Modern people spend most of their time in indoors, and indoors tend to have higher concentrations of volatile organic compounds (VOCs) than ambient due to the characteristic of closed spaces and various emission sources. Many studies have shown that VOCs can attribute to adverse health effects. The concentration of VOCs in indoor air is affected by human activities, building materials, and ventilation systems. Therefore, it is important to distinguish and quantify the dominant species in indoor air and define the factors affecting the indoor air quality by measuring the VOCs.

To classify the emission sources of various VOCs, Proton Transfer Reaction Time of Flight Mass Spectrometry(PTR-ToF-MS) was used for on-site and real-time measurements. PTR-ToF-MS ionizes VOCs and detects their product ions by proton-transfer-reaction with H₃O⁺(hydronium ions). Since PTR-ToF-MS has high sensitivities, low detection limits, and low fragmentation, it is suitable for real time measurement of various VOCs in the air. In this work, we have alternatively monitored the indoor air and supply air to compare the VOCs emission during work hour and off hour in the office. We have detected 200 of VOCs, and their concentrations showed the different pattern according to the emission sources. By comparing the change in the VOCs concentration according to the presence or absence of occupants and ventilation system, we could define the sources affecting the emission of VOCs into three categories ‘occupants’, ‘ventilation system’, and ‘building’. Some VOCs increased as the residents increases, some VOCs were released from buildings and increased after the closing hour. Most of the VOCs were diluted and decreased by the outside air as ventilation system was turned on.