Is it safe to keep dry ice in a cooler in the car or tent?

In a moving car with a window open, yes, for the length of the drive. In a parked car with the windows shut, or inside a tent while you sleep, no. Dry ice in a cooler turns into roughly 50 to 100 litres of carbon dioxide an hour. In a space the size of a car cabin or a small tent, with no fresh air, that passes the 40,000 ppm danger line in one to two and a half hours.

What dry ice is, and how much gas it makes#

Dry ice is carbon dioxide frozen solid. It sits at about minus 78 degrees C (minus 109 degrees F). It does not melt into a liquid. It turns straight into gas, which is called sublimation, and the gas is the same carbon dioxide (CO2) you breathe out. That is why it keeps a cooler dry. It is also the whole safety problem, because all of that gas has to go somewhere.

The amount of gas is fixed by chemistry, not by the cooler. CO2 weighs 44.0 g a mole (NIOSH). At 20 degrees C and sea-level pressure, a mole of any gas fills about 24.1 litres. So 1 kg of dry ice is 22.7 moles, which is about 547 litres, or 0.55 cubic metres, of gas. NIOSH's own conversion factor, 1 ppm = 1.80 mg per cubic metre at 25 degrees C, gives 556 litres. The two agree within 2 percent.

Turn that around and the gas weighs 1.83 kg a cubic metre at 20 degrees C, closer to 1.96 at 0 degrees C. Air is lighter. NIOSH lists CO2 as 1.53 times as dense as air. So it does not spread evenly at first. It runs downhill and lies along the floor, as the Asia Industrial Gases Association (AIGA) warns: leaking CO2 "settles at floor level enhancing danger".

The carbon dioxide limits: 5,000, 30,000 and 40,000 ppm#

Gas limits are given in parts per million (ppm). 10,000 ppm is 1 percent of the air. NIOSH, the US agency for workplace safety research, sets three numbers for carbon dioxide:

• 5,000 ppm is the long-term limit, called a time-weighted average (TWA). It is the most a worker should average over a working day of up to 10 hours. OSHA's legal limit (PEL) is the same figure.
• 30,000 ppm is the short-term limit (ST). It is the most for a short spell, averaged over 15 minutes, never to be passed during the day.
• 40,000 ppm is the IDLH, short for immediately dangerous to life or health. NIOSH bases it on human data: signs of intoxication after 30 minutes at 50,000 ppm, and unconsciousness after a few minutes at 70,000 to 100,000 ppm.

The gap between them is narrow. The danger line is only eight times the workday limit. A space does not have to fill with CO2 to hurt you. It only has to reach 4 percent.

Here is what each limit costs in dry ice, for a 3 cubic metre space. That is a model input: about the air in a car cabin, and the same figure the propane in a hot car page uses. For your own car or tent, scale every gram figure by your volume divided by 3.

A 5 kg block in a car or tent, worked through#

Take a 5 kg (11 lb) block. Put it in a hard cooler with the lid shut and the drain plug open. Put the cooler in a 3 cubic metre space. Here is what the published figures give.

Step one: the whole block. 5 kg makes about 2,730 litres of gas. The danger line for this space is 120 litres. So the block holds about 23 times what it takes. The question is never whether a block like this can make the air dangerous. It is how fast the gas comes off, against how fast fresh air takes it away.

Step two: the rate. FIU's dry ice guidelines say it sublimes at 5 to 10 lb every 24 hours. That is 2.3 to 4.5 kg a day, or 95 to 190 g an hour, which makes 52 to 103 litres of gas an hour. With no fresh air at all, the space reaches:

• 5,000 ppm in 9 to 17 minutes
• 30,000 ppm in 52 minutes to 1 hour 45 minutes
• 40,000 ppm in 1 hour 10 minutes to 2 hours 20 minutes
• 100,000 ppm in about 3 to 6 hours

An 8 hour night sublimes 0.76 to 1.5 kg. That is about 3.5 to 7 times the amount that reaches the danger line.

Step three: fresh air. No car or tent is perfectly sealed. Air leaks in and out, and the CO2 level climbs until the gas leaving equals the gas made. The level it settles at is simple: litres of CO2 made per hour, divided by litres of fresh air per hour.

• Parked car, windows shut, fan off. Ott and colleagues measured under 1 air change an hour, which is under 3 cubic metres for this space. Even at a full 1 air change an hour, the cabin settles at 17,000 to 34,000 ppm. Below that rate it settles higher, and past the danger line.
• Parked car, one window fully open. They measured 6.5 air changes an hour. The cabin settles at 2,700 to 5,300 ppm, around the workday limit.

