Containers made from Styrofoam or expanded polystyrene (EPS) foam are not safe to use in a microwave, unless they have a microwave-safe label. They may leach chemicals into food, which could increase the risk of some types of cancer.

People sometimes use the term Styrofoam, which the building industry uses in many materials, interchangeably with polystyrene foam. However, polystyrene is a different type of foam that makes up some disposable food containers and is not always safe to microwave.

Due to health and safety concerns, some types of food and drink containers are more suitable than others for microwaving.

This article discusses polystyrene foam containers in more detail and whether they are safe to microwave. It also provides tips for safely heating food and answers common questions about heating food containers.

Image of a polystyrene foam food container, some people mistakenly call these Styrofoam containers.Share on Pinterest
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Styrofoam is a brand of a particular type of foam that the building industry commonly uses in insulation and construction.

Some people in countries such as Canada and the United States often use the term when referring to disposable takeout containers, packaging peanuts, and disposable coffee cups. However, these items consist of a type of polystyrene foam that manufacturers inject into molds to set in shape.

The full name for this material is expanded polystyrene (EPS) foam.

EPS foam is a popular material for food and drink containers, as it is light and cheap to produce. It also acts as an insulator to keep food and beverages warm. However, cities in the United States — including Seattle and San Francisco — have begun to ban EPS containers in favor of more environmentally friendly options.

EPS foam does not decompose easily, and it is difficult to recycle. Animals may mistake the material for food and eat it, which can harm their health. Similarly, particles that end up in the ocean can negatively affect marine life.

Some scientists are also concerned that EPS foam, which contains a compound called styrene, may be carcinogenic or cancer-causing. Studies have linked the substance to incidences of cancer in both humans and other animals.

For more science-backed resources on the environment and sustainability, visit our dedicated hub.

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It may not be safe to microwave EPS foam containers unless they have a label showing they are microwave-safe.

The Food and Drug Administration (FDA) regulates plastic and polystyrene containers, plates, and cups and performs tests on their safety, including their use in microwaves.

FDA-approved EPS containers should have microwave-safe labels to show they have been tested and are safe for use in microwaves. Unless an EPS product displays this label, it may not be safe to microwave.

Research shows that heating may cause substances in the EPS to leak, or migrate, into the food inside the container. While the concentrations of substances that leak may not be large enough to cause harm in most typical situations, many factors may influence the safety outcomes, including:

  • fat content in the food
  • type of food
  • temperatures for heating
  • storage conditions of the containers
  • length of time heating

Additionally, some EPS foam containers may crack or lose structural integrity if someone heats them. This may cause hot foods or beverages to leak out and cause burns.

One substance that may transfer from the EPS foam container to the food inside is of particular concern to researchers. Styrene, which is a compound in EPS foam, has links to cancer.

According to the Centers for Disease Prevention and Control (CDC), high levels of styrene may affect a person’s nervous system and is a possible human carcinogen.

Human studies on the association between styrene and certain cancers often focus on long-term exposure, usually in people who regularly work with the substance.

Therefore, although there is a proven connection between occupational exposure to styrene and cancer, the results may not directly apply to styrene from microwaved EPS foam food or beverage containers:

However, a 2017 systematic review found no strong link between styrene exposure and certain types of cancer. Therefore, further research into this topic may be needed.

People may consider trying the following tips to ensure they are heating food containers safely:

  • heating an EPS foam container only if the product has a microwave-safe label
  • transferring food or beverages to microwave-safe containers made from Pyrex, ceramic, or glass
  • venting the microwave-safe EPS foam container to prevent pressure from building inside it
  • refraining from using old or damaged EPS containers, which may leak substances
  • using oven gloves, mittens, or a protective material when removing a heated container from the microwave
  • avoiding the microwave and transferring food to a pot or pan to heat on the stovetop or in the oven

Below are some common questions and answers about heating food containers.

What should you not put in the microwave?

A person should avoid putting items in the microwave that can melt, release toxic fumes, or cause a fire. These include:

  • plastic containers
  • aluminum foil
  • metal containers or cutlery
  • insulated coffee cups, bottles, or flasks
  • brown paper bags

Can Styrofoam go in the oven?

Individuals should not put EPS foam in the oven. The material may warp and soften at very high temperatures. Heating the EPS foam in the oven may also release harmful substances into the food, similar to the microwave.

Can you microwave a plastic bowl?

People should not heat plastic bowls in the microwave unless they have a microwave-safe label.

A person should only heat food in an EPS foam container in a microwave if the container has a microwave-safe label.

Microwaving EPS foam that is not microwave-safe can potentially cause the carcinogenic substance styrene to seep into the food. The container may also crack or soften, and hot food may leak.

To ensure safety, a person should transfer food to a glass, ceramic, or Pyrex container before heating.