The concentrations of CO, CO2, O3, and TVOCs were measured 2-3 times per every school day. The measurements were performed both in morning and in the afternoon for 15-20 min long periods by multi-gas sensor probe (model TG 502; GrayWolf Sensing Solutions, Shelton, USA) that was daily calibrated. The levels of formaldehyde were registered with Formaldemeter™ (model htV-M; PPM Technology, Caernarfon, UK) following the same methodology as for other indoor pollutants. In addition, indoor temperature (mean of 18.2 ± 1.1 ºC)

1 INTRODUCTION

As people spend 75-90% of time indoors (offices, schools, homes, etc.) indoor air quality has been recognized as an important factor for human health. The available evidence is rather alarming. According to World Health Organization (WHO) in 2012 4.3 million people died worldwide due to the effects of indoor air pollution (WHO, 2014). The potential health consequences of exposure to indoor pollution indicate the need for further studies in order to fully comprehend the respective health consequences. This is especially relevant for sensitive groups, such as young children who are particularly susceptible to air pollution. Children spend at schools up to 8-9 h per day, which raises an interest in better understanding of air pollution in these environments. Thus this work aims to investigate the levels of indoor pollution at one pre-school located in an urban area. Specifically, the concentrations of indoor health-relevant gases, namely Carbon Monoxide (CO), Carbon Dioxide (CO2), Total Volatile Organic Compounds (TVOCs), formaldehyde (HCHO), ozone (O3), Total Suspended Particulate matter (TSP) and particulate-bound Polycyclic Aromatic Hydrocarbons (PAHs) were evaluated. The risks associated with the exposure to PAHs for 3-5 years old children were also assessed.