concern that the indoor humidity cannot be extracted if the ventilation is off. The third hypothesis is that the start-up of ventilation will increase the concentrations of particulate matter in the morning. Continuous ventilation can be thought of as a solution to these problems, but at the same time, it can unnecessarily increase the energy consumption related to the management of indoor climate conditions. CO\({}_{2}\) usually indicates human-originated bio emissions in indoor environments. However, the material or biological emissions can be originated also from many other indoor and outdoor sources. Generally, organic and microbial metabolites may have considerable effects on health, but it is difficult to show which compounds are responsible for causing diseases and symptoms [25]. For this reason, a general trend of volatile organic compounds (VOCs) was monitored by measuring a total amount of those compounds (TVOC) by conducting the metal oxide semiconductor method. This means however that the individual VOCs cannot be identified. Regardless of that, it has been shown that TVOC can be a reasonable indicator for indoor material emissions [26; 27; 28]. In addition to TVOC, particulate matter (PM) is a common harmful contaminant of indoor environments. Particulate matter is defined as a widespread air pollutant, consisting of a mixture of solid and liquid particles suspended in the air [29].