Is It Really Safe to Sleep in a Car With Windows Closed?

We have all been tempted to catch a few Zs in our car before. Whether it is at a rest stop on a long road trip, on the side of the road when we just can’t go on any longer, semi-squatting in a Walmart parking lot, or just at our campsite on a car camping trip.

Sometimes you just want to tilt the seat back, turn the AC on and doze off for a little while in your comfy seat. Despite many of us participating in this activity routinely, conventional wisdom tells us that we should not sleep in our cars with the windows closed.

boy sleeping in car

Is it safe to sleep in a car with the windows shut or not?

No, it’s generally not safe to sleep in a car with the windows closed. With the engine running, the risk of carbon monoxide accumulation in the cabin due to an exhaust malfunction or blockage is just too high.

As always, the situation dictates sleeping in the car might be your only safe response to encroaching exhaustion, or due to severe weather keeping you off the road.

It would be best if you understood all the variables so you can make an informed decision. We will share those with you just below.

Can You Get Carbon Monoxide Poisoning from Sleeping in Your Car?

Yes, and easily with the car running. The best way to avoid it is to not sleep in your car at all with the engine running, or at least, to not do so with car shut up tightly.

How About When the Engine is Off?

It is possible to get carbon monoxide poisoning when sleeping in a car even with the engine off.

This, though, is largely dependent on other factors such the creation of CO gas from inside the car (a liquid or solid fuel heat source) or from other sources near the car, such as running vehicles.

CO Buildup is Deadly and Happens Fast

The main danger from sleeping in any automobile with the windows up and the engine running comes from carbon monoxide buildup in the cabin due to a failure or blockage of the exhaust system.

Carbon monoxide, or CO, is just one of the many byproducts of combustion carried out of the engine in the exhaust.

As you probably know already, CO buildup is extremely hazardous and insidiously dangerous. The gas is odorless, colorless and tasteless, and you might not necessarily smell exhaust fumes in the cabin while the deadly gas is building up.

This is especially true if you are asleep. In many countries, carbon monoxide is one of the leading causes of airborne poisoning, and this includes the United States and Europe.

Carbon monoxide joins with hemoglobin in the blood, forming carboxyhemoglobin. This prevents your blood from carrying oxygen to all the many oxygen hungry tissues throughout your body.

As carbon monoxide starts to accumulate in your body, symptoms such as headache, vomiting, dizziness, nausea and fatigue will appear along with pronounced weakness. This is often mistaken for the flu, food poisoning, or some other illness.

Mental aberrations and symptoms will also manifest, including visual anomalies, fainting, seizure and pronounced confusion.

As blood CO concentration starts to approach 50%, coma and death will be very near. Death via carbon monoxide poisoning is fairly common in acute cases when victims are asleep. Much of the time, they will never wake up.

The cabin of any automobile is a small volume, often tightly sealed. With only a moderate blockage or malfunction of the exhaust system, enough carbon monoxide could accumulate to fatally poison in as little as an hour. Even a short cat nap in such circumstances could prove fatal.

Preventing CO Buildup in Cabin

One common cause of CO build up in an automobile cabin is due to a blockage of the exhaust tail pipe or tail pipes.

Snow, high water, or even significant accumulations of dirt, leaves and other detritus could be sufficient to cause back pressure and a subsequent ingress of carbon monoxide.

If you are forced to sleep in your vehicle with the windows up or choose to ensure that nothing is blocking or will block the tailpipe before settling down, especially if you are going to sleep.

The other major cause of CO infiltration is malfunction. This is much more difficult to detect even in vehicles with significant emissions control suites. The best way to prevent this unfortunate occurrence is regular inspection and attendant maintenance.

Of course, it is possible to entirely prevent the accumulation of carbon monoxide via exhaust emissions if you turn the car off when staying inside it while resting.

The only other generally reliable way to prevent the buildup of carbon monoxide in the passenger cabin is by keeping all the windows fully open or nearly fully open.

A Cracked Window is Not Much Help

Many drivers and passengers are aware of this carbon monoxide hazard and will opt to slightly open a window while resting in order to maintain a semblance of security while encouraging airflow and hopefully preventing the accumulation of dangerous gases. Unfortunately, this is only marginally helpful.

A cracked window, even multiple cracked windows allowing cross ventilation, will not move enough air to significantly impede the buildup of carbon monoxide.

