Can you leave propane or fuel canisters in a hot car?

Not in a parked car in the sun once the air outside is above about 25 degrees C (77 degrees F). Measured cabins run 22 to 25 degrees C above outdoor air within an hour, which reaches every storage limit printed for camping fuel: 49 degrees C on bear spray, 50 on gas canisters, 52 on propane. A correctly filled 1 lb propane cylinder turns liquid-full at about 64 degrees C. Dashboards exceed that, and the relief device then vents gas into the car.

How hot does the inside of a parked car get?#

How fast, and how far above outside. McLaren, Null and Quinn logged a dark sedan for 60 minutes on 16 clear days with outdoor air at 72 to 96 degrees F (22 to 36 degrees C). The cabin rose about 3.2 degrees F every five minutes, 80 percent of the rise in the first 30 minutes, and ended on average about 40 degrees F (22 degrees C) above outdoor air. On the coolest day, 72 degrees F, it still reached 117 degrees F (47 degrees C). Windows opened 1.5 inches made no significant difference.

Where in the car. Vanos and colleagues measured six vehicles in Tempe, Arizona, in sun and in full shade, on three days between 25 June and 11 July 2014, with outdoor air of 32.8 to 41.5 degrees C. After about an hour:

A canister on the floor in the shadow of a seat sits near cabin air. On the dashboard, a parcel shelf or a seat in a sunbeam, the sun heats it directly: on average 4 to 22 degrees C above cabin air.

What temperature is too hot for propane and gas canisters?#

Each fuel's own documents give a limit, and they agree within 3 degrees C.

Federal cylinder rules design to a slightly higher point: 49 CFR 173.301 requires enough empty space that a cylinder "will not be liquid full at 55 degrees C (131 degrees F)", and a DOT-39 is proof tested at its contents' pressure at 130 degrees F. That is a transport design margin, not a storage temperature.

Against the first table, average sunny cabin air (46.7 degrees C) sits 2 to 5 degrees C under the limits, the seat sits on them, and the dashboard and steering wheel are well past them.

The pressure inside, at the temperatures a car reaches#

The pressure in a propane cylinder or gas canister is the vapour pressure of the liquid inside. While any liquid remains, it depends only on temperature, not on how full or how big the container is. NIST's reference data give it for the three gases camping fuel is made from:

The canister gases climb fastest in proportion: from 20 to 69 degrees C, propane's gauge pressure multiplies by 3.3, isobutane's by 4.8 and n-butane's by 6.5. Real propane also runs a little high: AmeriGas lists commercial propane, which may contain up to 7 percent ethane, at 287 psig at 130 degrees F, 11 percent above the pure figure.

Will a propane cylinder explode in a hot car?#

Two answers circulate: "it is a bomb" and "the relief valve makes it safe". The first is wrong about the usual failure and the second is wrong about what venting means.

Why a correctly filled cylinder does not usually burst. A DOT-39 lot is rejected if a burst test fails below 2.0 times test pressure, and test pressure is at least the contents' pressure at 130 degrees F: about 517 psig or more for propane. At the hottest dashboard reading, 85 degrees C, pure propane's vapour pressure is 484 psig, still under that floor.

Why the cylinder vents anyway. Vapour pressure only holds while there is vapour space. Under 49 CFR 173.304a, propane (specific gravity 0.504 to 0.510) is filled to 42 percent of the cylinder's water capacity by weight, which is about 420 kg of propane per cubic metre of cylinder. NIST's saturated liquid density falls to that figure at 63.6 degrees C (146 degrees F): at that temperature the liquid fills the cylinder. Any further warming has no gas to compress, the pressure climbs far faster than the vapour pressure curve, and the pressure relief device opens. The same 42 percent rule applies to a 20 lb tank, so its liquid-full point is the same.

Harrisonburg's deputy fire marshal told WHSV the same in June 2026: "It won't burst, the relief valve, if working properly... but by venting that pressure, it just releases gas into the area that it's in." AmeriGas adds both failure cases: vented propane "can also become ignited", and a container "may rupture suddenly and violently without warning if pressure relief devices fail to function properly."

An overfilled or refilled cylinder gets there sooner. On the same arithmetic, a cylinder 10 percent overfilled is liquid-full at about 43 degrees C, below average sunny cabin air. Whether refilling a 1 lb cylinder is legal has the full overfill table.

How much vented propane makes a car flammable?#

Our arithmetic, with one assumed input:

Propane's lower flammability limit is 2.15 percent in air (AmeriGas). Assume 3 cubic metres of cabin air, a model input to scale to your own vehicle. 2.15 percent of 3,000 litres is 64.5 litres of gas, and at about 1.83 g per litre (44.1 g per mole over 24.1 litres per mole at 20 degrees C) that is 118 g. A full 1 lb cylinder holds 465 g, so about a quarter of one cylinder, evenly mixed, makes the cabin air flammable.

Propane vapour is heavier than air and, in AmeriGas's words, "may collect at low levels", so a real leak gathers in footwells and the luggage floor before the cabin average reaches 2.15 percent. WHSV's advice if you smell propane on opening the car is to back away and call 911.

Canister gas is no better: the Primus sheet gives the blend a lower limit of 1.8 percent. That sheet mentions no relief device and warns the canister "may explode if heated", so for canisters the printed 50 degrees C is the only line to work to.