So the drive home with a window down sits near the workday limit. A shut, parked car or a closed tent overnight heads well past it. The industry warnings are blunter. AIGA says a dangerous level "can build up in less than 20 minutes". FIU says that after 15 minutes with dry ice in a bag on the passenger seat and no fresh air, people "may start to feel symptoms such as increased respiration and headaches". A bag runs faster than a cooler: FIU keeps dry ice in insulated containers to slow the gas.

How fast does dry ice sublime in a cooler?#

FIU's figure is "10%, or 5 to 10 pounds every 24 hours, whichever is greater". For a 5 kg block, 10 percent is only 0.5 kg, so the 5 to 10 lb figure is the one that applies. Grizzly, a cooler maker, says dry ice lasts 18 to 24 hours in its hard coolers, and up to 48 hours with excellent insulation and few lid openings. It does not say how much dry ice that is.

Heat speeds it up. AIGA says "higher temperatures in the vehicle and poor ventilation will increase the speed of carbon dioxide gas build up". So plan on the fast end in a hot car or in sun, until a scale tells you your own rate (see the check below).

The cooler ice calculator and the coolers and ice guide work with melting water ice, at 334 kilojoules a kilogram. They size water ice, not dry ice. For dry ice, the sublimation rate is the number that matters.

How much fresh air keeps you under the limits?#

The settled level is the CO2 made per hour divided by the fresh air per hour. Turn that around and you get the fresh air each limit needs.

Ott's team made more than 100 measurements on four vehicles. Parked and shut, under 1 air change an hour, fails the top row. One window fully open, 6.5, just about holds it. Two more results matter on the road:

• Moving, windows shut, fan off or AC on Max: under 6.6, rising with speed from 15 to 72 mph. So recirculation is no better than the fan off, and at town speeds a shut car falls short of the top row.
• One window open 3 inches: 8 to 16 times the shut figure. This is the easy fix, and it is what Grizzly says: drive with the windows down.

The level does not jump to its settled value at once. At 1 air change an hour it gets 63 percent of the way in the first hour and 95 percent in three. So a short stop with the windows shut is not the same as a long one.

A tent has no published figure. Its fresh air depends on the vents, the doors and the wind. The table says what it would need: 10 to 21 cubic metres of outside air an hour, all night, to hold a 3 cubic metre tent under the workday limit. A closed tent on a still night cannot be counted on for that.

Why the tent is worse, and where the cooler goes#

Three things make a tent the worst place for dry ice.

You are lying on the floor. CO2 pools low, and in a tent your face is a few centimetres above the ground. A cabin-average figure understates the air you are breathing.

You are asleep. The early warning signs NIOSH lists, headache, dizziness and fast breathing, are things you notice awake. FIU's advice is plain: "never store near a sleeping area".

The cooler vents at ground level. The drain plug sits at the bottom of the cooler. Opened, it lets the gas out low, right where it then spreads. That is good outside and bad under a fly sheet. A vestibule shares the tent's air and sits at floor level, so it counts as inside the tent.

Where the cooler should go instead:

• Outside, away from the tent or camper, as Grizzly says. No source gives a distance.
• Downhill of anyone sleeping, and not beside a door, since the gas runs downhill and sinks into hollows.
• In the shade, which slows the rate.
• Drain plug open, or lid ajar. Grizzly says to open the plug "so pressure does not build up". AIGA and FIU both warn that a sealed container can build up dangerous pressure. FIU says an airtight one "may explode".

The same rule covers a camper, van or roof tent you sleep in. Grizzly puts "tent or camper" in the same line. Heating in a tent has its own gas problem, covered in using a Mr. Heater Buddy in a tent. That one is carbon monoxide, a different gas, and a CO alarm will not detect CO2.

Is it really dangerous, or is it just CO2?#

Two answers go round, and both are half right.

"It is only CO2, you breathe it out all day." True about the gas, wrong about the amount. NIOSH puts the danger line at 4 percent of the air. A pound of dry ice makes 248 litres of gas, and a 3 cubic metre cabin holds 3,000 litres of air.

"Dry ice in the car will knock you out in minutes." That fits loose dry ice in a shut car, the case FIU and AIGA describe. A cooler in a car with a window open settles near the workday limit, not near unconsciousness.

What decides it is the gas made per hour against the fresh air per hour. AIGA goes further: "Carrying Dry Ice in passenger cars is not recommended". A pickup bed or a trailer keeps the cooler out of the cabin air, which Grizzly also suggests where possible.

