Car exhaust can absolutely kill you, and carbon monoxide is the primary reason. CO is an odorless, colorless gas produced by incomplete combustion of fuel, and it works by hijacking your blood’s ability to carry oxygen. In an enclosed or poorly ventilated space, a running car engine can produce enough CO to cause fatal poisoning in minutes to hours, depending on the concentration. The danger is made worse by the fact that you often cannot tell it is happening until you are too impaired to save yourself.
How Carbon Monoxide Poisons You
When you breathe in carbon monoxide, it enters your lungs and crosses into your bloodstream just like oxygen does. The problem is that CO binds to hemoglobin, the protein in red blood cells that normally carries oxygen to your tissues, with a far higher affinity than oxygen itself. The result is a molecule called carboxyhemoglobin (COHb), which is a more stable complex than the normal oxygen-hemoglobin bond, effectively locking oxygen out of its seat on the hemoglobin molecule.1Journal of Chemical Theory and Computation. Binding of Carbon Monoxide to Hemoglobin in an Oxygen Environment: Force Field Development for Molecular Dynamics This means your blood is still flowing, your heart is still pumping, but progressively less oxygen is actually reaching your brain, heart, and other organs.
CO does not just block the spots where oxygen would normally attach. It also changes the shape of the hemoglobin molecule in a way that makes the remaining oxygen-carrying sites hold on to their oxygen more tightly, releasing less of it to the tissues that need it. Research has shown that as COHb levels climb, the pressure needed for hemoglobin to release half its oxygen drops dramatically, meaning the oxygen that is still being carried gets delivered less efficiently.2PubMed. Influence of carbon monoxide on hemoglobin-oxygen binding So CO hits you with a double blow: less oxygen picked up and less oxygen delivered.
Beyond the blood, CO also targets mitochondria, the energy-producing structures inside your cells. Because mitochondria contain heme-proteins similar to those in hemoglobin, CO can interfere with cellular energy production directly.3PubMed Central. Carbon monoxide and mitochondria-modulation of cell metabolism, redox response and cell death This means even if some oxygen is reaching your cells, CO can still impair how your cells use that oxygen to generate energy. The brain and heart, being the most energy-hungry organs, are the first to suffer.
What CO Poisoning Feels Like and Why People Miss It
The symptoms of carbon monoxide poisoning are notoriously vague and easy to mistake for the flu, a hangover, or simple tiredness. At low to moderate levels, you might feel a dull headache, mild nausea, dizziness, or fatigue. As exposure continues and COHb levels rise, symptoms intensify to confusion, impaired judgment, difficulty walking, and eventually loss of consciousness. The progression can be gradual enough that by the time you realize something is seriously wrong, you may lack the coordination or mental clarity to get yourself out of the situation.
One of the most dangerous aspects of CO poisoning is how difficult it is for standard medical equipment to detect. A normal pulse oximeter, the clip placed on your finger at a doctor’s office or hospital, cannot distinguish between hemoglobin carrying oxygen and hemoglobin carrying carbon monoxide. Studies have shown that pulse oximetry readings stayed above 96% even in patients whose COHb levels were as high as 44%, severely overestimating how much oxygen was actually available.4PubMed. The pulse oximetry gap in carbon monoxide intoxication The device essentially reads carboxyhemoglobin as if it were oxyhemoglobin, giving a falsely reassuring number. Separate research confirmed that standard pulse oximeters consistently overestimate arterial oxygenation in CO-poisoned patients, with the error growing as COHb levels climb.5PubMed. Pulse oximetry in severe carbon monoxide poisoning
This means a person who is dangerously poisoned can appear to have normal oxygen saturation on a pulse oximeter. Accurate diagnosis requires a blood test, specifically a co-oximetry panel that can separately measure COHb levels. If you or a doctor suspects CO exposure, a finger clip reading of 98% does not rule it out. This is worth remembering if you ever feel sick in a way that improves rapidly once you leave a particular building or car.
