Most drivers know cars can get dangerously hot in the sun — but few realise that this danger extends to many items we commonly leave in our vehicles.

From batteries to vapes and even glasses, items we commonly leave behind can explode, ignite, or damage the interior under high heat. There is a growing number of incidents each summer involving cabin fires, melted equipment, and even injuries — all caused by the items we leave behind.

This article explores the science behind in-car heat build-up, outlines the 9 most common at-risk items, and discusses the potential consequences for both everyday drivers and commercial fleet operators.

Greenhouse Effect: Why Cars Become Danger Zones in Hot Weather

Park a car in the sun for even a short period, and the temperature inside can soar dramatically — often reaching 50–70°C or more, depending on the outside temperature, colour of the vehicle, window tint, and interior materials.

This heat rise happens due to the greenhouse effect. Sunlight enters through the windows and is absorbed by interior surfaces such as the dashboard, seats, and carpet. These surfaces then re-radiate the energy as infrared heat, which cannot easily escape. 

Since vehicles are effectively sealed environments with limited airflow, this heat accumulates fast.

On a 25°C day, cabin temperatures can exceed 45°C (113°F) within 30 minutes. On hotter days nearing the 30°C threshold, the temperature can rise even faster, sometimes hitting 60°C in just 10–15 minutes, turning that seemingly harmless item you left on the passenger seat into a fire risk.

Common Everyday Items That Can Become Fire Hazards in Cars

Here’s a breakdown of the most problematic items and what makes them so dangerous when left in a hot vehicle:

1. Disposable Lighters and Vapes

These small, often forgotten items are among the most dangerous.

Lighters contain pressurised butane, which expands rapidly when heated. In extreme heat, they can crack, rupture, or even explode.

Vapes and e-cigarettes contain lithium-ion batteries and often flammable liquids — a double threat. Overheating can cause battery failure (thermal runaway) or leakage, and the liquid may ignite.

2. Aerosol Cans (Deodorant, Cleaning Products, Paint, Air Fresheners)

Aerosols are sealed under pressure, and nearly all have a “do not store above 50°C” warning. In-car temperatures can easily exceed this. When the internal pressure builds up past the safety limit, cans may burst, releasing flammable gas or harmful chemicals.

3. Hand Sanitiser and Alcohol-Based Cleaners

These contain high concentrations of ethanol or isopropyl alcohol — both extremely flammable. In a sealed, hot environment, vapour can accumulate. If exposed to a spark, static discharge, or direct sunlight through a magnified lens, ignition is possible.

4. Pens

Many pens are made with thin plastic barrels and pressurised ink cartridges. In high heat, the ink can expand, leak, or even rupture the barrel. This can result in messy stains, damage to upholstery or electronics, and in rare cases, chemical fumes or flammable vapour depending on the ink type.

5. Power Banks and Lithium Batteries

Lithium-ion batteries are used in power banks, phone chargers, tablets, and other gadgets. High heat can cause the electrolyte inside to break down, leading to leaks, swelling, or thermal runaway.

6. Canned Drinks

Like aerosol cans, carbonated drinks are stored under pressure. Heat causes the liquid and gas inside to expand to bursting. While this may not cause a fire directly, it can create a sticky, corrosive mess that damages electronics, upholstery, and internal fittings — and could create a hazard if a driver is startled while entering the vehicle.

7. Petrol Canisters

Even small amounts of petrol in portable containers can release flammable vapours. In a hot, enclosed vehicle, vapour pressure increases significantly, posing a high risk of explosion or ignition, especially if the canister is not fully sealed or stored correctly.

8. Reading Glasses and Sunglasses

While not a flammable item, glass lenses left on the dashboard or seat can act like magnifying glasses, focusing sunlight into a narrow point. This can ignite flammable materials such as paper, tissues, seat fabric, or plastic components.

9. Plastics

Soft plastics can warp, melt, or emit fumes when exposed to extreme temperatures. Like the lenses of glasses, clear plastic bottles can also refract sunlight, potentially focusing heat onto flammable surfaces and increasing fire risk.

There are documented cases of smouldering seats caused by sunlight through a half-full bottle left on the dashboard.

What Can Happen When These Items Ignite?

Vehicle Damage

Interior fires, whether small or extensive, can destroy dashboards, seats, and electronics. Smoke damage alone can render a vehicle undriveable. Replacing upholstery, repairing scorched dashboards, or cleaning up after an aerosol explosion can cost hundreds or even thousands of pounds or dollars.

Injuries and Safety Risks

In worst-case scenarios, these fire risks can lead to injuries — especially if a driver re-enters the vehicle unaware that an item is overheating, or if an object explodes during a drive. There’s also risk to passersby or nearby parked vehicles in densely packed areas.

Fleet and Commercial Vehicle Implications

For businesses running commercial fleets, from logistics and courier firms to taxi services and maintenance providers, the consequences multiply:

• Downtime: Even minor fires or clean-ups can sideline vehicles for days.
  
• Lost productivity: Fewer vehicles available = reduced service capacity.
  
• Contract penalties: Lease agreements may impose fines for damage beyond normal wear and tear.
• Insurance complications: Not all policies will cover fire or damage caused by preventable heat exposure.

How to Prevent Fires and Heat Damage in Your Vehicle During Summer

The good news? These incidents are almost entirely preventable with simple precautions.

Daily Clean-Out Routine

• Get in the habit of removing high-risk items — especially lighters, vapes, aerosol cans, and batteries — at the end of each day or when the car will be idle for more than a few minutes.
• Encourage family members or employees to do the same.

Store Smart, Not Hot

• Keep necessary items in insulated bags or shaded compartments, and never leave flammable items on the dashboard or seats.
• Use purpose-built storage cases for electronics and battery packs with heat-resistant liners.

Use Sunshades and Park in the Shade

• Reflective windscreen sunshades can drastically lower dashboard temperatures.
• Parking in covered or shaded areas reduces heat exposure, especially during peak sunlight hours.

Educate All Drivers — Personal or Professional

• For fleet operators: Include summer safety reminders in driver communications or toolkits.
• For everyday motorists: Share this knowledge with family, especially teen or new drivers who may not be aware of the risks.

Implement Seasonal Vehicle Checks

• If you’re managing a fleet, set up scheduled summer checklists for removing high-risk items, inspecting battery-powered devices, and refreshing safety signage.

For fleet operators, tools like Healthy Working: Safe Driving can reinforce these safety messages. Our Fleet Risk Management platform helps reduce road incidents by assessing driver behaviour and delivering tailored advice, support and training — and it’s easy to customise with your own branding and content.

Get a free trial of Healthy Working: Safe Driving.

A Few Minutes of Prevention Could Save a Car — or a Life

It’s easy to overlook the things we leave behind in our cars — a vape in the cupholder, a can of deodorant in the glovebox, a power bank under the seat. But when summer arrives, those everyday items need to be top of mind when exiting the vehicle.

Whether you’re a commuter, a delivery driver, a parent running errands, or a fleet manager overseeing dozens of vehicles, a little awareness goes a long way. Take the time to clear out potentially dangerous objects, educate drivers and passengers, and stay vigilant during the warmer months.