Where can the cooler go? The short version#

What this answer does not cover#

The figures here are a model, built on one assumed space of 3 cubic metres, an even mix of gas, and FIU's cooler rates. Real CO2 does not mix evenly. It pools, so the floor of a tent or a footwell can be well above the average this page works out. A smaller tent than 3 cubic metres reaches every limit sooner, in proportion. Heat pushes the rate up, and dry ice outside a cooler sublimes faster than any figure in the rate table. The arithmetic counts only the dry ice. Your own breath, a dog and a stove all add CO2 too. NIOSH limits are workplace limits, so treat them as the outer edge for children, pets or anyone with heart or lung disease. Grizzly's warning about pets low in a vehicle matters for that reason. Nothing here covers handling: dry ice at minus 78 degrees C burns skin, so Grizzly, AIGA and FIU all say to use gloves, a towel or tongs. If anyone in a car feels unwell, AIGA's advice is to stop, get out and open all the doors. If a parked car may hold built-up gas, do not get in until it has aired out.

Check your own cooler and car#

Two checks, both with things you may already own.

1. Find your cooler's rate. Weigh the dry ice when you buy it, on a luggage or kitchen scale. Weigh it again 24 hours later. Divide the loss in grams by 24 for grams an hour.
2. Turn that into gas. Multiply grams an hour by 0.55 for litres of CO2 an hour.
3. Find the fresh air you need. Divide litres an hour by 5 for cubic metres of fresh air an hour that holds 5,000 ppm. So 150 g an hour makes 82 litres, and needs about 16 cubic metres of fresh air an hour.
4. Measure your car on the drive. Put a CO2 monitor low, near the footwell, and drive with your usual window setting. Check its stated range first. One that reads only to 5,000 ppm can tell you that you passed the limit, but not by how much.
5. Act on it. If the reading passes 5,000 ppm, open a window wider. Never test a parked, shut car with anyone inside it.

Common mistakes#

Sealing the cooler. A closed drain plug and a tight lid gasket trap the gas. Open the plug, as Grizzly says.

Leaving the cooler in the car at the trailhead or the shop. A parked, shut car is the case with under 1 air change an hour. It is also the hot case, which speeds the rate.

Using the vestibule as "outside". It is under the same fly, at floor level, beside your head.

Trusting the smell. CO2 has no smell. NIOSH calls it a "colorless, odorless gas". The first sign is how you feel.

Putting the cooler where the dog lies. Grizzly flags pets because the gas sinks to the floor of a vehicle, and a dog's head is there.

Planning a long trip on dry ice alone. At 2.3 to 4.5 kg a day, a 5 kg block lasts one to two days. For longer trips, a 12 volt fridge has no gas problem at all.

Frequently asked questions#

Can you leave dry ice in a cooler in a closed car?#

Not for long, and not with anyone inside. A parked car with the windows shut gets under 1 air change an hour. At FIU's cooler rate of 5 to 10 lb a day, a 3 cubic metre cabin passes the 5,000 ppm workday limit in 9 to 17 minutes and settles at 17,000 ppm or more. AIGA warns a dangerous level "can build up in less than 20 minutes".

Can you keep a dry ice cooler in your tent overnight?#

No. FIU says never to store dry ice near a sleeping area, and Grizzly says to keep the cooler away from the tent. An 8 hour night sublimes 0.76 to 1.5 kg of dry ice. That is 3.5 to 7 times what brings a 3 cubic metre space to NIOSH's 40,000 ppm danger line. CO2 also pools at floor level, where you sleep.

Should the drain plug be open when using dry ice in a cooler?#

Yes. Grizzly says to open the drain plug so the gas can escape and pressure does not build up. AIGA and FIU both warn that dry ice in a sealed container can build up unsafe pressure, and FIU says an airtight container may explode. Leaving the lid slightly open does the same job.

How much CO2 does 1 lb of dry ice make?#

About 248 litres, or 8.8 cubic feet, at 20 degrees C. That comes from the molar mass of carbon dioxide, 44.0 g a mole, and about 24.1 litres of gas a mole at that temperature. One kilogram makes about 547 litres. Half a pound is about enough to bring a sealed car cabin of 3 cubic metres to 40,000 ppm.

Is it safe to drive with dry ice in the car?#

For a cooler, with a window open, the arithmetic says yes for the drive. Opening one window 3 inches raised a car's air change rate 8 to 16 times in Ott's measurements, and Grizzly says to drive with the windows down. With windows shut, recirculation is no better than the fan off. AIGA does not recommend carrying dry ice in passenger cars at all.

What are the symptoms of breathing too much CO2?#

NIOSH lists headache, dizziness, restlessness, tingling, breathing difficulty, sweating, a fast heart rate and higher blood pressure, then coma and asphyxia. FIU says to leave the area if you start to pant and breathe quickly, get a headache, or your lips or fingernails turn blue. At 70,000 to 100,000 ppm, NIOSH records unconsciousness within a few minutes.

How long does dry ice last in a cooler?#

FIU gives 5 to 10 lb lost every 24 hours, so a 5 kg (11 lb) block lasts about one to two days. Grizzly says 18 to 24 hours in a hard cooler, and up to 48 hours with excellent insulation and few openings. Heat speeds it up, so a cooler in a hot car or full sun lasts less.