The Garage Scenario and How Quickly It Can Happen
The classic lethal scenario involves a car idling in a closed garage. Even a small, attached garage with the door shut can accumulate dangerous CO concentrations within minutes. But the risk extends beyond fully enclosed spaces. A case study documented fatal carbon monoxide poisoning from a running vehicle in an open outdoor area, demonstrating that sufficiently long exposure combined with unfavorable weather conditions, such as still air that prevents the gas from dispersing, can cause lethal poisoning even without walls and a ceiling.6PubMed Central. Suicidal Carbon Monoxide Poisoning Using Motor Vehicle Exhaust in an Open Space
Semi-enclosed spaces like carports, partially open garages, or drive-through areas adjacent to living spaces also present meaningful risk. So does sitting in a parked car with the engine running and a window cracked, particularly if the exhaust pipe is partially blocked by snow, mud, or another obstruction that routes fumes back into the vehicle. Tailgating parties where people sit behind idling trucks, or children playing near running vehicles in driveways, represent real-world exposure scenarios that people rarely think about.
How quickly poisoning becomes lethal depends on several variables: the size of the space, ventilation, the concentration of CO in the exhaust, and the person’s size and health. In a tightly sealed single-car garage, concentrations can reach dangerous levels in under ten minutes with an older vehicle. With a newer car producing less CO per minute, it takes longer, but “longer” does not mean “safe.” A few hours of running a newer car in a sealed garage can still be fatal.
Modern Cars Produce Less CO, but They Are Not Safe
Catalytic converters, standard on gasoline vehicles since the late 1970s in the United States, substantially reduce the amount of carbon monoxide in exhaust. A properly functioning catalytic converter on a modern car converts most CO into carbon dioxide before it leaves the tailpipe. This has changed the landscape of CO poisoning from vehicle exhaust. A case report documented how the decreased CO concentrations in late-model vehicles equipped with catalytic converters are changing the clinical presentation of exhaust inhalation, with lower peak COHb levels than would have been seen in older vehicles.7PubMed. The role of catalytic converters in automobile carbon monoxide poisoning: a case report
This reduction is real but not a guarantee of safety. Catalytic converters take time to warm up, and during cold starts the device does not function effectively. Research on modern diesel vehicles found that during the cold-start and warm-up phase, vehicles emit considerably more pollutants, including carbon monoxide, because of incomplete combustion and reduced performance of exhaust aftertreatment devices.8PubMed. Development of a cold-start emission model for diesel vehicles using an artificial neural network trained with real-world driving data A car started on a freezing morning in a closed garage pumps out substantially more CO during those first several minutes than it will once the engine and catalytic converter have reached operating temperature. Older vehicles, vehicles with damaged catalytic converters, or vehicles that have failed emissions testing produce much higher CO levels at all times.
In Denmark, the introduction and spread of catalytic converters correlated with a significant decline in CO-related deaths from vehicle exhaust. During the period from 1969 to 1987, annual cases rose steadily as car ownership increased. After 1987, as newer vehicles equipped with converters replaced older ones in the fleet, annual cases dropped from roughly 190 to 63 per year by the end of the 1990s.9PubMed. Suicide by carbon monoxide from car exhaust-gas in Denmark 1995-1999 The technology clearly saves lives, but the fact that fatal poisonings continued to occur throughout the study period underscores that modern cars are not harmless in enclosed settings.
Why Some People Are More Vulnerable
Pregnant women face an especially dangerous situation with CO exposure because the gas affects both mother and fetus. Carbon monoxide is described as the leading cause of poisoning in the United States and is associated with high maternal and fetal mortality.10PubMed. Carbon Monoxide Exposure During Pregnancy Fetal hemoglobin has a higher affinity for CO than adult hemoglobin, meaning the fetus accumulates the gas to higher levels than the mother and eliminates it more slowly. A multicenter study of pregnancy outcomes after accidental CO poisoning found that severe maternal toxicity was associated with significantly worse fetal outcomes, including stillbirth and cerebral palsy with brain damage consistent with oxygen deprivation.11PubMed. A multicenter, prospective study of fetal outcome following accidental carbon monoxide poisoning in pregnancy Mild accidental exposure generally resulted in normal fetal outcomes, but the line between “mild” and “severe” is not always obvious in real time.
Young children and older adults are also at elevated risk. Children breathe faster relative to their body size, meaning they inhale more CO per kilogram of body weight in the same contaminated room. Older adults, particularly those with pre-existing heart or lung disease, have less physiological reserve to tolerate even modest drops in oxygen delivery. A forensic case series from Verona found that among 24 CO poisoning fatalities, three individuals had concurrent cardiovascular disease, and COHb levels were below 50% in six of the cases, suggesting that pre-existing conditions can make lower levels of CO exposure lethal.12PubMed. Carbon monoxide related deaths: A Verona case series. When cooperation becomes compulsory People who are asleep or intoxicated face heightened danger simply because they are unable to notice symptoms and remove themselves from the environment.
