This experimental study focusses on air quality monitoring by low-cost and accurate sensors to provide a rank of air quality indices for citizens community in smart cities. Inter-comparison long-term measurements of CO, NO ₂ , SO ₂ and PM ₁₀ are performed in a real scenario using referenced chemical analyzers at air-quality monitoring stations. The accuracy of the low-cost sensors is assessed in order to address the Data Quality Objectives (DQO) of the EU Air Quality Directive 2008/50/EC for indicative measurements. These preliminary results show that the low-cost sensors, when accurate, are suitable to define a rank of individual air quality index (AQI) to inform effectively general public and enhance environmental awareness. Finally, we show that the developed multiparametric sensor-system NASUS can be integrated both in a distributed city-network based on cost-effective fixed nodes, and in portable handheld sensor-systems to monitor air-pollution personal exposure.