This is a tricky question! The ventilation of the Pop 'N Go ® Pets is excellent! We have mesh siding, holes in the roof, etc. But, for example, a dog in a car with the windows cracked in the middle of summer could still suffer from the heat even though the ventilation is ok! Please do not EVER leave your pet locked inside our product or any other product while outdoors during hot or cold days!! This should be common sense, but we have to say it!