Vinegar kills the mildew smell.

Vinegar and a thorough air-dry will fix that mildew smell. You don’t need to buy a new lunch box.

First, wash the whole thing — including the lid and any rubber seals — with hot water and dish soap. Scrub out any visible gunk. Mildew feeds on food residue, so if there’s leftover crumb or sauce, the smell will come back no matter what you do.

Then wipe the inside down with white vinegar (undiluted is fine) and let it sit for a few minutes. Vinegar kills mold spores better than soap alone. If the smell is really stubborn, sprinkle baking soda inside and leave it overnight before wiping it out.

The most important step is drying. Mildew can’t survive without moisture, so leave the lunch box open in a dry, airy spot for a full day. Not an hour. A day. Put it near a window or a fan if you can. Once it’s bone dry, it shouldn’t come back — as long as you don’t seal it up wet again.

If the smell survives all that, the smell is probably baked into the plastic, and that’s when you replace it. But vinegar plus drying fixes 9 out of 10 lunch boxes.