The microwave-safe label is the only test

The only way to know a lunch box is safe to microwave is to check the bottom for a microwave-safe label — if it isn’t there, don’t microwave it.

Look for the little symbol that looks like wavy lines, or the words “microwave safe” printed on the plastic. That’s the manufacturer telling you the plastic was tested and won’t break down at microwave temperatures. Material tricks like “it’s plastic, plastic is fine” don’t work — different plastics have completely different melting points.

The risk with unlabeled containers isn’t an explosion. It’s that cheap or unknown plastic can melt, warp, and leach chemicals like BPA and phthalates into your food when heated. Old, scratched, or cloudy containers are worse, because the plastic degrades over time. And if the container has any metal trim — even a thin decorative edge — it can spark and start a fire. That’s the one that actually gets people.

Reheating leftovers isn’t worth eating melted plastic.