Sensor systems, also known as microsensors or low-cost sensors (LCSs) because of their small size and low initial purchase cost, have been developing rapidly in recent years. Their popularity can be explained by several factors: • the relatively low initial purchase cost of these technologies compared to the measuring instruments used in reference methods; • their ease of use and adaptability, which offer a variety of application areas to a broad range of users; • growing public awareness of air pollution and its health impact; • the development of "crowdsourcing"1 citizen sciences and a growing interest among the population in producing and sharing data; • growing demand and interest in connected objects (the Internet of Things2) from parts of the population; • a need to improve knowledge of the large-scale spatial and temporal distribution of air pollution, and to cover different microenvironments; • advances in electronic engineering and computer science enabling management of the large amounts of data generated. These technologies have been and still are the subject of a multitude of studies in different potential application areas. However, the use of sensor systems raises several questions, such as their metrological reliability, the management, use and interpretation of the resulting data and, ultimately, their relevance in addressing air quality issues. In this context, the DGS, DGEC and DGPR issued a formal request to ANSES on 21 December 2018, asking it to: 1. conduct a review of studies using microsensors and the profiles of their users; 2. assess the strengths and limitations, and the complementarity with respect to conventional measurement, of data from microsensors used by citizens to characterise exposure in order to interpret the health implications; 3. discuss the legal status of the data generated by microsensors. In the remainder of this document, the term "sensor system" will be used instead of microsensor, according to the definition proposed by the French Standardisation Organisation (AFNOR).