Can Sleeping in a Car Cause Carbon Monoxide? What You Need to Know

Yes. Sleeping in a car can lead to carbon monoxide (CO) poisoning—sometimes rapidly—especially if the engine is running, the vehicle is in an enclosed or semi-enclosed space (like a garage), the tailpipe is blocked (by snow, mud, or debris), or there are exhaust leaks. Cracking the windows or using the fan is not a reliable safeguard. Health and transportation agencies consistently warn against sleeping in a vehicle with the engine on or in situations where exhaust can accumulate.

What Experts Say About Carbon Monoxide and Cars

Carbon monoxide is a colorless, odorless gas produced by incomplete combustion, including from gasoline and diesel engines. It binds to hemoglobin far more strongly than oxygen, impairing the body’s ability to deliver oxygen to tissues. The U.S. Centers for Disease Control and Prevention (CDC), the National Highway Traffic Safety Administration (NHTSA), and emergency medicine organizations warn that CO can build up to lethal levels in vehicles, sometimes within minutes, and that symptoms can be subtle before becoming life-threatening.

How Carbon Monoxide Builds Up in Vehicles

Engine Running or Idling

Even modern cars with catalytic converters emit carbon monoxide. Idling to heat or cool the cabin can allow exhaust to seep into the vehicle through seals, vents, or floor openings. If you fall asleep, you may not notice symptoms as CO accumulates.

Enclosed or Semi-Enclosed Spaces

Running a vehicle in a garage—attached or detached—can be deadly, even with the door open, because CO can pool and migrate into the cabin or adjoining structures. The gas mixes with air (it’s about the same density) and can permeate quickly.

Hidden Risks in Modern Vehicles

Keyless ignition vehicles can be left on unintentionally, and hybrids may start the engine automatically to maintain battery charge or cabin temperature, surprising sleeping occupants. Remote start features can run the engine when no one is paying attention, and exhaust system leaks can route CO directly under the cabin.

Weather and Environmental Factors

Snow, ice, or mud can block the tailpipe, forcing exhaust back under the vehicle and into the cabin. High winds and certain parking angles can also push exhaust toward air inlets, while extreme cold can encourage prolonged idling—raising risk.

High-Risk Scenarios

The following situations commonly lead to dangerous CO buildup in vehicles; understanding them can help you avoid the most hazardous circumstances.

• Sleeping in a car with the engine on for heat or air-conditioning.
• Idling in an attached or detached garage, even with the door partially or fully open.
• Keyless or hybrid vehicles left “on” inadvertently; hybrids that restart the engine automatically.
• Tailpipe blocked by snow, ice, mud, tall grass, or debris.
• Exhaust leaks, faulty catalytic converters, or damaged floor/seal grommets.
• Using fuel-burning heaters, stoves, or generators near the vehicle.
• Parking next to other idling vehicles or generators that can vent exhaust toward your car.
• Relying on “windows cracked” or the HVAC fan as a safeguard; neither reliably prevents CO buildup.

Any one of these can create a hazardous environment; combined, the risk escalates quickly. The safest approach is to avoid sleeping in a vehicle, particularly with any possibility of exhaust exposure.

Common Symptoms and What to Do

Recognizing early symptoms can save lives, but be aware that CO can incapacitate quickly and while asleep. If you suspect CO exposure, act immediately.

• Dull headache, dizziness, or lightheadedness.
• Nausea or vomiting.
• Weakness, confusion, or difficulty thinking clearly.
• Shortness of breath or chest pain.
• Blurred vision, loss of coordination, or fainting.
• Loss of consciousness; in severe cases, death.

Symptoms can escalate fast and may be mistaken for fatigue or illness. Don’t wait for multiple signs—err on the side of caution.

If you suspect CO exposure in or around a vehicle, take the following steps immediately and in order.

1. Turn off the engine (if running) and move everyone to fresh air right away.
2. Call emergency services; mention suspected carbon monoxide exposure.
3. If available, administer 100% oxygen while awaiting help; do not re-enter the vehicle.
4. Seek medical evaluation even if symptoms improve—delayed effects are possible.
5. Do not use the vehicle again until a qualified mechanic checks the exhaust and seals.

Rapid removal from the source and medical assessment are critical; delayed treatment can increase the risk of neurological and cardiac complications.

Prevention and Safer Alternatives

Preventing CO buildup is the most effective protection. The following practices reflect consensus guidance from safety and health agencies.

• Do not sleep in a vehicle with the engine on, ever—even with windows cracked.
• Never run a car in an enclosed or semi-enclosed space (garages, carports, barns).
• Keep the tailpipe completely clear; check for snow, ice, mud, or debris before starting.
• Maintain the exhaust system; fix leaks, rust, or damage promptly.
• Avoid relying on the HVAC “fresh air” setting or slightly open windows for safety; they’re not reliable CO controls.
• Consider a portable CO alarm rated for vehicle/RV/marine use as an added layer—not a primary protection.
• Plan alternatives: designated rest stops, shelters, motels, or safe indoor spaces in extreme weather.
• Do not use fuel-burning heaters or stoves inside or near the vehicle.

These steps reduce risk substantially, but none makes sleeping in a running vehicle safe. Eliminating engine use and avoiding enclosed spaces remain the core safeguards.

