While people are exposed to micro-plastics from food, water, beverages and air every day, the extent to which these plastics accumulate in the human body and the potential health risks remain unknown. Researchers report in the American Chemical Society's journal Environmental Science and Technology that a new model of micro-plastic exposure has been developed that can explain variable levels across sources and across populations. The new model shows that the average mass of micro-plastics accumulated is lower than previous estimates.

There are various sources of ingestion of micro-plastics, such as bottled water, salt and seafood. Once they enters the human body, their fate and flow are unknown. In addition to potentially causing tissue damage and inflammation, micro-plastics can also be a source of carcinogens and other harmful compounds.

Previous studies have attempted to estimate human exposure to these micro-plastics and their leached chemicals, but these studies have had limitations, including differences in the databases used, failure to consider the entire range of micro-plastic sizes, and average exposure rates used that did not reflect global intake.

Nur Hazimah Mohamed Nor, Albert Koelmans and their colleagues wanted to develop a comprehensive model to estimate lifetime exposure of adults and children to micro-plastics and their associated chemicals. The model consists of two parts: a plastic model to assess the exposure and accumulation of micro-plastics in the human body, and a chemical model. For the chemical model, four representative chemicals were investigated: BaP, DEHP, PCB126 and lead.

"When assessing the role of micro-plastics as chemical carriers in the body, researchers have tended to assume a worst-case scenario in their calculations, i.e. 100% instantaneous leaching of the chemical," the researchers wrote.

To build their model, the researchers identified 134 studies that reported concentrations of micro-plastics in fish, mollusks, crustaceans, tap or bottled water, beer, milk, salt and air. They revised the data for accurate comparisons across studies. The team then used data on food consumption in different countries by age group to estimate the extent of micro-plastic intake. This information, combined with micro-plastic absorption rates from the gastrointestinal tract and hepatic excretion rates, was used to estimate the distribution of micro-plastics in the gut and tissues.

The model predicted that an average of 8,300 (6.4 ng) micro-plastic particles would accumulate in the tissues of an 18-year-old adolescent, while an average of 50,100 (40.7 ng) micro-plastic particles would accumulate in the tissue of a 70-year-old adult. The researchers concluded that the estimates of the four chemicals leached from plastic were small compared to a person's total intake of these compounds.

The researchers say that previous studies may have overestimated micro-plastic exposure and possible health risks, but it will be important to assess the contribution of other food types to intake and accumulation.