Atmospheric Microplastics in Indoor and Outdoor Air: Abundance and their Potential as Vectors for Heavy Metals

Microplastics (MPs) have emerged as pervasive atmospheric pollutants, yet their occurrence and characteristics in urban air environments remain poorly characterized. This study quantified the abundance, morphological features, polymer composition, and elemental associations of airborne MPs in indoor and outdoor environments in Marikina, Philippines. Airborne particles were collected using a respirable dust sampler and analyzed through microscopic observation, attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX). Results revealed significantly higher MP concentrations in outdoor air (0.341±0.038 MP/m³) than indoors (0.155±0.106 MP/m³) (p=0.0454), with estimated daily intakes ranging from 0.5-3.2 MP/day indoors and 2.1-4.3 MP/day outdoors. Fibers were the predominant morphology in both environments, accounting for 80.0-95.7% indoors and 61.3-84.0% outdoors. Polymer analysis identified indoor MPs mainly as polyvinyl chloride (PVC, 50.0%), polyethylene (PE), and polyurethane (PU), whereas outdoor samples showed a broader diversity, including polyamide (PA), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), and high-density polyethylene (HDPE). Elemental analysis revealed the adsorption of toxic metals such as titanium (Ti), chromium (Cr), lead (Pb), mercury (Hg), and cadmium (Cd), with Pb concentrations reaching up to 18.13% in indoor PE fibers and Cr up to 18.29% in outdoor HDPE. These findings demonstrate that outdoor environments harbor higher concentrations and more chemically complex MPs, emphasizing the potential risk of human inhalation exposure. The study underscores the urgent need for monitoring airborne MPs and developing mitigation measures to reduce human and environmental health in urban areas.