Tracing 3D printing emissions: Micro- and nanoplastic formation and the trade-off of mass-based mitigation.

The widespread adoption of fused deposition modeling (FDM) 3D printing introduces undocumented inhalation risks from airborne micro- and nanoplastics (MNPs) in indoor microenvironments. While prior studies have focused on non-specific nanoparticles and volatile organic compounds (VOCs), the precise chemical identity, emission mechanisms, and respiratory fate of printing-emitted MNPs remain critical knowledge gaps. This study systematically quantified MNP emissions across five commercial filament types using size-resolved monitoring, morphological analysis and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). Substantial MNP emissions were observed for ASA and ABS filaments (2.3 ± 1.0 and 0.6 ± 0.3 mg kg-1, respectively). Crucially, morphological evidence of droplet condensation combined with chemical characterization revealed that these emitted MNPs predominantly comprised polyethylene (PE) and polyvinyl chloride (PVC) not originating from the parent polymer matrices. Furthermore, respiratory dosimetry modeling unveiled a mitigation trade-off: while standard FDM enclosures removed > 90% of total nanoparticle mass, they selectively allowed ultrafine nanoparticles to escape, inadvertently driving the pulmonary deposition fraction of inhaled mass from 10.3% to 19.8% in adults, and up to 29.5% in children. These findings highlight that mass-based metrics alone do not fully capture changes in inhalation exposure and respiratory deposition patterns. We advocate for integrating particle number concentrations into indoor air quality standards and requiring disclosure and regulation of all filament constituents to reduce potential MNP exposure at the source.