Mice are a common model organism for evaluating the health effects of potentially toxic or harmful agents in humans, and thus the effect of microplastics on them has been analyzed. In pooled data with zebrafish, intestinal bacteria were significantly perturbed in the presence of microplastics; populations of Chlamydia, Proteobacteria, and Firmicutes in the GI tract increased while the population of Bacteroides in the GI tract decreased. 5 m polystyrene particles at 1 mg/L concentrations in mice decreased intestinal mucus, perturbed the intestinal microbiome, and perturbed their livers (with decreased liver weight and altered gene expression and metabolic enzymes amounts). With larger doses (15 and 20/mg/kg/d polystyrene, greater than estimates of human doses of 14-700 mg/d), oxidative stress, GI inflammation, decreased intestinal mucus, and liver toxicity were observed, though the larger doses may make the results less applicable at ordinary levels of consumption. In addition, changes in the uterus, fibrosis, and altered gender ratio and metabolism in children were observed, indicating that microplastics may exert estrogenic effects. In addition, thyroid disruption was also observed. AChE enzyme inhibition and alterations in its gene expression were noted as well. The internal wear of implants containing plastics has been a known source of microplastic deposition in people and in other mammals such as dogs. The particles induce fibrin deposition and necrosis in the nearby joint areas, while macrophages take up the polymer. Particles were found in the aorta of a dog who had received an implant eighteen months earlier, indicating that sufficiently small microplastic particles can be transported throughout the organism. Polyethylene particles formed by implant wear in humans were taken up by macrophages and suppressed their ability to clear bacterial infections. The effect of the particles depends on their composition. Researchers used a simulated multicompartment digestive tract with human intestinal bacteria to characterize the effect of PET microplastic particles on humans. At doses of 166 mg/d, degradation of the PET particles was observed, and the colonic bacteria in each compartment were altered distinctly. PET degradation and biological effects of microplastics on the human intestinal microbiome are thus reasonably expected. In ground glass nodules (GGN) (lesions in human lungs), cotton, rayon, and microplastic fibers (>20 m) were found in a larger fraction of the lesions than in lung tissue. The incidence of microplastic-containing GGN correlated positively to age and to the detection of cancer. The microfibers were more degraded in GGN than in lung tissue; rougher fibers may either be more toxic or immunogenic themselves or may release toxic chemicals into the surrounding tissue. Finally, a recent article studied microplastics found in the feces of people with inflammatory bowel disease (IBD) (either Crohn's disease or ulcerative colitis) and without IBD. While most of the microplastics found were > 300 m, more of the smaller particles (< 30 m) were found in people with IBD than in people without IBD, with PET, PVC, and polyamide particles the most prevalent. The number of particles found in feces correlated positively with the severity of IBD, likely due to increased leakage through the gastrointestinal tract linings of IBD patients.