"On average, people could actually be ingesting approximately 5 grams of microplastics every week - that’s the equivalent of a credit card." That's according to a study by WWF, Dalberg, and the University of Newcastle, Australia.
From the report (PDF):
The long-term effects of plastic ingestion on the human body are not yet well documented. But studies have shown that beyond a certain exposure level, inhalation of plastic fibres seem to produce mild inflammation of the respiratory tract. In marine animals, higher concentrations of microplastics in their digestive and respiratory system can lead to early death. Research studies have demonstrated toxicity in vitro to lung cells, the liver, and brain cells.
Some types of plastic carry chemicals and additives with potential effects on human health. Identified health risks are due to production process residues, additives, dyes and pigments found in plastic, some of which have been shown to have an influence on sexual function, fertility and increased occurrence of mutations and cancers. Airborne microplastics may also carry pollutants from the surrounding environment. In urban environments, they may carry PAHs – molecules found in coal and tar − and metals.