How long can food shopping stay in a hot car?

The reason this question comes up so often is that a car does not feel like a dangerous place to leave a bag of shopping. It is shaded from rain, it is your own space, and twenty minutes does not sound long. But a parked car in summer sun behaves like a small greenhouse: sunlight enters through the glass, is absorbed by dark interior surfaces, and the heat cannot escape.

Safety

Step by step

1. Understand how fast a parked car heats up.With the engine off and the windows closed, the interior climbs rapidly in sunshine — most of the rise happens in the first fifteen to thirty minutes, and the interior can reach temperatures far above the outdoor air. Leaving a window slightly open makes surprisingly little difference. This is the same mechanism that makes leaving a dog in a car so dangerous.
2. Know which parts are hottest.The cabin is hotter than the boot in direct sun, and dark surfaces in direct sunlight — the parcel shelf, the rear seat, the dashboard — are hotter still. A bag on the back seat in the sun is in the worst place in the vehicle. A boot in the shade is the least bad.
3. Apply the shortened safe window.Chilled food must be kept at or below 8°C. Once above that, the clock starts, and in genuine heat the working rule from the hot-weather food guide applies: treat the safe window as about an hour rather than two. A twenty-minute detour on a hot day is a meaningful part of that budget, not a rounding error.
4. Treat the high-risk foods as the deciding ones.Cooked meats, pâté, sandwiches with meat or fish or egg, soft and unpasteurised cheeses, prepared salads, milk, cream, yoghurt, raw meat and fish, and anything labelled to be kept refrigerated. Tins, dry goods, bread and most whole fruit and vegetables are unaffected. Judge the load by its most vulnerable item.
5. Make the supermarket the last stop.The whole problem disappears if the shopping goes straight home. Do other errands first, then shop, then drive home. This is easier than any mitigation and it costs nothing.
6. Carry an insulated bag as insurance.If a detour is unavoidable, chilled and frozen goods in an insulated bag with a frozen pack, in the shaded footwell rather than a sunlit seat, buys a great deal of the margin back.
7. Do not rely on how it looks or smells.The bacteria that cause food poisoning generally do not change the appearance, smell or taste of food. A chicken sandwich that has been at thirty degrees for two hours looks exactly like one that has not. This is why the time and temperature rule exists rather than a sniff test.
8. Throw it away if you are unsure.If chilled high-risk food has been in a hot car for a length of time you cannot account for, discard it. The cost of a wasted packet of cooked chicken is trivial against the alternative, and no judgement made afterwards can recover the information.

Tips

• Most of a car's heat gain happens in the first half hour, so the difference between a ten-minute and a forty-minute stop is much larger than it sounds.
• A windscreen sunshade and parking in shade both reduce the peak substantially. Neither makes a car safe for chilled food over a long period.
• If you shop online, chilled and frozen goods arrive in a refrigerated van, which removes this problem entirely on hot days.
• Keep a folded insulated bag permanently in the car. The trips that catch people out are the unplanned ones.
• The same physics is why you must never leave a child or a dog in a parked car — the interior reaches lethal temperatures fast, and cracking a window does not prevent it.

Common mistakes

• Assuming a cracked window keeps the car cool enough — It makes very little difference to the peak temperature. The heat is arriving as radiation through the glass and being trapped, not simply a lack of draught.
• Judging the food by smell afterwards — The organisms that cause food poisoning usually do not alter smell, appearance or taste. Food that has been warm for too long looks and smells entirely normal.
• Putting shopping in the boot and forgetting it — Bags left in a boot overnight or through an afternoon are the classic case. Unload chilled and frozen items first, every time.
• Treating the standard two-hour guidance as applying in a hot car — That figure assumes ordinary room temperature. In genuine heat the window shortens to roughly an hour, and a parked car in sun is well beyond ordinary room temperature.

If it doesn't work

Questions people ask