4 | CONCLUSION
Exposure to high levels of air pollution and microplastics can have an adverse effect on the fetus14 possibly due to deterioration in placental endocrine, metabolic and immunological functions, while a range of effects have been shown in animal models including altered growth and protein expression. There is also evidence that nanoparticulate material may bind viruses and facilitate their entry into cells. Well-controlled epidemiological and experimental studies distinguishing direct and indirect effects of NP are now required as well as determining the timing (early or late) of exposures in pregnancy. More sophisticated 3D models of the human placental barrier are also required. Appropriate use of animal species will enable the possible longer-term impacts on offspring to be evaluated. In human, effects may alter after 11 weeks of gestation with the onset of maternal blood flow to the placenta. Most work to date on transplacental transfer has focussed on solutes, and so methods for the quantification and accurate tracing of particles into and across the placental barrier need improving, including monitoring of location, accumulation and persistence in placental tissue, and metabolic and inflammatory adaptation.