The number on the bottom is not a safety rating.

No, you can’t just microwave any plastic lunch box — “microwave-safe” on the label is the only thing you should trust, and even then there are catches.

The little triangle with a number is a resin identification code. It tells recyclers what plastic it is, not what the microwave is going to do to it. Number 5 is polypropylene (PP), which is the good news: it handles heat well and most sturdy lunch boxes and takeout tubs are made of it. If it says microwave-safe and it’s a 5, you’re basically fine for normal reheating.

Number 7 is a catch-all for “everything else.” That includes polycarbonate (old-school hard plastic, often BPA-based) and PLA (bioplastics made from corn). PLA will absolutely deform in the microwave, and polycarbonate is not something you want leaching into hot food. So with a 7, the number alone tells you nothing — you really need the explicit microwave-safe label.

As for the fatty foods warning: that’s physics. Fats get way hotter than water in the microwave — oil can easily hit 400°F, which is above polypropylene’s melting point. So a container that’s perfectly fine for reheating soup can warp or droop when you heat something greasy. Manufacturers add that warning because they know you’ll blame them when your container melts into a pancake.

When in doubt, use glass.