What should I never leave in a hot car?

The reason this list is longer than people expect is that a parked car is not a slightly warm room. Sunlight passes through the glass, is absorbed by dark interior surfaces, and the heat cannot escape — the interior climbs rapidly, most of it within the first half hour, and dark surfaces in direct sun go far higher still. A dashboard in summer sun is hotter than almost anywhere else in daily life.

Safety

Step by step

1. Never leave a child or an animal, for any length of time.This is categorically different from everything else on the list. A car interior can reach lethal temperatures within minutes, children overheat far faster than adults, and dogs cannot sweat. Cracking a window makes very little difference to the peak temperature. There is no errand short enough. If you see a dog in distress in a locked car, call 999.
2. Take out lithium batteries and power banks.Power banks, spare tool and camera batteries, e-cigarettes and anything with a lithium cell degrade permanently in heat and, in the worst case, fail dangerously. A power bank on a dashboard in summer sun is a genuine fire risk as well as a wasted purchase. This includes the laptop in the boot.
3. Take out aerosols, gas canisters and lighters.Aerosols carry a maximum storage temperature, commonly around 50°C, which a car interior exceeds readily. Deodorant, dry shampoo, spray paint and camping gas canisters can vent or burst. Disposable lighters left on a dashboard are a known cause of interior fires.
4. Take out medicines.Most medicines have a stated storage range, typically below 25°C, and heat degrades many of them — including some inhalers, insulin, liquid antibiotics, EpiPens and ordinary painkillers. A medicine left in a car may simply be less effective, which matters most for exactly the medicines people carry for emergencies. Do not use a glovebox as a medicine cabinet.
5. Take out sun cream.Ironic and common. Heat degrades the UV filters, so a bottle that has spent a summer in a car door pocket may not deliver its stated protection. Most bottles state a storage temperature and say to keep out of direct sun. Keep it in a bag, not in the car.
6. Take out electronics.Phones, tablets, laptops, dashcams and camera bodies all have operating and storage limits well below what a car reaches. Screens can suffer permanent damage, batteries degrade, and a phone left on a dashboard will refuse to work until it cools. A dashcam is designed for the environment; a phone is not.
7. Take out chilled and frozen food immediately.Covered in detail in the food guides — the short version is that chilled food above 8°C has a short safe window, and in a hot car it is far shorter. This is the most common way people make themselves ill in summer without realising.
8. Know what is merely ruined.Chocolate and anything with a low melting point. Candles. Vinyl records, which warp readily. Photographs and printed documents, which fade and cock. Plastic items on the dashboard, which distort and off-gas. Fizzy drinks in cans, which can burst. Glasses with plastic frames and lenses, which distort in a case on the dashboard.
9. Treat the dashboard as the worst location.Direct sun on a dark surface behind glass produces the highest temperatures in the vehicle — far above the cabin air. The order from worst to best is dashboard, parcel shelf, seats in direct sun, boot, then footwell in shade. If something must stay in the car, the shaded footwell is the least bad place.
10. Use a windscreen shade and park in shade.Both reduce the peak substantially and neither makes a car safe for the items above. They are damage limitation for the interior itself — the dashboard, the seats and the steering wheel — rather than a licence to leave things behind.

Tips

• A folded reflective windscreen shade takes ten seconds and noticeably reduces dashboard temperature, which protects the car itself even when nothing is left inside.
• Glovebox and door pockets are not cool — they are unventilated spaces inside a hot car, and the door pocket in the sun is worse than the cabin.
• A steering wheel or a seatbelt buckle in direct sun can be hot enough to burn skin, which matters particularly with children getting into a car.
• If you must transport something heat-sensitive, put it in an insulated bag in the shaded footwell with the air conditioning running, and take it out the moment you arrive.
• Anything you leave in a car all summer — a first aid kit, an emergency phone charger, a bottle of water — should be checked and replaced periodically, because it has been heat-cycled for months.

Common mistakes

• Leaving a window cracked and believing the car is now safe — It makes surprisingly little difference to the peak temperature, because the heat arrives as radiation through the glass and is trapped. It does not make a car safe for a child, an animal or anything on this list.
• Keeping medicines or an EpiPen in the glovebox — Most medicines have a stated storage limit around 25°C and lose effectiveness above it. An emergency medicine that has been cooked all summer may not work when it is needed.
• Storing a power bank in the car permanently — Heat degrades lithium cells and in the worst case causes thermal runaway. It is one of the few items on this list that can start a fire.
• Leaving sun cream in the car door — Heat degrades the UV filters, so the protection may be well below the stated factor exactly when it is being relied on.

Checklist

• Children and animals — never, for any length of time
• Power banks, spare batteries, e-cigarettes
• Aerosols, gas canisters, lighters
• Medicines, inhalers, EpiPens, insulin
• Sun cream
• Phones, tablets, laptops, cameras
• Chilled and frozen food
• Chocolate, candles, vinyl, photographs, plastics

Questions people ask