Ziploc bags have become an essential item in American kitchens over the years, but they are actually a fairly new creation. The origin of these now-common zipper bags can be traced back to Danish inventor Borge Madsen, who initially designed a new zipper system intended for clothing. Although this initial plan didn’t succeed, the invention eventually evolved into the well-known Ziploc bags, which made their official debut in 1968.
Nowadays, Ziploc bags are used for storing leftovers, preserving food freshness, and marinating meats. These versatile bags are even handy for storing non-food items. However, according to a class action lawsuit filed in 2025, using Ziploc bags in freezers or microwaves could pose a significant risk. The lawsuit claims that S.C. Johnson & Son, Inc — the company that has owned Ziploc since 1998 — makes misleading statements about their bags being “suitable” or “safe” for freezer and microwave use. The complaint further alleges that these claims could expose consumers to microplastics — tiny plastic particles less than 5 millimeters in length, formed when larger plastics degrade.
Ziploc bags are manufactured using low-density polyethylene and polypropylene plastic, which are generally more durable than the materials used in other plastic resealable bags. However, the complaint argues that these two types of plastics can degrade under the extreme temperatures associated with microwaves and freezers. The lawsuit cites “scientific and medical evidence” to support this claim, so let’s examine some of the relevant science.
Polyethylene likely isn’t as safe as you’d hope
A 2023 study published in Environmental Science and Technology examined the release of microplastics from plastic containers. Researchers discovered that microwave heating resulted in the highest release of microplastics into food compared to other scenarios, with some containers releasing as many as 4.22 million microplastic and 2.11 billion nanoplastic particles from just one square centimeter of plastic within three minutes in the microwave. Perhaps even more concerning, the study found that refrigeration and room-temperature storage for over six months led to the release of millions to billions of microplastics and nanoplastics. The study also found that food pouches made with polyethylene — similar to Ziploc bags — released more particles than polypropylene-based plastic containers.
Regarding the use of Ziploc bags in a freezer, the belief is that freezing the plastic can make it more brittle, leading to an increased release of microplastics. As Carmen Marsit, a professor of environmental health at Emory University’s Rollins School of Public Health explained to Health, “The freezing process might make the plastic more brittle, so then you might have more shedding of those microplastics into the food products.”
Ziploc’s official website states that their bags can be used in the microwave, but “only to defrost or reheat food.” The company also advises leaving the zipper open at least one inch to let steam and heat escape. However, there is no mention of the bags being dangerous or leaching microplastics when used in the microwave, which the lawsuit claims is a significant oversight. An S.C. Johnson spokesperson addressed these claims in a statement to Health, stating, “We believe Ziploc products are safe when used as directed and that the lawsuit is without merit.” Meanwhile, an article in Plastics Today argues that the researchers conducted an “extremely brief” chemical analysis of the particles, which was insufficient to confirm they were plastic (though it’s notable that this article comes from a “community for plastics professionals”).
What’s the problem with microplastics?
You might have heard the term “microplastics,” and even if you haven’t explored it deeply, you likely have a general sense that they are not beneficial. But what exactly are microplastics, and why are they problematic? As the name suggests, microplastics are small pieces of plastic. Anything measuring less than 5 millimeters long is typically considered microplastic. While most of these pieces are created when larger plastic breaks down, there are intentionally created microplastics. For example, the beads in hand sanitizer are technically microplastics. The cosmetic and self-care industry often adds microplastic beads to their products as exfoliating elements. Unfortunately, these small pieces end up being washed down the drain and enter the ocean and other marine habitats. These microplastics are then consumed by fish and other creatures, eventually making their way back into our bodies when we consume these animals.
This isn’t ideal for us, as microplastics can carry contaminants, including synthetic chemical compounds linked to cancer or birth defects. A 2023 study published in the Yonsei Medical Journal explains how experiments suggest that microplastics can significantly affect humans, causing digestive and respiratory problems and disrupting various systems, including immunity and reproduction.
Furthermore, chemicals in plastic food packaging, such as ortho-phthalates, PFAS, and perchlorate, can disrupt our endocrine system, which regulates hormone creation and release. These chemicals can transfer from plastic into our food but can also be consumed as microplastics, potentially causing hormonal system disruptions affecting everything from appetite and metabolism to cell growth regulation and neurodevelopment. Indeed, a 2024 study published in Environmental Science and Technology found that microplastics were “suspected” to adversely affect reproductive, digestive, and respiratory systems. Even more concerning, the researchers identified a “suggested link” between microplastics and colon and lung cancer.
Are Ziploc bags really unsafe to microwave and freeze?
Research is ongoing into how microplastics affect humans, and there is still much we don’t understand. While studies have identified numerous correlations between microplastics and health issues in humans, establishing a direct causal link remains somewhat elusive. Even with this uncertainty, it’s likely best to avoid potential additional exposure to microplastics.
The lawsuit against Ziploc is ongoing, so it will be interesting to see how it unfolds. At this point, it remains unclear whether microwaving or freezing Ziploc bags actually generates microplastics. According to the company’s statements following the lawsuit, their bags are safe, and some argue that the medical evidence cited in the complaint has its own issues. However, many experts in the field believe that microwaving or freezing any plastic, including Ziploc bags, is unwise. As Brad Younggren of Circulate Health told CNET, “Repeated exposure to extreme temperatures — cold or hot — stresses plastic materials and can lead to the breakdown of their surface layers, releasing tiny plastic particles into food.”
In any case, it’s probably wise to err on the side of caution. A 2019 study published in Environmental Science and Technology assessed the number of microplastic particles in popular foods, considering these foods’ recommended daily intake. Based on their findings, the researchers estimated that Americans consumed between 39,000 and 52,000 particles annually, depending on age and sex, and between 74,000 and 121,000 when inhalation was taken into account. Water intake via plastic bottles was also thought to introduce an additional 90,000 microplastics into the equation (though we could potentially eliminate microplastics in water with a simple trick). The team also noted that while these estimates are subject to significant variation, they were likely to be underestimates. In short, if you can avoid microwaving your Ziploc bags, it’s probably a good idea.