The Damage That Comes After Survival
Surviving a serious CO poisoning episode does not necessarily mean you walk away unscathed. One of the more unsettling aspects of CO toxicity is the phenomenon of delayed neurological sequelae, where a person appears to recover from the acute poisoning only to develop neuropsychiatric symptoms days or weeks later. These can include memory problems, difficulty concentrating, personality changes, movement disorders, and depression.13PubMed Central. Delayed neurologic sequelae of carbon monoxide intoxication
A large study examining over 2,300 victims of acute CO poisoning found that delayed neurological problems developed in roughly 3% of the entire group, but in about 12% of those who were serious enough to be admitted to the hospital. The period between apparent recovery and onset of these delayed symptoms ranged from 2 to 40 days, with an average of about 22 days.14JAMA Neurology. Delayed Neurologic Sequelae in Carbon Monoxide Intoxication Imagine feeling fine for three weeks after a garage exposure incident, then gradually losing the ability to remember recent conversations or navigate a familiar route. The delayed nature of these symptoms makes them easy to miss and difficult to connect back to the original exposure without a high index of suspicion.
The brain is the organ most vulnerable to lasting injury from CO because of its enormous oxygen demands. The damage likely involves a combination of direct oxygen deprivation, inflammatory responses triggered by the reperfusion of blood after treatment, and oxidative stress from CO’s effects on mitochondria.
Treatment After Exposure
The first and most important step for anyone suspected of CO poisoning is to get into fresh air immediately. If you can move, get out. If you find someone unconscious near a running car in a garage, get them out before calling for help if you can do so safely, or call emergency services and let them know CO is suspected so they can take precautions.
Once in a medical setting, the standard treatment is high-flow oxygen delivered through a tight-fitting mask. Breathing pure oxygen speeds up the displacement of CO from hemoglobin, cutting the time it takes to clear the gas from many hours to roughly an hour. For more severe cases, hyperbaric oxygen therapy, which involves breathing oxygen at higher-than-normal atmospheric pressure inside a specialized chamber, has been used to push CO off hemoglobin even faster and potentially reduce the risk of delayed neurological damage. However, the evidence for hyperbaric oxygen remains mixed. A review of the literature noted that while hyperbaric therapy should be considered within six hours for patients with neurological deficits, unconsciousness, cardiac involvement, pregnancy, or very high COHb levels, there is currently no general recommendation for the treatment due to inconsistent results across trials.15PubMed Central. The Diagnosis and Treatment of Carbon Monoxide Poisoning
This is one of those areas where the science has not caught up with the severity of the problem. Hyperbaric chambers are not available everywhere, the window for treatment is narrow, and high-quality trials comparing hyperbaric to standard oxygen therapy are lacking. Many emergency physicians use it when available for the most severe cases, but the decision is often based on clinical judgment rather than definitive protocol.
No Reliable Threshold for “Safe” Exposure
One common misconception is that there is a clean, predictable relationship between how much CO is in your blood and how sick you get. In reality, the progression from impairment to incapacitation to death is not as stepwise as older medical guidelines suggested. Toxicology modeling has found that the blood COHb levels at which a person becomes too impaired to escape an environment are difficult to distinguish from the levels that predict death, and no specific modeled COHb level could be reliably linked to the onset of incapacitation.16PubMed. Acute inhalation toxicity of carbon monoxide and hydrogen cyanide revisited This means you cannot count on feeling a clear “warning stage” before things become life-threatening. By the time CO is affecting your thinking, you may already be too confused to take action.
Individual variation makes this worse. A healthy young adult may tolerate COHb levels that would trigger a heart attack in someone with coronary artery disease. Alcohol and other sedating substances reduce a person’s ability to notice and respond to symptoms. The Verona forensic series mentioned earlier found that 11 of 24 CO fatalities tested positive for substances including alcohol, benzodiazepines, or morphine, suggesting that being under the influence substantially increases the risk of a fatal outcome from CO exposure.12PubMed. Carbon monoxide related deaths: A Verona case series. When cooperation becomes compulsory
Carbon Monoxide Detectors and Vehicle Cabin Monitoring
The single most effective prevention tool for accidental CO poisoning in homes is a working carbon monoxide detector. These devices are inexpensive and widely available, and they belong on every floor of any home with an attached garage, gas appliances, or a fireplace. Many building codes now require them. The detector does what your nose cannot: it identifies rising CO levels before they become dangerous and alerts you with an alarm.
