Direct µ-FTIR analysis of microplastics deposited on silicon in indoor air environments

To date, research on microplastic (MP) pollution has focused on its presence, characteristics, and impacts in aquatic and terrestrial environments. Airborne MPs, particularly in indoor environments, have received far less attention, despite the fact that this is where people spend most of their time, and indoor MP concentrations typically exceed those found outdoors. Here, we investigated MP fallout onto silicon in residential areas and an academic chemistry building in Oxford, Mississippi. Because silicon is mostly transparent in the mid-infrared range, this sampling approach allows for direct analysis using micro-Fourier transform infrared spectroscopy (µ-FTIR), negating contamination from sample preparation and speeding analyses. The direct analysis approach reduced particle loss compared to a traditional method where fallout in a container is transferred by rinsing and filtering its contents before analysis. The deposition rate (MPs/m2/d) among the fifteen sampling sites ranged from 12.6 × 103 to 159 × 103. The abundance of MPs for the bedrooms was significantly (p < 0.05) higher than those in the other room types. MPs < 100 μm constituted ~90% of the total number of MPs, and most of these were fragments, whereas MPs > 100 μm were primarily fibers. The dominant polymers (~83.6%) were those of the nylon family, especially polyamide and polyarylamide, which were sourced to carpets, clothing, and blankets. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) revealed rough MP surfaces predominantly composed of carbon and oxygen, with varying O/C ratios across individual particles. Overall, we conclude that indoor environments are prone to MP pollution that varies with MP sources and characteristics of indoor settings, and that direct deposition onto silicon can be a useful approach for chemical imaging of fallout by FTIR microscopy.