Keywords: 
• liquid crystal monomers /
• physicochemical properties /
• pollution characteristics /
• environmental behaviors /
• human exposure

Abstract: Liquid crystal monomers (LCMs) are a class of synthetic organic chemicals with a diphenyl or bicyclohexane backbone with hydrogen atoms on the phenyl rings replaced with functional groups such as bromine, chlorine, cyano groups, or fluorine. LCMs have been used widely in liquid crystal displays (LCDs) in electrical and electronic products such as computers, mobile phones, and televisions. The rapidly developed LCD industry has also led to a continuous increase in the annual production volume for LCMs around the world. It has been estimated that demand for LCMs use in LCDs have rose to ~1 300 t by 2021. It has recently been suggested that LCMs should be considered to be emerging persistent organic pollutants of the environment because they may be persistent, bioaccumulative, and toxic. LCMs are not covalently bonded to any of the other materials used in LCDs, so can be unintentionally released into the environment during manufacturing, use, disposal, and recycling of LCD devices and pose serious risks to ecology and human health. Environmental and toxicological research on LCMs in the scientific community has just recently been initiated, and information is deficient in many fields. In this paper, information published since 2018 about LCMs physicochemical properties, production, and emissions; analytical methods for accurately quantifying LCMs; LCMs concentrations, behaviors, and fates in various environmental matrices; and the risks posed by LCMs to humans is reviewed. Recent studies have mainly focused on target LCMs concentrations and distributions, and most studies have involved gas chromatography mass spectrometry with target ions or ion pairs monitored. Many unknown LCMs may be overlooked using such methods because data for LCMs production volumes and use in commercial LCDs are limited. It has previously been found that E-waste dismantling caused large amounts of LCMs in waste LCD panels to be emitted in E-waste recycling areas. LCMs have been detected in sediment, soil, municipal landfill leachate, indoor and outdoor dust, air, and other environmental media. This indicates widespread LCMs contamination of the environment, particularly in E-waste processing areas. Most LCMs are semi-volatile organic compounds, so air is expected to be the main medium in which LCMs released from e-products are transported to other areas. These results also provide indirect evidence that humans are exposed to LCMs in environmental media through ingestion, dermal contact, and inhalation. To date, LCMs have been detected in human tissues in only four studies, and the systematic exploration of bio-monitoring regarding LCMs in the human matrices is still scant. Fluorinated LCMs have been found to be the dominant LCMs in both environmental media and biota, and exhibit higher persistent, bioaccumulative, or even toxic potential than other LCMs. Future studies should focus on developing methods combining target and non-target analysis for identifying and quantifying novel LCMs. It is suggested that large-scale field research should be performed to investigate LCMs migration and transport mechanisms from E-waste recycling areas to other environmental matrices. Studies on bioaccumulation characteristics of LCMs in biota should also be performed to allow the risks posed by LCMs to humans to be effectively assessed.