Inside vehicles, the situation is more technologically challenging but not ignored. Researchers have developed prototype cabin air quality monitoring systems using gas sensors that can detect CO and hydrocarbon leakage inside a vehicle cabin. One such system used sensors to detect elevated CO levels and triggered both an alert notification and automatic window opening to ventilate the vehicle.17RSF Conference Series: Engineering and Technology. Case Study Carbon monoxide and Hydrocarbon Leakage Detectors Cabin Automotive Vehicle These systems are not yet standard in consumer vehicles, but they illustrate the direction technology could go. Some people who spend long periods in parked, running vehicles, such as rideshare drivers or people living in their cars during cold weather, have started carrying battery-powered CO detectors as a low-cost precaution.
For practical purposes, the simplest and most reliable prevention remains behavioral: never run a car engine in a closed or poorly ventilated space, and never sleep in a running vehicle without confirming the exhaust system is intact and the environment allows for adequate airflow. If you warm up your car on cold mornings, back it out of the garage first. If you are stuck in snow, clear the tailpipe before running the engine for heat. These habits sound basic, but they account for the majority of preventable CO deaths from vehicle exhaust.
CO Exposure from Pregnancy Complications to Fetal Harm
Returning to pregnant women specifically, the risk profile is worth understanding in more detail because the fetus faces dangers the mother does not. Fetal hemoglobin binds CO even more avidly than adult hemoglobin, and the placenta does not filter CO out. Once CO crosses the placenta, it accumulates to higher concentrations in fetal blood and clears more slowly than from the mother’s circulation. A case report documented a pregnancy complicated by CO poisoning that resulted in fetal bladder complications from anoxic lesions, illustrating that CO can cause organ-specific damage to the developing baby beyond the more commonly discussed neurological harm.18PubMed Central. Carbon Monoxide Poisoning during Pregnancy: Presentation of a Rare Severe Case with Fetal Bladder Complications
The multicenter study on pregnancy outcomes found that mild accidental exposure usually led to normal fetal outcomes, which offers some reassurance for the many pregnant women who worry after brief, incidental exposure to exhaust fumes or a malfunctioning heater.11PubMed. A multicenter, prospective study of fetal outcome following accidental carbon monoxide poisoning in pregnancy The key word there is “mild.” Severe poisoning, marked by loss of consciousness or very high COHb levels in the mother, carried serious fetal risk. Because the fetus takes longer to clear CO, hyperbaric oxygen has been recommended for pregnant patients with significant exposure on the rationale that it may reduce fetal oxygen deprivation, though the same evidence gaps that affect hyperbaric therapy recommendations generally apply here too.
When the Source of CO Is Not Obvious
While this article focuses on car exhaust, it is worth noting that vehicle exhaust is only one common source of CO. Gas furnaces, water heaters, stoves, generators, charcoal grills, and camp stoves all produce CO. Every winter, emergency departments see cases of CO poisoning from portable generators run indoors during power outages, or from charcoal grills used inside tents or apartments for heat. The mechanism of harm is identical regardless of the source.
What makes vehicle exhaust particularly dangerous is the combination of high CO output, confined spaces like garages that connect to living areas, and the normalcy of the activity. People who would never dream of burning charcoal in their bedroom will routinely warm up a car in a closed garage with the door to the house open. The gas is invisible and odorless. The other components of exhaust that you can smell, like unburned hydrocarbons, are present in much smaller amounts in modern vehicles and may not alert you to the danger. By the time you smell something off, CO levels may already be high.
If multiple people in the same household develop headaches, nausea, or flu-like symptoms simultaneously, and the symptoms improve when everyone goes outside, CO should be the first suspect. Pets, being smaller and closer to the ground where CO can settle in some ventilation patterns, sometimes show symptoms first. A dog or cat that seems unusually lethargic or disoriented in a house with a running vehicle nearby is a signal worth taking seriously.