Outdoor air quality guidelines have been constantly implemented during the last decades. Nonetheless, no international regulations have been put into action in terms of indoor air quality standards and standardized procedures for indoor pollution measurements. In this study, we investigated the chemical composition of PM 2.5 collected outdoors and indoors at six dwellings located in two Italian areas. The selected sites concerned inland/central and southern Italy, including urban, peri-urban, rural and coastal settings. The seasonal and site-specific particulate matter (PM) variations were analyzed outdoors and indoors, by estimating the impact of the main macro-sources and the contribution of the macro-and micro-components. Outdoors, organic matter represented the main contribution at inland and coastal sites, respectively during winter and summer. A clear, seasonal variation was also observed for secondary inorganic species. A site-specific dependence was exhibited by traffic-related components. Indoors, organic and soil-related species were influenced by the presence of the inhabitants. Some specific tracers allowed to identify additional local source contributions and indoor activities. Although the sampling season and site location defined the outdoor air quality, the higher PM concentrations and the chemical composition indoors were influenced by the infiltration of outdoor air and by the indoor activities carried out by its inhabitants.Atmosphere 2020, 11, 368 2 of 27 discussion of the scientific evidences about the adverse health effects of air pollution. This technical paper highlights that several components contribute to the health effects of PM, but that there is not enough evidence to associate the health outcomes to specific emission sources. The report underlines that most of the evidences are accumulated on cardiovascular and respiratory outcomes, that are generally related to traffic emissions, coal and oil combustion and biomass combustion, particularly in indoor environments located in low-income countries [5].Although scientific literature demonstrated that people spend more than 90% of their time in indoor environments, notably in dwellings, schools, offices and means of transport, no supranational legislation has been implemented in order to encompass indoor air quality standards [6,7]. In addition, no standard procedures have been introduced yet, in order to monitor indoor contaminants.Indeed, the evaluation of the exposure to indoor pollutants is far from being a simple task. The concentration and chemical composition of indoor air, in fact, depends on the release of contaminants from indoor sources as well as on their penetration from outdoors. As far as PM is concerned, the few studies whose focus was set on the chemical properties of particles suspended in indoor environments have required remarkable efforts. For this reason, they mostly concerned specific group of hazardous components such as elements, carbonaceous compounds or classes of species like polycyclic aromatic hydrocarbons [...