If You Must Sleep in a Car: Harm-Reduction Tips

Sleeping in a vehicle is not recommended. If it is unavoidable, these measures can reduce—but not eliminate—CO risk.

1. Park outdoors in open air, away from other running vehicles or generators; keep the engine off.
2. Before resting, verify the tailpipe is clear and the exhaust system is intact (no strong exhaust smell while previously running).
3. Do not rely on cracked windows for CO control; use them only for comfort/ventilation with the engine off.
4. Avoid alcohol and sedatives; they can mask early symptoms and deepen sleep.
5. Use non-fuel heat solutions: appropriate cold-weather sleeping bags, insulated pads, extra layers, and battery-powered (not combustion) warming devices designed for enclosed use.
6. If available, use a portable CO alarm rated for RV/marine environments; place it near breathing level, following the manufacturer’s instructions.
7. Set periodic alarms to wake and reassess conditions, especially in cold or changing weather.

These steps prioritize engine-off rest in open air. Treat them as last-resort measures, and seek safer accommodations when possible.

Special Cases: Hybrids and Electric Vehicles

Different drivetrains change—but do not eliminate—risk dynamics around CO.

• Hybrids: Even if the cabin seems quiet, the engine can start automatically to maintain the battery or temperature, producing exhaust unexpectedly.
• Electric vehicles (EVs): EVs do not emit CO; cabin preconditioning uses electric heat pumps/resistance heaters. However, CO can still infiltrate from nearby idling vehicles or generators. Never run any combustion source in an enclosed space, even with an EV.
• Attached garages: Any combustion engine (another car, lawn tool, generator) can raise CO that migrates to an EV’s cabin or into a home.

The safest practice for all vehicle types is to avoid sleeping in enclosed spaces and to keep engines and combustion sources off near resting occupants.

Legal and Safety Considerations

Many jurisdictions restrict idling and prohibit sleeping in vehicles in certain areas. Beyond CO risk, sleeping in cars carries risks of hypothermia, hyperthermia, theft, and traffic hazards. Check local laws, prioritize well-lit designated rest areas, and inform a trusted contact of your location.

Sources and Expert Guidance

Guidance in this report aligns with recommendations from the CDC, NHTSA, Ready.gov/FEMA, emergency medicine bodies, and automotive safety organizations. Their core message is consistent: avoid running engines in or near enclosed spaces, don’t sleep in idling vehicles, and treat CO as a silent, fast-acting hazard.

Summary

Sleeping in a car can cause carbon monoxide poisoning, particularly with the engine running, in enclosed or semi-enclosed spaces, or when exhaust is obstructed or leaking. Cracked windows and HVAC settings are not reliable protection. The safest approach is simple: don’t sleep in a running vehicle; park only in open air; keep exhaust clear; and consider a portable CO alarm as a supplemental precaution. When in doubt, move to fresh air and seek medical help immediately.

What are the side effects of sleeping in a car?

Health Risks: Poor ventilation can lead to carbon monoxide buildup, especially if the engine is running. Sleeping in a cramped position can also cause discomfort or strain. Legal Issues: Some places have laws against sleeping in your car, which could lead to fines or being asked to move.

Is it safe to sleep in a car overnight with the windows closed?

Sleeping in a car with all windows closed is **generally safe**, as cars are not airtight and allow for some air circulation. Carbon monoxide poisoning If the engine is running, especially in an enclosed space like a garage, exhaust fumes can build up and become dangerous.

Can I get carbon monoxide from sleeping in a car?

Protect Yourself and Your Loved Ones
The recent incidents of people dying in their cars from carbon monoxide poisoning highlight the importance of car safety and the potential dangers of sleeping in parked vehicles with the engine running. Carbon monoxide is a silent killer – it can’t be seen, smelled, or tasted.

Do I need to crack my windows when sleeping in my car?

Yes, you should crack your car windows slightly when sleeping in your car to ensure proper ventilation, prevent moisture buildup from condensation, and avoid stuffiness. Cracking the windows also helps to prevent carbon monoxide from accumulating, even though it’s not an airtight seal; this is especially true if the engine is turned off. However, you must do this only enough to allow airflow, not so much that it compromises your security or privacy. 
Benefits of cracking windows

• Ventilation: Opens in new tabCracking windows provides fresh air circulation, which is essential for a comfortable sleep and to prevent the air from becoming stale. 
• Moisture control: Opens in new tabCracking a window helps to reduce the moisture from your breath and sweat, which can cause condensation, fogging, and mold growth inside your car. 
• Safety: Opens in new tabWhile not a significant cooling effect on a hot day, ventilation is important for your health and comfort. 

Considerations when cracking windows

• Security: You must crack the window just enough to allow air to flow in, but not so much that it creates an opportunity for someone to reach in and access your car or person. 
• Weather: In rainy weather, you might need to use window visors or screens to prevent water from getting in while still allowing air to circulate. 
• Bugs: If bugs or mosquitoes are a concern, you can use mesh screens that cover the window openings to keep them out while still enjoying ventilation. 
• Safety: To prevent potential carbon monoxide poisoning, ensure your vehicle’s engine is completely off before sleeping, and always turn off the engine when sleeping. 

Alternative Solutions 

• Window screens: Opens in new tabThese mesh covers can be placed over the windows to allow for airflow without letting in bugs or debris.
• Sunroof: Opens in new tabOpening the sunroof slightly can also provide a good amount of ventilation.