The safe duration you can stay in a car with the doors closed depends almost entirely on whether the engine is running and the climate control is on. With the engine off, it's generally unsafe to stay inside a sealed car for more than a few hours due to the risk of carbon dioxide (CO2) buildup from your own exhaled breath, not a lack of oxygen. In a typical sedan, CO2 levels can rise to uncomfortable and potentially dangerous levels well before oxygen depletion becomes a concern. The biggest danger, however, is temperature. A car can become lethally hot or cold incredibly quickly, making weather the most critical factor.
The primary risk isn't oxygen depletion but the accumulation of carbon dioxide. As you breathe, you exhale CO2. In an airtight space, the concentration of CO2 increases, which can lead to headaches, dizziness, confusion, and eventually loss of consciousness. The time this takes varies based on the car's interior volume and the number of people inside. A larger SUV will take longer to fill with CO2 than a small coupe.
Extreme temperatures drastically shorten the safe timeframe. On a warm day (70°F/21°C), the interior temperature can reach over 115°F (46°C) within an hour. In winter, a car offers little insulation against freezing temperatures, leading to hypothermia. Never sleep overnight in a car with the engine off for these reasons.
The only way to stay safely in a parked car for an extended period is to crack a window open for ventilation. Even an inch of opening allows for sufficient air exchange to prevent CO2 buildup and helps moderate the interior temperature. Running the engine to operate the climate control is a solution, but it must be done in a well-ventilated, open area to prevent deadly carbon monoxide (CO) poisoning from the exhaust.
| Factor | Impact on Safe Duration | Key Consideration |
|---|
| Engine & Climate Control | Indefinite (if in open area) | Risk of carbon monoxide (CO) poisoning if in an enclosed space like a garage. |
| Ventilation (Windows Cracked) | Several hours to a full day | Prevents CO2 buildup; critical for managing temperature. |
| Outside Temperature | Drastically variable | Hot weather (90°F/32°C) can make a car unsafe in under 1 hour. Cold weather can lead to hypothermia. |
| Vehicle Size | Moderate impact | A larger vehicle (SUV, van) has more air volume, slowing CO2 buildup compared to a small car. |
| Number of Occupants | Significant impact | More people breathing means CO2 builds up much faster. |
