PS6, or polystyrene (#6 plastic), is not considered microwave safe unless the specific container carries a microwave-safe label. Most PS6 containers, including the familiar white foam takeout boxes and clear plastic cups, have not been tested for microwave use and can release styrene, a chemical linked to cancer, when heated.
Why PS6 Reacts Poorly to Heat
Polystyrene starts to soften at around 100°C (212°F), which is roughly the temperature of boiling water and well within the range food reaches in a microwave. As the material softens, residual styrene monomer trapped in the plastic can migrate into your food. Research testing styrene migration found that food containers released up to 6 micrograms per milliliter of styrene into food simulants, and that both higher temperatures and higher fat content significantly increased how much styrene leached out. So microwaving something like leftover curry or cheese-covered pasta in a PS6 container is a worse scenario than reheating plain rice.
Beyond chemical leaching, PS6 foam containers can warp, crack, or partially melt in the microwave, especially if the food inside has hot spots. Oils and sugary sauces heat well above the water boiling point and can push the container past its softening threshold in seconds.
The Health Concern With Styrene
Styrene is the chemical building block of polystyrene plastic, and small amounts of unreacted styrene remain in finished containers. The National Toxicology Program lists styrene as “reasonably anticipated to be a human carcinogen,” and the International Agency for Research on Cancer classifies it as a possible human carcinogen. At high exposure levels (far above what you’d get from a single container), styrene causes nervous system effects including changes in color vision, slowed reaction time, and concentration problems.
California’s Proposition 65 sets a no-significant-risk level for styrene at 27 micrograms per day. While a single reheated meal in a PS6 container is unlikely to push you past that threshold, repeated use adds up, and there’s no practical reason to accept the exposure when safer options exist.
How to Tell If Your Container Is PS6
Flip the container over and look for a small triangle made of chasing arrows with the number 6 inside it. You may also see the letters “PS” stamped beneath the triangle. Common PS6 items include white foam clamshell takeout boxes, foam coffee cups, clear brittle deli containers, and disposable plastic cutlery. The material is lightweight, breaks easily, and often makes a squeaky sound when rubbed.
Some PS6 containers do carry a microwave-safe symbol, typically a small icon of wavy lines or a microwave. These have been tested under FDA protocols that simulate worst-case heating conditions: 130°C (266°F) for 15 minutes with a fatty-food simulant and 100°C for 15 minutes with a water-based simulant. If a PS6 container passes those tests, it has demonstrated that chemical migration stays within acceptable limits. If there’s no microwave-safe label, assume the container hasn’t been tested.
Safer Alternatives for Reheating
Glass is the top recommendation from toxicologists for reheating food. It’s chemically inert, doesn’t react with acids or fats, and contains no plasticizers or stabilizers that could migrate into food. Borosilicate glass containers are safe for the microwave, freezer, and conventional oven up to about 450°F.
If you prefer plastic, polypropylene (#5 PP) is the best option for hot food. It has a much higher softening point than polystyrene and contains low levels of extractable chemicals compared to other plastics. Look for the number 5 inside the recycling triangle. Ceramic plates and bowls without metallic glazes or trim also work well.
Materials to specifically avoid in the microwave, alongside unlabeled PS6, include PVC (#3 plastic) and older polycarbonate containers (#7 plastic). These can release endocrine disruptors or other harmful compounds when heated. Pure food-grade silicone is another safe choice for microwave use, particularly platinum-cured silicone, which has the lowest migration risk among flexible materials.
What to Do With PS6 Takeout Containers
If your food arrives in a PS6 foam box, transfer it to a glass or ceramic dish before reheating. This takes about 10 seconds and eliminates the risk entirely. PS6 containers are fine for holding cold or room-temperature food for short periods, since styrene migration drops dramatically at lower temperatures. The problem is specifically the combination of heat and food contact, which is exactly what microwaving creates.