The lethal consequences of CO in engine exhaust is tragically illustrated by the hundreds of persons who die each year from carbon monoxide poisoning caused by a running vehicle inside a closed garage. Others die or become ill in homes with attached garages, while stranded in their car, or while driving or riding in a vehicle with a defective exhaust system.

What causes carbon monoxide poisoning from vehicles?

  • Operating a vehicle with a defective exhaust system.
  • Operating a vehicle with a defective emission system or poorly tuned engine.
  • Driving a vehicle with the trunk lid or rear tailgate open.
  • Driving a vehicle with holes in the car body.
  • Allowing children to ride under a topper on a pick-up truck.
  • Warming up a vehicle in a garage, even with the outside garage door open.
  • Operating vehicles in a garage, carwash, or any enclosed building.

How does carbon monoxide from vehicles affect the air we breath? Before catalytic converters, a 1973 medical study found that a 90-minute ride on a Los Angeles freeway produced EKG irregularities in 40% of patients with preexisting cardiovascular disease. Expressway CO levels approached 25-100 ppm. The EPA emission standards have reduced the amount of carbon monoxide produced by over 95%. Still, a single vehicle emitting high concentrations of CO can leave a plume (cloud) of carbon monoxide. Following the dirty vehicle and driving in the plume can cause health problems for some people. Iowa does not require state emission checks and it’s common to encounter individual vehicles emitting excessive amounts of CO.

Why are defective exhaust systems so dangerous? Internal combustion gasoline engines produce extremely high carbon monoxide concentrations. Even a properly tuned gasoline engine, will produce more than 30,000 parts per million (ppm) of CO in the exhaust stream before the catalytic converter. An exhaust leak can allow escape of CO before it is converted to non-toxic CO2 in the catalytic converter. The CO leaking from the exhaust system can enter the vehicle through holes in the body or open windows or doors. Exhaust systems must be gas tight from the engine to the end of the tailpipe.

How does the catalytic converter reduce the risks of CO poisoning? The typical catalytic converter found on most newer cars and trucks combines oxygen with carbon monoxide to form non-poisonous carbon dioxide (CO2) reducing the high concentrations in the exhaust manifold (typically 30,000 ppm or more) to low concentrations (typically below 1,000 ppm after the catalytic converter). Tailpipe concentrations of carbon monoxide in gasoline engines without catalytic converters are typically from 30,000 to over 100,000 ppm, depending on the condition of the engine.

How can CO poisoning occur if the engine has a catalytic converter? Exhaust gas that leaks out before the catalytic converter has high CO concentrations. Out-of-tune or misfiring engines produce elevated concentrations of carbon monoxide and unburned fuel that can destroy the catalytic converter. During cold starts the catalytic converter is ineffective. And if there is insufficient oxygen (caused by operation in a closed building or with a defective oxygen system), there will not be enough oxygen for oxidizing the CO to CO2.

What is the problem with pick-up toppers, open tailgates, and holes in the vehicle body? For carbon monoxide poisoning to occur, a person must breath the CO. Holes allow the CO to enter the vehicle. Every year several people die while sitting in old vehicles with defective exhaust systems and holes rusted through the vehicle floor. When a vehicle is moving, holes or openings in the rear of the vehicle are under a suction which pulls in exhaust fumes. All holes in the car body must be sealed. The suction effect applies when a rear tailgate window or the trunk is left open or when persons ride in the back of a pick-up truck under a topper. The suction produced as the truck is driven and the lack of ventilation in the topper combine to produce a potentially deadly combination. Normally active children who sleep while in the back of a pick-up may be sleepy because they are breathing carbon monoxide. In California, several cases of children dying in the back of pick-ups under a topper have been documented.

When stranded in a snowstorm we are told to open a window on the downwind side of the car, to operate the engine for only a short time until the car warms, then shut it off. Is this correct? Caution should be used with the mentioned procedures. With the engine off snow may cover the exhaust pipe. An open window on the downwind side of the car will likely be in a low pressure area where exhaust gases could collect and be pulled into the car. Since the amount of carbon monoxide is much higher during initial start-up and decreases dramatically after the catalytic converter warms, continually starting the engine produces more CO than letting the engine run.

Does CO affect driving ability? Yes. Studies show that elevated CO in the body interferes with driving skills. At high carbon monoxide concentrations CO intoxication occurs and severely impairs driving ability. People suffering from CO intoxication think slowly and irrationally, are confused, and are unable to safely operate a motor vehicle. Because they do not realize they are impaired, the condition is extremely dangerous.

We would like to thank Iowa State University, ISU College of Engineering and ISU College of Agriculture and Life Sciences for this material. Visit Iowa State University on line at