The team also found that the higher the temperature in their preparations, the more particles were released. Water heated to 25 degrees C—that is, room temperature—and shaken in the bottle caused the plastic to shed 600,000 particles per liter. “So mechanical agitation of the liquid against the surface of the polymer is enough to pull off some of the actual microplastics that are not properly attached to the plastic itself,” says Boland.
At 70 degrees C—the WHO’s recommendation for proper sterilization—the number of particles shed was up to 16.2 million per liter. Bumped way up to 95 degrees C, 55 million particles per liter came off the bottle and infused the solution. “The sterilization process itself exacerbates the level of microplastic formation,” says Boland, “such that, if you leave out the sterilization step, even though it’s unsafe, you’ll reduce the number of microplastics that are actually generated.”
Boland and his colleagues tested the same bottles repeatedly over the course of three weeks, and they found that the bottles kept releasing particles over time. One day a bottle might release fewer, but the next day its levels might skyrocket again, sometimes higher than after its very first treatment. “We saw that every bottle actually showed these kinds of cycles in the level of release,” says Boland. “What we think is happening is that as you use a bottle, you slowly start abrading the polymer.”
Polypropylene is a rubbery polymer made of layers of a tougher crystalline material and a more amorphous material. “The amorphous material calves off very easily,” says Boland. “And then the more robust crystalline material that’s exposed is much more resistant to being undercut, if you will, by the water.” That could produce a cycle in which particle release peaks one day and then hits a trough the next. As you repeatedly prepare formula in a bottle, the hot water eats through the layers of the polypropylene. Boland can actually see these layers inside the bottle walls with a powerful imaging technology called atomic force microscopy. “It’s almost like when you look at the side of the Grand Canyon, you see these strata,” Boland says.
“The numbers are, well, frightening,” says Deonie Allen, who studies microplastics at the University of Strathclyde, who wasn’t involved in the research. “They’re terrifying. They’re huge. They’re bigger than any exposure tests that have been done before for human uptake.”
Previous research, for instance, has estimated that adults consume between 39,000 and 52,000 microplastics particles per year. If the Trinity team’s calculations are correct, babies fed with plastic bottles are getting as many as 4 million per day, or 1.5 billion particles per year.
To be clear, this study did not determine any implications for children’s health. “The impact of what these particles have on human health is still something that’s basically unknown,” says Allen. “We know that if you breathe it, then it can end up in your lungs, in lung tissue. We know that if you ingest it, it ends up in your stool.”
So we know microplastic particles are in our bodies—we just don’t know what they’re doing in there. “We don’t have a norm for maximum tolerable daily intake, because we’re years away from that stage in human risk assessment for plastic particles,” says Vrije Universiteit Amsterdam microplastic researcher Heather Leslie, who wasn’t involved in the research. “So it’s impossible to say if there are or are not going to be toxicological effects at the doses reported here.”
Once in the human body, microplastics might release their component chemicals as “leachates,” which have previously been shown to impair the behavior of animals like marine snails. Scientists have found that a range of oceanic species we eat, including crabs, squid, and shellfish, have microplastics in their muscle tissues, meaning the particles may have migrated through the gut wall. Researchers have fed polyvinyl chloride (aka PVC) particles to dogs and found that the microplastics showed up in their blood. Fish exposed to plastic nanoparticles ended up with brain damage after the particles crossed the blood-brain barrier, a sort of biological force field meant to keep nasties like pathogens out.