Can you leave a gas canister or other camping fuel in the car?#

Item by item, for a car parked in the sun on a day above about 25 degrees C:

• 1 lb propane cylinders and 20 lb tanks. Drive them, then take them out on arrival: "Don't leave it in there to get hot," as the fire marshal put it. Carry them upright and secured so the relief valve sits in the vapour space; on its side a tank vents liquid, which expands 270 times as gas (AmeriGas).
• Isobutane and butane canisters. Same answer, with less margin: the maker's limit is 50 degrees C and the warning is bursting, not venting.
• Liquid fuel bottles. The fuel starts to boil above 35 degrees C and its flash point is below 0. No numeric limit is published; treat it like a canister.
• Bear spray. 120 degrees F (49 degrees C) on the label, bursting possible above 130 degrees F. How long bear spray lasts covers what heat does to its propellant.

In a cooler, at night, or in full shade on a mild day, all four are within their limits. The answer is about temperature, not cars. In practice: buy fuel last and unload it first; carry it upright and wedged, on the car camping checklist; keep it out of the sun, ideally inside the cooler; and at camp store it in outdoor shade, not in the car or the tent. Storing gear between trips covers the months between, and how long a 1 lb propane cylinder lasts lets you count spares instead of stockpiling them.

What decides it: outdoor temperature, sun and an hour#

The decision rule comes from adding McLaren's measured rise to the printed limits. Cabin air in sun is roughly outdoor air plus 22 degrees C on average, and plus 25 degrees C on their coolest day.

The right-hand column adds Vanos's surface readings, 4 to 22 degrees C above cabin air on average (seat to dashboard).

Where the rule stops applying. It describes a closed car in clear sun. Shade cut Vanos's cabins by 8.4 degrees C, and a running engine with air conditioning changes everything. Vehicle size changes the rate more than the end point: Vanos's vehicles heated at 0.30 to 0.42 degrees C per minute, mostly because of their air volumes, and reached similar equilibria. We found no measurement of boot (trunk) temperatures, so this page does not rank the boot against the cabin.

Check your own car before a summer trip#

A cheap minimum and maximum thermometer answers the question for your vehicle.

1. Put the thermometer where the fuel rides: floor behind a seat, boot or roof box.
2. Park in the sun on a clear day, windows closed, from late morning.
3. Read the maximum after 60 and 120 minutes, and note the outdoor temperature.
4. Subtract. That rise replaces 22 degrees C in the table above.
5. Repeat in direct sun on the seat or shelf. If it passes 50 degrees C, nothing pressurised rides there.

Common mistakes#

Cracking a window. McLaren found windows opened 1.5 inches made no significant difference to how fast or how far the cabin heated. Shade does work.

Trusting "it has a relief valve". The relief valve keeps the cylinder intact by putting propane into the car. About a quarter of a cylinder, on our 3 cubic metre model, is enough to reach the flammable limit.

Laying a 20 lb tank on its side. The relief valve must sit in the vapour space; on its side it vents liquid propane.

Putting a canister on the dashboard to warm it. Warming a cold canister is a real technique, covered in canister stoves in the cold, but a sunny dashboard averaged 68.9 degrees C in Arizona and reached 85.

Frequently asked questions#

Can you leave a propane tank in a hot car?#

Not parked in the sun on a warm day. An AmeriGas propane data sheet says containers should never exceed 125 degrees F (52 degrees C), and a sunny car's cabin passes that within an hour once outdoor air is above about 27 to 30 degrees C. Seats and dashboards run hotter still. Drive the tank upright and secured, then take it out when you arrive, as fire officials advise.

Will a 1 lb propane cylinder explode in a hot car?#

Usually it vents instead. A correctly filled cylinder becomes liquid-full at about 64 degrees C (146 degrees F), on NIST densities and the federal 42 percent filling rule, and its pressure relief device then releases propane into the car. That gas can ignite. A cylinder whose relief device fails can rupture, and an overfilled or refilled cylinder reaches the liquid-full point much sooner.

What temperature is too hot for an isobutane gas canister?#

The Primus PowerGas data sheet says store below 50 degrees C (122 degrees F), protect from sunlight, and warns the canister "may explode if heated". Isobutane's pressure rises from about 2.0 bar gauge at 20 degrees C to 5.8 bar at 50 degrees C and 9.6 bar at 69 degrees C, the average sunny dashboard temperature measured in Arizona.

How hot does a car get inside on a 25 degree day?#

About 47 to 50 degrees C (117 to 122 degrees F) of cabin air after an hour in clear sun, based on McLaren and colleagues' measured rise of about 22 degrees C over outdoor air. Most of the rise comes in the first 30 minutes. Surfaces in direct sun are hotter: dashboards averaged 68.9 degrees C in an Arizona study.

Is it safe to keep propane in the trunk in summer?#

The trunk keeps the cylinder out of direct sun, which matters, since sunlit surfaces run 4 to 22 degrees C above cabin air on average. We found no published measurement of trunk temperatures, so it is not known to stay under the 52 degrees C limit. Treat it like the cabin: fine for the drive, not for a parked afternoon on a warm day.

Is a 20 lb propane tank safer in a hot car than a 1 lb cylinder?#

No. Pressure depends only on the temperature of the propane, not on the size of the container, so both reach the same pressure in the same car. Both are filled to the same 42 percent rule and go liquid-full at the same temperature. The larger tank simply holds about 19 times more propane to vent.