Abstract
Indoor air is likely a major route of human exposure to microplastics (MPs), yet indoor airborne concentrations from active sampling span nearly four orders of magnitude, from below 1 to more than 1000 particles m−3. We assessed whether this variability reflects real differences or methodological heterogeneity by synthesizing 33 primary studies (2017–2026) on MPs in indoor air, deposition, settled dust and air-conditioner filters, recording the sample type, lower size limit and identification technique. Reported concentrations were negatively associated with the lower size limit (Spearman’s ρ = −0.58, 95% CI −0.88 to 0.05, n = 12), and the particles counted changed with it: studies resolving particles of a few micrometers were dominated by fragments, whereas those limited to larger particles found mainly fibers. Sample type and spectroscopic confirmation further shaped reported abundances. Polyester, polyamide, polypropylene and polyethylene prevailed across settings, pointing to textiles and furnishings as the main sources. MPs have been detected in human lung tissue and sputum, but estimated inhalation doses range from tens to tens of thousands of particles per day. Comparing indoor MP concentrations without accounting for methodological factors risks mistaking measurement differences for differences in exposure. We identify priorities for harmonized reporting, size-resolved sampling and exposure-relevant toxicology.