Indoor and Outdoor Air Fibrous Microplastics in Khuzestan, Iran: Meteorological Influences, Physicochemical Properties, and Health Risks from Associated Heavy Metals

Rising fibrous plastic production necessitates evaluating different aspects of microplastics especially their attributed health effects. In the current study deposited and respirable fibrous microplastics (FMPs) were collected from nine cities across Khuzestan province, Iran, as well as one site with no human activity (total seasonal samples, n = 40) from February 2024–February 2025. Passive sampling was performed using glass cylinders, while active sampling employed a high-volume PM2.5 sampler. Meteorological and air pollution data were collected from NASA and Department of Environment during the study period, respectively. Polymer identification was performed using confocal Raman spectroscopy and microscopy, while elemental analysis was carried out with scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX). Both analyses were conducted by experienced spectroscopists and artificial intelligence (AI) approaches. Statistical analyses included Spearman correlation and one-way ANOVA, with statistical significance set at p < 0.05. The results showed that mean deposited FMPs were 71.5 #/m2.h indoors and 25.5 #/m2.h outdoors, while respirable FMPs averaged 1.284 #/m3 indoors and 0.107 #/m3 outdoors. The indoor-to-outdoor deposition ratio was 2.4, whereas the ratio during vacuum cleaning compared to after vacuum cleaning reached 27.2. For FMPs longer than 5000 μm, significant negative correlations were observed with air temperature (R = -0.394, p < 0.05), while a significant positive correlation was found with relative humidity (R = 0.378, p < 0.05). Black fibers dominated indoor environments (> 32%), whereas semi-transparent fibers prevailed outdoors (> 45%). Polycarbonate was the most frequent polymer. AI successfully identified the polymer for the FMPs that remained undetected by the spectroscopists. Concentrations of Cr, Pb, Co, Ni, Zn, Cd, As, and Cu associated with respirable FMPs ranged from 8.7 × 10−5 to 3.2 × 10− 3 µg/m3. The possible carcinogenic risk was found for Cd, As, Co and Cr (i.e. 1.3×10-6 to 7.5×10-6). Concurrently, hazard quotients for non-carcinogenic effects of all heavy metals did not pose potential health concerns (i.e. HQ < 1).