Ice packs buy you about 4 hours of safe lunch

Ice packs keep yogurt and cheese safe in an insulated lunchbox for about four hours—if you pack it right.

The trick is to put the frozen pack on top of your food, not on the bottom. Cold air sinks, so it washes down over the yogurt and cheese as it melts, keeping everything colder for longer. Use a real gel pack or a frozen juice box, and keep the lunchbox out of direct sun.

If you’re packing yogurt, freeze it the night before—then it doubles as its own ice pack. But remember the clock starts when you pack it, not when you open it. After four hours, throw away anything perishable. The risk isn’t worth a case of food poisoning from leftover lunch.

On brutal 90°F+ days, cut that to three hours unless the lunchbox is truly insulated. When in doubt, feel the yogurt—if it isn’t cold to the touch, don’t eat it.