Ah, indeed. Here a few excerpts from a literature review:

A major issue when determining the risks of microplastics to human health is the lack of information on human exposure. Adequate analytical tools to sample, isolate, detect, quantify, and characterize small microplastics (<10 µm), especially nanosized plastic particles, are urgently needed.

Increased exposure through indoor air, direct swallowing of house dust or dust settling on food (10), and direct exposure to particles released from plastic food containers or bottles, such as polypropylene infant feeding bottles (11), are of special concern. Larger microplastics are likely excreted through faeces, or after deposition in the respiratory tract or lungs through mucociliary clearance into the gut (1, 2). Given the methodological limitations and measurement bias toward larger particles, existing analyses probably underestimate human external exposure and generally do not include the fraction of smaller-sized particles <10 µm, which are likely more relevant to toxicity (1, 12). Notably, internal exposure measurements of plastic particles in human body fluids and tissues are still in their infancy.

Reported concentrations of microplastics in tap and bottled water vary between 0 and 104 particles/litre, with generally greater particle counts for small-sized microplastics (8). The first atmospheric measurements of larger-sized, predominantly fibrous microplastics indicate that plastic particles are a relevant component of fine dust, with, for example, deposition rates in central London ranging between 575 and 1008 microplastics per square meter per day (9).

Limited in vitro and in vivo data suggest that only small fractions of administered microplastics are capable of crossing epithelial barriers of lungs and intestines, with specific uptake profiles and generally increasing uptake efficiency with decreasing particle size (2).

Studies with human cells in culture, and in rodents and aquatic species indicate translocation of microplastics <10 µm from the gut cavity to the lymph and circulatory systems, causing systemic exposure and accumulation in tissues including liver, kidney, and brain (12). Al though the smallest particles (<0.1 µm) may be capable of accessing all organs, crossing cell membranes (12), the placenta (13), and also the brain (14), major knowledge gaps regarding absorption, distribution, metabolism, and excretion (ADME) still exist. Whether there are dose-dependent effects of microplastics in humans also remains unknown.

An additional intriguing, yet understudied, but potentially hazardous property of microplastics is the presence of an eco- or biocorona, i.e., biomolecules and other substances on the surface of the plastic particle, which may influence particle uptake, fate, and effects (6, 13). [Microplastics could act as vehicle for toxic substances]

Vethaak, A. D., & Legler, J. (2021). Microplastics and human health. Science, 371(6530), 672-674. https://doi.org/10.1126/science.abe5041 [but of course it's fucking paywalled]