Watch out! The high temperature will make the microplastics in your bowl more turbulent.

According to the team's calculations, parents who hold baby milk powder in plastic bottles and soak and shake in hot water may unwittingly feed their children more than a million microplastic particles a day.

Just as everyone ingests soil or dust, researchers are not sure whether ingestion of microplastics is harmful. Tamara Galloway, an ecotoxicologist at the University of Exeter (the University of Exeter) in the UK, thinks it is not a big problem: "most of the microplastics you ingest will pass directly through your intestines and then excreted." But Li Yunzhu has reservations about this, saying that the potential risk of microplastics may be high.

For nearly 20 years, researchers have been worried about the potential harm of microplastics, but most of the research has only focused on the risks of microplastics to marine life. In 2004, Richard Thompson, a marine ecologist at the University of Plymouth (University of Plymouth), defined plastics smaller than 5mm as "microplastics" after his team found small particles of plastic on many beaches in the UK.

Since then, scientists have found microplastics everywhere they go, the deep sea, snow in the Arctic and ice in the South Pole, shellfish, salt, drinking water and beer, drifting in the air or falling over mountains and cities with rain. These tiny fragments may take decades or more to fully degrade. "to be sure, almost all species are exposed to some degree," Galloway said.

The earliest studies of microplastics focused on beads found in personal care products, particles of raw materials used to produce plastics, and fragments slowly eroding from discarded bottles and other large fragments. All of this flows into rivers and oceans, and in 2015, oceanographers estimated that 15 to 51 trillion microplastic particles were floating in the world's surface water. Since then, more and more sources of microplastics have been identified, such as plastic particles cut off from car tires on the road and synthetic microfibers in clothing. These particles spread between the sea and land, and people are likely to inhale or eat them no matter where they live.

The workers are cleaning up the plastic on the beach

Figure | Tom Camacho/Science Photo Library

Albert Koelmans, an environmental scientist at the University of Wageningen (Wageningen University) in the Netherlands, reported in March that children and adults may consume tens to 100, 000 microplastics a day, according to some surveys of microplastics in air, water, salt and seafood. He and his team believe that in a worst-case scenario, people may consume a total amount of microplastic the size of a credit card each year.

Assessing the effects of plastic particles on people or animals is the other half of the puzzle, which is easier said than done. More than 100 laboratory studies have exposed animals (mainly aquatic organisms) to microplastic environments and found that exposure may lead to reduced reproductive efficiency or physical damage to some organisms. However, this is difficult to explain clearly, because microplastics have a variety of shapes, sizes and chemical composition, and the materials used in many studies are very different from those found in the environment.

One thing is clear-the problem of microplastics will only get worse. The world produces nearly 400 million tons of plastic every year, and plastic production is expected to more than double by 2050. Even if all plastic production is miraculously stopped tomorrow, existing plastics in landfills and the environment (estimated at about 5 billion tons) will continue to degrade into tiny fragments that cannot be collected or cleaned up, increasing the content of microplastics.

How do microplastics cause harm?

Researchers have several theories about how microplastics cause harm.

If they are small enough to enter cells or tissues, they may irritate our bodies because they are foreign bodies-such as long, thin asbestos fibers, which can inflame lung tissue and cause cancer. This is potentially similar to air pollution: PM10 and PM2.5 (particles smaller than 10 μ m and 2.5 μ m) from power plants, car exhaust and forest fires, which deposit in the respiratory tract and lungs and can damage the respiratory system at high concentrations. Of course, Koelmans points out that the concentration of PM10 is still thousands of times higher than the concentration of microplastics found in the air.

Larger microplastics are more likely to have a negative impact on us through chemical toxicity. Manufacturers add compounds such as plasticizers, stabilizers and pigments to plastics, many of which are harmful, such as disrupting the endocrine (hormonal) system. However, whether ingestion of microplastics significantly increases our exposure to these chemicals depends on the speed at which they leave the plastic particles and the speed at which they spread through our bodies, and such research is still in its infancy.

Another idea is that microplastics in the environment may absorb chemical pollutants and then transport them to animals that eat contaminated microplastics. Jennifer Lynch, a marine biologist at the National Institute of Standards and Technology (US National Institute of Standards and Technology in Gaithersburg, Maryland), has a theory. He believes that animals absorb pollutants from food and water anyway, and if plastic particles are not contaminated when swallowed, they may even help remove pollutants from animal internal organs. At present, researchers are still unable to agree on the major issue of whether microplastics carry pollutants.

However, the most common harm to marine life may be that swallowing plastic fragments with no nutritional value prevents them from ingesting other essential foods to survive. In 2020, Lynch, director of the Marine debris Research Center (Marine Debris Research at Hawaii Pacific University in Honolulu) at the University of the Pacific Hawaii in Honolulu, conducted an autopsy on sea turtles found dead on the beach, examining plastic in their internal organs and chemicals in their tissues. Her team conducted a series of analyses of nine hawksbill turtle larvae under three weeks old and found that microplastics hindered the growth of young sea turtles. One of the turtle larvae is only 9 centimeters long, while there are 42 pieces of plastic in the gastrointestinal tract, mostly microplastics, which may prevent it from eating more food to maintain its growth.

The 9cm baby sea turtle studied by Lynch's team and the plastic particles in its body

Figure | Jennifer Lynch

Temperature is the key to reduce the intake of microplastics

At present, even the most leading research has not directly examined the effects of microplastics on the human body. The only available research is to rely on laboratory experiments to expose cells, human tissues, mice, rats and other animals to microplastics. For example, in one study, mice fed large amounts of microplastics caused inflammation of the small intestine; in two other studies, mice exposed to microplastics had fewer sperm and fewer cubs than those in the control group. Some in vitro studies on human cells or tissues have also shown the toxicity of microplastics.

However, as with marine studies, it is not clear how these effects relate to the concentration of plastic exposed to mice or humans. Most studies use polystyrene balls, which does not represent the diversity of microplastics that people consume in real life. At the same time, the researchers also said that the current research is more in vitro research, and it can not be inferred from the effects of microplastics on some tissues that it will lead to health problems of the whole animal.

One of the questions surrounding risk is whether microplastics remain in the body and accumulate in some tissues. Studies in mice have found that microplastics about 5 μ m in diameter can be left in the intestines or reach the liver. Based on limited experimental data o