Human exposure characteristics of liquid crystal monomers (LCMs) and their mechanisms to cross the skin barrier

INTRODUCTION: Liquid crystal monomers (LCMs) are persistent, bio-accumulative, and toxic substances widely used in liquid crystal displays of smart devices. However, little has been known about the human exposure and health risks of LCMs. Here, the human exposure characteristics of LCMs and their mechanisms to cross the skin barrier were explored.

METHODS: A suite of 60 LCMs were selected as target compounds. A total of 149 dust samples and 43 paired hand–forehead wipe samples of workers were collected from October to November 2020 in an e-waste recycling industrial park in central China. Forty-eight indoor and ninety-seven outdoor dust samples were collected across mainland China from March to August 2017. The estimated daily intake (EDI) based on the above samples was evaluated to assess the workers’ and residents’ exposure risks of LCMs. Three-dimensional human skin equivalents were used to quantitatively assess the percutaneous penetration of LCMs. Transporters in the skin were explored via molecular docking analysis.

RESULTS: LCMs were widely detected not only in e-waste areas but also in residents both indoors and outdoors. The median EDI of the total LCMs (ΣLCMs) via dust ingestion and dermal contact of workers were 48.3 and 16.5 ng/kg body weight/day, respectively, indicating a high occupational exposure risk of LCMs. The median EDI of ΣLCMs via dust ingestion, dermal contact, and inhalation of residents were 1.50 × 10^-2, 2.90 × 10^-2, and 8.57 × 10^-6 ng/kg body weight/day, suggesting exposure risks of LCMs to residents. LCMs with higher log K_ow and molecular weight had more difficulty in crossing the skin barrier. Additionally, ABCG2 (an efflux transporter) may be responsible for the percutaneous penetration of LCMs.

CONCLUSIONS: There were exposure risks of LCMs to both workers and residents. Passive diffusion and active transport may be involved in the percutaneous penetration of LCMs.