Can you microwave styrofoam?

Expanded polystyrene foam softens near 100 °C (212 °F), which is below the temperature hot food reaches in a microwave. A cup or clamshell can slump, split and drop hot food before the food is even warm through. Containers marked microwave-safe have been tested by the maker against the FDA food-contact rules at microwave temperatures; unmarked foam has not been tested for that use at all. The number 6 in the recycling triangle identifies the plastic, not its heat rating, and tells you nothing about whether it can be microwaved.

How to do it properly

1. Look on the base for the words microwave-safe or the three-wavy-lines symbol
2. Heat in 30-60 second bursts and stir between them rather than one long run
3. Leave the lid off or vented so steam is not trapped
4. Stop immediately if the container softens, warps or smells of plastic

What goes wrong

• The base softens and the container collapses while you are lifting it
• Oily and sugary foods run hotter than water and melt foam in under a minute
• Unlabeled foam has never been tested for food contact at these temperatures

Other options

• Tip the food into a glass or ceramic bowl first
• Cover with a plate instead of a lid
• Reheat anything greasy on the stove

Other "can you microwave" questions

• Can you microwave aluminum foil? No — No — foil reflects microwaves and the edges arc, which can start a fire.
• Can you microwave plastic containers? It depends — Only ones labeled microwave-safe — cold-storage tubs are not among them.
• Can you microwave eggs? It depends — Beaten or pricked eggs, yes. A whole egg in its shell will explode.
• Can you microwave Tupperware? It depends — Yes, if that specific piece carries the microwave-safe symbol on its base.
• Can you microwave paper plates? It depends — Plain white paper plates are fine; anything with metallic trim or a foam layer is not.
• Can you microwave metal? No — No — metal reflects microwaves, arcs at edges, and can damage the oven or start a fire.
• Can you microwave breast milk? No — No — microwaves create hot spots that scald a baby even when the bottle feels cool.
• Can you microwave takeout containers? It depends — Only if the container is marked microwave-safe — most are not, and metal-handled ones never are.
• Can you microwave cardboard? It depends — Plain dry cardboard is usually fine briefly; anything with glue, staples, wax or foil is not.
• Can you microwave glass? It depends — Yes for ordinary heatproof glass; no for leaded crystal, and never straight from the freezer.
• Can you microwave water? It depends — Yes, but very clean water can superheat and erupt when you move the cup.
• Can you microwave ceramic? It depends — Yes for plain glazed ceramic; no if it has metallic trim or is unglazed and porous.