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 on a long road trip when we just can’t go on any longer or due to a bout of laziness on a 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? It is 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. 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.

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.


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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