Air pollution has been a serious environmental problem in China for decades, affecting air quality, climatic condition, and human health. People spend most of their time indoors, and due to the existence of indoor pollution sources and relatively low indoor air exchange level, indoor particulate matter sources tend to be more extensive than outdoor, resulting in higher concentration level, more complex composition and toxicity, and greater harm to human health, which requires more attention and research.

In this study, 65 indoor particulate samples, 36 outdoor particulate samples, and 7 dust samples outside the building in different zones and floors were collected. Experimental analysis methods such as heavy metal content determination, stable isotope analyses (δ¹³C and δ³⁴S), morphology and mineral composition analyses (SEM/EDS, particle size, and XRD), and data analysis methods such as principal component analysis, Pearson correlation analysis, potential ecological risk and health risk assessment methods were used to investigate morphology and mineral composition, distribution and enrichment characteristics of heavy metals, source apportionments, and risk assessment of indoor particulates in urban areas. The major findings of the present research work are as follows:

(1) Indoor particulates were concentrated in 10~100 μm in size and different in shape. Compared to outdoor particulates which contained more main components of the earth's crust (O, Si, Al, and Ca), indoor particulates contained more C, N, and some heavy metal elements. For indoor particulates, besides quartz and calcite, there were dolomite, gypsum and some minerals containing iron and lead.

(2) Concentrations of Cu, As, Cd, Pb, Sb, and Zn in indoor particulates were generally higher than the background values, while concentrations of V, Cr, Co, and Mn were lower than the background values. The concentrations of elements were generally high in indoor particulates in Yaohai Industrial Park (YI), Binhu new district (BH), Government District (GD) and Railway Station (RS). And the concentrations were relatively low in Dongpu Reservoir (DR). The enrichment of heavy metals in indoor particulates was higher in BH, Economic Development Zone (ED) and RS. In comparison to outdoor particulates, heavy metals in indoor particulates were mostly in the form of non-residuals.

(3) Among all the sources of indoor particulates in each zone, indoor sources were dominated. Among indoor sources, cooking and smoking were significant sources. During cooking, both fuel and food had an influence. Outdoor sources (traffic sources, industrial sources, natural sources, and so on) were also important sources of indoor particulates, and different zones were affected by different outdoor sources. YI and Mingzhu Square (MS) were more affected by industrial sources, and other zones were more influenced by traffic sources. Besides, residents acted as "sinks" of heavy metals in particulates.

(4) Indoor particulates in all the zones expressed extremely high potential ecological risks. The potential ecological risks were higher for indoor particulates in the 1st to 5th floors, in the case of smoking, and with the smaller family size. Compared to road dust and air particulates, the potential ecological risk was ranked in the order of indoor particulates > road dust > PM_2.5 > PM₁₀. The carcinogenic risks decreased in the order of Ni > As > Cr > Pb > Cd for both children and adults in different zones, and the carcinogenic risks from Ni, As, and Cr exposure were not negligible. Compared to road dust and air particulates, the carcinogenic risks were ranked in the order of indoor particulates > PM_2.5 > PM₁₀ > road dust.