For the Love of God, Stop Microwaving Plastic

A study of baby-food containers shows that microwaving plastic releases millions upon millions of polymer bits.

At the start of his third year of graduate school, Kazi Albab Hussain became a father. As a new dad and a PhD student studying environmental nanotechnology, plastic was on his mind. The year before, scientists had discovered that plastic baby bottles shed millions of particles into formula, which infants end up swallowing (while also sucking on plastic bottle nipples). “At that time,” Hussain says, “I was purchasing many baby foods, and I was seeing that, even in baby foods, there are a lot of plastics.”

Hussain wanted to know how much was being released from the kinds of containers he’d been buying. So he went to the grocery store, picked up some baby food, and brought the empty containers back to his lab at the University of Nebraska—Lincoln. In a study published in June in Environmental Science & Technology, Hussain and his colleagues reported that, when microwaved, these containers released millions of bits of plastic, called microplastics, and even tinier nanoplastics.

Plastics are complex cocktails of long chains of carbon, called polymers, mixed in with chemical additives, small molecules that help mold the polymers into their final shape and imbue them with resistance to oxidation, UV exposure, and other wear and tear. Microwaving delivers a triple whammy: heat, UV irradiation, and hydrolysis, a chemical reaction through which bonds are broken by water molecules. All of these can cause a container to crack and shed tiny bits of itself as microplastics, nanoplastics, and leachates, toxic chemical components of the plastic.

The human health effects of plastic exposure are unclear, but scientists have suspected for years that they aren’t good. First, these particles are sneaky. Once they enter the body they coat themselves with proteins, slipping past the immune system incognito, “like Trojan horses,” says Trinity College Dublin chemistry professor John Boland, who was not involved in this study. Microplastics also collect a complex community of microbes, called the plastisphere, and transport them into the body.

Our kidneys remove waste, placing them on the front lines of exposure to contaminants. They are OK at filtering out the relatively larger microplastics, so we probably excrete a lot of those. But nanoplastics are small enough to slip across cell membranes and “make their way to places they shouldn’t,” Boland says.

“Microplastics are like plastic roughage: They get in, and they get expelled,” he adds. “But it’s quite likely that nanoplastics can be very toxic.”

Once they’ve snuck past the body’s defense systems, “the chemicals used in plastics hack hormones,” says Leonardo Trasand, a professor at the NYU Grossman School of Medicine and the director of the Center for the Investigation of Environmental Hazards. Hormones are signaling molecules underlying basically everything the body does, so these chemicals, called endocrine disruptors, have the potential to mess with everything from metabolism to sexual development and fertility.

“Babies are at greater risk from those contaminants than full-grown people,” Hussain says. So to test how much plastic babies are exposed to, Hussain’s team chose three baby-food containers available at a local grocery store: two polypropylene jars labeled “microwave-safe” according to US Food and Drug Administration regulations, and one reusable food pouch made of an unknown plastic.

They replaced the original contents of each container with two different liquids: deionized water and acetic acid. Respectively, these simulate watery foods like yogurt and acidic foods like oranges.

They then followed FDA guidelines to simulate three everyday scenarios using all three containers: storing food at room temperature, storing it in the refrigerator, and leaving it out in a hot room. They also microwaved the two polypropylene jars containers for three minutes on high. Then, for each container, they freeze-dried the remaining liquid and extracted the particles left behind.

For both kinds of fluids and polypropylene containers, the most microplastics and nanoplastics—up to 4.2 million and 1.2 billion particles per square centimeter of plastic, respectively—were shed during microwaving, relative to the other storage conditions they tested.

In general, they found that hotter storage temperatures cause more plastic particles to leak into food. For example, one polypropylene container released over 400,000 more microplastics per square centimeter after being left in a hot room than after being stored in a refrigerator (which still caused nearly 50,000 microplastics and 11.5 million nanoplastics per square centimeter to shed into the stored fluid). “I got terrified seeing the amount of microplastics under the microscope,” Hussain says.

To test what these plastics do to our bodies once they’re consumed, the team bathed human embryonic kidney cells in the plastic roughage shed by the baby-food containers. (The team chose this kind of cell because kidneys have so much contact with ingested plastic.) After two days of exposure to concentrated microplastics and nanoplastics, about 75 percent of the kidney cells died—over three times as many as cells that spent two days in a much more diluted solution.

While the concentration of plastic used in these solutions was higher than what a baby would be exposed to by eating from a microwaved food jar in real life, Hussain notes that the full extent of plastic particle accumulation over time—from food and from the air and surfaces—is unknown, and might be high. So, he says, it’s important to study the health effects of high levels of exposure.

While Hussain’s team was the first to test the toxicity of plastics on cells using the particles released from commercially available food containers, a review published in the Journal of Hazardous Materials last year found that exposure to microplastics can cause cell death, inflammation, and oxidative stress. “Plastics are a huge problem for human health,” says Trasand. “This study just pushes the concern even further.”

Micro- and nanoplastics aren’t the only particles leaking out of plastic containers and into food. When plastic is broken apart by heat, tons of chemical additives fly out as well. Boland notes that while the techniques used in Hussain’s experiment could not distinguish between plastic polymers and chemical additives, “both are probably toxic.” We don’t know whether chemical additives are as bad as nanoplastics (or worse), but “at the end of the day,” he says, “none of the stuff that’s emerging from these plastics is very good for anybody.”

Judith Enck, a former EPA regional administrator and the president of Beyond Plastics, a policy and advocacy group against plastic pollution, stopped microwaving plastic 30 years ago. She thinks that you should, too: “My goodness, especially if you have kids or if you’re pregnant, do not put plastic in the microwave.” 

“It’s a pain in the neck,” she acknowledges, but “even this one study should be a wake-up call—not just to new parents but to the FDA. They need to be far more proactive.” Transand agrees: “The FDA is glacially behind.”

To get a plastic product approved for food or beverage packaging, a manufacturer needs to submit a limited amount of self-reported data to the FDA. But the agency doesn’t have the resources to test the safety of all plastic products before they go on the market or to spot-check them once they’re available in stores.

Polypropylene is considered safe for food contact—even in the microwave—by the FDA, which allows companies to use it for packaging things like baby food. Boland disagrees: “I don’t believe that there are microwave-safe plastics.” Trasand and Enck agree that while independent studies should continue testing how much plastic is being released from food packaging, there is already enough evidence to show that “microwave-safe plastic” isn’t really safe. “I think the FDA needs to tell companies that they can no longer say any plastic is microwavable,” says Enck.

Broadly reducing human exposure to plastics will require government action and sweeping corporate change, says Trasand. After all, they’re in the air, in the water, and inside you. Enck doesn't think manufacturers are likely to make the first move. “Corporations will continue to use plastic for as long as they can, because it’s cheap. That motivates them more than anything,” she says.

Even if a new technology emerged that could prevent plastic containers from shedding particles, Boland suspects that companies wouldn’t adopt it without being forced to do so by regulation. In principle, food companies and plastics manufacturers could be “opening themselves up to litigation for past products,” he says, since changing their packaging would imply that they had been knowingly producing something that released microplastics before.

Enck says that one potential solution could be to create a third-party certification program connecting food companies to independent scientists who can test their products and report results to the FDA. On an individual level, there are still some things people can do: Opt for reusable glass and stainless steel. Don’t pour hot liquids into plastic containers. And, please, stop microwaving plastic.

Boland says scientists should keep doing research to understand exactly what particles are being released from plastics under specific conditions. “If you can’t measure,” he says, “you can’t legislate.”

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