The presence of contaminants in indoor environments can affect the health of occupants and expose them to several kind of risks (respiratory diseases, allergies, skin/eyes/throat irritation, etc.). During winter season, in addition, concentration of chemicals indoor increases in new or refurbished buildings due to limited ventilation and energy-efficient insulation (windows, doors, walls, etc.) with accentuated negative effects on health. Recent studies have demonstrated that concentration of some air pollutants indoor can exceed that found outdoor. The impacts of indoor air quality on occupant health and comfort are ultimately determined by levels of contaminants and comfort parameters (i.e., temperature, humidity and metabolic substances). It is well known that most of chemicals accumulating indoor are volatile or semi-volatile organic compounds (VOCs and SVOCs, respectively) and their presence is either as gas mixture in indoor air or adsorbed on airborne particulate matter and settled dust. This work represents an initiative of sustainability on campus in which available low-cost technology has been employed to continuously monitor basic parameters, such as carbon dioxide, humidity and temperature inside classrooms of the University of Ferrara. There are a number of relationships that could be useful in discussing carbon dioxide and indoor air quality, such as the impact of carbon dioxide on occupant negative perception of the indoor environment (i.e., low comfort level, high relative humidity levels, body odour) or the relationship between carbon dioxide concentration and the concentration of other indoor contaminants. In this work, carbon dioxide concentration has been used as an indicator of indoor air quality due both to its use as tracer gas for air change rate estimation and to its adverse effects on student's performance and productivity (e.g., concentration, learning ability, specific mental tasks). The study has been conducted on different typologies of classrooms (in terms of maximum number of allowed occupants, indoor volume, architectonic structure, etc.) during teaching activities with the purpose to evaluate, on the one side, if air quality and comfort are appropriate for teaching/learning activities (according to WHO guidelines) and, on the other, to establish whether air ventilation rate is adequate or implementation is required. Acquired data can thus also be useful in defining good practices that help creating and maintaining healthy indoor environments.