Lung inflammation found at lowest test dose, study warns safe threshold is below 40 micrograms

Can micro- and nanoplastics — particles too small to see with the naked eye — damage the lungs when inhaled? A South Korean research team has analyzed pulmonary toxicity from repeated exposure to such particles and identified a threshold at which harmful effects begin.

A research team led by Drs. Yoon Seok-ju and Han Hyeong-yun of the Biosimulation Research Center at the Korea Institute of Toxicology (KIT) said Wednesday that laboratory animals exposed to micro- and nano-sized polyethylene particles over 13 weeks showed lung inflammation across all exposure groups.

Harmful effects were detected even in the lowest-dose group, which received 40 micrograms — leading the team to conclude that the lowest observed adverse effect level (LOAEL) is likely below 40 micrograms.

The LOAEL is the lowest exposure dose at which adverse effects are first observed in toxicity testing. It is a key indicator used to assess the risk of a given substance and to establish safety standards.

Concern has been growing over the entry of micro- and nanoplastics into the human body and their health effects, yet quantitative data on the exposure levels at which toxic changes actually occur with repeated exposure remain scarce. Nanoplastics in particular have long been flagged as a potential hazard because their extremely small size allows them to penetrate deep into the lungs through the respiratory tract.

The research team repeatedly administered micro- and nano-sized polyethylene particles — supplied by the Korea Research Institute of Standards and Science (KRISS) — directly into the airways of rats over 13 weeks. They then monitored a four-week recovery period, conducting a comprehensive analysis of pathological changes in lung tissue alongside key inflammatory markers, including inflammatory cytokines and C-reactive protein (CRP).

Inflammatory cells in lung tissue increased and inflammation-related markers rose across all exposure groups. Most significantly, lung inflammation was confirmed even at the lowest test dose of 40 micrograms. The team said this indicates the actual LOAEL may be below 40 micrograms.

Sex-based differences in toxic response were also observed. Inflammation-related markers and lung tissue reactions differed between male and female animals, raising the need to account for sex differences in future risk assessments of micro- and nanoplastics.

The study also applied an intratracheal instillation method that South Korean researchers are developing as a new international standard test under the ISO framework for nanotoxicity assessment. The method is designed to distribute nanomaterials evenly throughout rodent lungs so that toxicity can be evaluated on a consistent basis. The team confirmed that the method can be applied to toxicity assessments of a wide range of nanomaterials, including micro- and nanoplastics.

The researchers said the 40-microgram figure should not be applied directly to humans. Because it represents a dose administered directly to laboratory animals, it does not correspond to typical human microplastic inhalation levels or human safety standards. Its significance, they said, lies in providing foundational data for future human health risk assessments and safety standards that take actual environmental exposure levels into account.

"This research is meaningful in that it presents the LOAEL — a reference point for assessing the pulmonary risk of micro- and nanoplastics," Han said. "We will continue to build the scientific evidence needed to establish human health risk assessments and safety standards through future research that reflects real-world environmental exposure levels."