This means that the driver and all passengers inside the cabin will still be at risk should a leak or back pressure event occur and carbon monoxide begin accumulating.

With the engine turned off a cracked window might improve overall air quality and comfort, however.

Running the AC Will Not Prevent CO Buildup

Some people assert that you can prevent the buildup of carbon monoxide when sleeping in a vehicle with the windows up by simply running the air conditioner.

Though this will bring fresh or at least fresher air into the cabin it will not do so in a great enough quantity to offset the accumulation of carbon monoxide unless the carbon monoxide leak is very slow and very small.

In the end, the only way to completely mitigate the risk of carbon monoxide build-up when sleeping in a vehicle with the windows rolled up is to keep the engine turned off.

Is it Safe to Sleep in Your Car Overnight?

It can be safe, but as mentioned you must take great care to avoid carbon monoxide poisoning from any and all sources, whether or not the engine is running.

Obviously, leaving windows significantly lowered, tops down or doors ajar creates other safety hazards, too. But so long as you are cautious and in a safe area it is possible to get a safe and sound night’s sleep in your vehicle.

How Long Before You Run Out of Air in a Closed Car?

This is a much-debated topic, but the reality is that in most passenger vehicles you will not truly run out of air and suffocate since they are not air tight.

Now, oxygen levels as a volume vary from vehicle to vehicle and some vehicles are sealed up much more tightly than others, so suffocation is not out of the question.

In any case, if all doors, windows, tops, trunks and other ingress points are tightly closed you can notice a serious degradation in air quality long before you asphyxiate. This is because your own exhaled carbon dioxide is building up in the air you are breathing.

You will first notice shortness of breath, feelings of sluggishness, dizziness, headache and eventually nausea long before you reach a critical level of oxygen depletion in such an instance.

This is nature’s way of telling you to get some fresh air and stat! In small car that is tightly closed, you might notice a really “stuffy” feeling in as little as 30 minutes, and even less with a passenger or three.

Keep in mind that other factors like snow, water, a running engine and others can greatly accelerate the degradation of air quality in the cabin!

What About Sleeping a Car During Cold Weather?

One of the biggest concerns and challenges associated with sleeping in a car or other vehicle is in times of cold weather. We have all seen highways and interstates be turned into parking lots for hours or even days thanks to severe winter storms.

There is also always a possibility of winding up stranded in your car due to accident or some other mishap in the same conditions.

In such times, when your car is both your best possible shelter and also a heat source, most will try to wait things out or sleep it off snugged up in their vehicle.

This is entirely understandable, and even prudent, but it is especially dangerous for all the reasons already stated.

If you run the engine to stay warm you run the risk of passing out from carbon monoxide or being overcome in your sleep…

If you don’t run the engine, you face the very real danger of freezing to death and may still be facing CO hazards from other nearby drivers who are running their cars. Talk about a tough call.

But not to worry: there are steps you can take to overcome these hazards while staying snug as a bug inside your vehicle. The first and most important step is to be prepared for the eventuality with an onboard winter survival kit. This will include a number of items like a folding shovel, some warm blankets and maybe sleeping bags, clothing (gloves and hats, too) flares, food and water, carbon monoxide detector, etc.

The main idea is to have sufficient insulation to stay toasty warm even with the engine and heater off and leaving a window or two open a crack (at least periodically) for cross ventilation.

If it is snowing, you can use your shovel and scraper to clear snow away from the door jambs, tailpipe and engine compartment to ensure exhaust gasses evacuate normally.

Lastly, you should not hesitate to run the engine in short intervals in order to heat the cabin to tolerable levels, if required. Just don’t leave it running while you are asleep unless the situation is very desperate!


It is generally not safe to sleep in any vehicle with the windows up, and the engine running.

Though generally small, the chances of carbon monoxide build-up due to exhaust emissions cannot be ruled out, and a blocked tailpipe, damage to the exhaust system or faulty sealing can allow carbon monoxide inside in quantities that can make driver and passengers ill or even result in fatal poisoning.

The only safe way to sleep in a vehicle with the windows up engine turned off.

1 thought on “Is It Really Safe to Sleep in a Car With Windows Closed?”

  1. Seems to me one could simply purchase a battery powered CO alarm to keep in the vehicle. For 10-20 bucks you can sleep without worrying at all.

    I also can’t help but wonder, if this is such a risk, do big rigs with sleepers come with CO monitors standard, since drivers are often sleeping while the engine idles?

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