Nebraska study highlights potential hazards of microwaving plastic containers

But they may want to think twice before hitting the start button, based on findings from a study recently published by University of Nebraska-Lincoln researchers.

In a set of experiments, the researchers found that microwaving plastic baby food containers available in grocery stores can release large numbers of tiny plastic particles, in some cases more than 2 billion nanoplastic and 4 million microplastic particles for every square centimeter of container.

Although the health effects of consuming micro- and nanoplastics remain unclear, the Nebraska team also found that three-quarters of cultured embryonic kidney cells had died after two days of being exposed to the same particles.

In a 2022 report, the World Health Organization concluded that although limited data provide little evidence that such particles have adverse effects in humans, the organization recommended limiting exposure to them. The organization also called for additional study of the particles on multiple fronts.

Kazi Albab Hussain, the study’s lead author and a UNL doctoral student in civil and environmental engineering, said previous research by other scientists had found that microplastics were released from plastic baby bottles, particularly when they were heated. Another study by Canadian researchers reported that some premium plastic tea bags release microplastic particles when steeping in hot water. Both Hussain’s study and the Canadian research were published in the American Chemical Society journal Environmental Science & Technology.

Hussain said he and his colleagues realized that no studies had looked at other plastic containers used for baby food other than baby bottles. They began their research in 2021, the year Hussain’s now 21-month-old son was born.

“We thought it was important to see if those plastic products were also releasing micro- and nanoplastics,” Hussain said, noting that less is known about nanoplastics than microplastics. Nanoplastic particles are smaller than microplastics and difficult to detect. The researchers defined microplastics as particles at least 1/1000th of a millimeter in size; nanoplastics were anything smaller.

“This is a really new thing, so even the definition is not clear yet,” Hussain said. “The range is not quite fixed yet, because this is a new thing in the discussion.”

They also chose to look at baby food containers because infants and toddlers typically are more at risk from exposure to any type of contaminant because of their smaller relative size and the fact their organs are still developing, he said.

The researchers conducted experiments with two baby food containers made from polypropylene and a reusable pouch made of polyethylene, both plastics approved by the Food and Drug Administration.

In their tests, they sought to replicate the kind of everyday scenarios in which people might use plastic containers, including refrigeration, room temperature storage for six months, shorter term high-temperature storage and microwaving, he said.

In one experiment, they heated the liquids in the plastic containers at full power for three minutes in a 1,000-watt microwave. The researchers did not microwave the pouch because it was not suitable for microwave use.

For the tests, they filled the containers with deionized water or a liquid intended to simulate mildly acidic foods, such as dairy products, fruits and vegetables.

Afterward, they filtered the liquids and analyzed them for evidence of micro- and nanoparticles.

Hussain said they found the microwave heating released the most particles.

The actual number of each size of particle released by the microwaving depended on a number of factors, including the plastic container and the liquid inside it. Using a mathematical model to analyze various scenarios, they also calculated that infants drinking products with microwaved water and toddlers consuming microwaved dairy products would have the highest intake of particles.

Simulated refrigeration, room temperature storage and high-temperature storage also could lead to the release of micro- and nanoplastics, he said.

Under the supervision of Svetlana Romanova of the University of Nebraska Medical Center’s College of Pharmacy, the team cultured and exposed embryonic kidney cells to the actual plastic particles released from the containers. Hussain said previous research has used commercially produced or lab-made particles. The researchers also exposed the cells to particle concentrations that infants and toddlers might encounter over days or from multiple sources.

After two days, 23% of kidney cells exposed to the highest particle concentrations survived, a mortality rate higher than previous studies involving other types of cells. The researchers suspect that kidney cells may be more susceptible to the particles than the cells used in those studies. Earlier studies also tended to use larger particles, some too large to enter cells.

The question of cell infiltration, Hussain said, is one of many areas where more research is needed before researchers can determine the true risk of consuming micro- and nanoplastics. He already is doing some of those studies.

The American Academy of Pediatrics advised against microwaving and dishwashing plastic containers and bottles in a 2018 policy statement. At that time, the concern was that heating plastics could increase the chances that the containers would leach chemicals.

Dr. Leonardo Trasande, the lead author of the statement, said micro- and nanoplastics are different because they are plastic particles, not the chemicals themselves. But the particles could bring chemicals used as additives in plastics along for the ride. Trasande directs the Center for the Investigation of Environmental Hazards at the New York University Grossman School of Medicine.

The study, however, could not fully distinguish any effects from the particles themselves from those of any ride-along chemicals, he said, however good a job the researchers did in filtering and cleaning the particles.

But Trasande, who was not involved in the UNL study, said it suggests that plastics pose more of a concern than researchers anticipated. He, too, said more studies are needed to resolve the ins and outs of the findings.

Hussain advised other parents to follow the steps he takes with his son. He doesn’t microwave plastic containers or put hot water in them. When he buys pureed baby and toddler food, he seeks out products in glass jars.

“It is hard to avoid plastic 100%,” Hussain said. “When you start deliberately trying to avoid it, you can see how much plastic you are using in your daily life. I wasn’t even aware of how much plastic I was using every day.”