Cars release a mix of gases from their tailpipes, but the bulk of exhaust is surprisingly ordinary. Most of it is nitrogen (the same gas that makes up 78% of the air you breathe), carbon dioxide, and water vapor. The harmful pollutants that cause health and environmental problems make up less than 0.5% of total exhaust volume. That small fraction, though, adds up to an enormous impact when multiplied across hundreds of millions of vehicles.
The Main Gases in Car Exhaust
When gasoline burns completely inside an engine, it produces just two things: carbon dioxide (CO2) and water vapor. These two gases, along with nitrogen that passes through the engine unchanged, make up the vast majority of what comes out of the tailpipe. A typical passenger car emits about 400 grams of CO2 per mile driven, which works out to roughly 4.6 metric tons per year for someone driving about 11,500 miles annually on a vehicle averaging 22.2 miles per gallon.
But combustion is never perfectly complete. Some fuel doesn’t fully burn, and the extreme heat and pressure inside the engine trigger chemical reactions between nitrogen and oxygen from the air. That’s where the harmful pollutants come from.
Carbon Monoxide
Carbon monoxide (CO) forms when carbon in the fuel doesn’t get enough oxygen to fully convert into CO2. Instead of producing carbon dioxide, the reaction stops partway, leaving behind this colorless, odorless, and toxic gas. The amount produced depends heavily on the fuel-to-air ratio in the engine. When the mix is too fuel-rich, CO levels spike. Highway vehicles and other mobile sources account for roughly 80% of all carbon monoxide emissions.
Nitrogen Oxides
Nitrogen oxides, often written as NOx, form when the intense heat inside an engine forces nitrogen and oxygen from the air to combine. These gases don’t come from the fuel itself. They’re a byproduct of the high temperatures that combustion creates.
NOx is a serious respiratory irritant. Breathing elevated levels can inflame your airways, worsen asthma, and trigger coughing, wheezing, or difficulty breathing. Longer exposure may increase susceptibility to respiratory infections and contribute to the development of asthma, with children and older adults facing the greatest risk. NOx also plays a key role in forming ground-level ozone, the main ingredient in smog.
Unburned Hydrocarbons
Not every molecule of fuel combusts inside the engine. These leftover fuel fragments, called unburned hydrocarbons, escape through the exhaust. Some also enter the air through gasoline evaporation from the fuel system, especially in warm weather. Research in Toronto found that vehicle exhaust was the most dominant source of hydrocarbons measured at urban monitoring sites, with gasoline evaporation contributing an additional share that increased noticeably during summer months.
Some of these hydrocarbons are directly toxic. Benzene and 1,3-butadiene, both known carcinogens, are present in exhaust. Others, like formaldehyde and acetaldehyde, irritate the eyes and respiratory tract. As a group, hydrocarbons react with nitrogen oxides in sunlight to produce ground-level ozone, which is why smog tends to be worst on hot, sunny days in cities with heavy traffic.
Particulate Matter
Car exhaust also carries tiny solid particles, collectively called particulate matter. These particles are a complex mixture of carbon (soot), organic molecules, metal oxides, and traces of sulfate and nitrate compounds. Chemical analysis of exhaust particles has found metals including aluminum, cadmium, chromium, copper, iron, lead, nickel, and zinc. The smallest particles, particularly those under 10 micrometers (called PM10) and under 2.5 micrometers (PM2.5), are the most dangerous because they’re small enough to penetrate deep into the lungs. Diesel engines generally produce more particulate matter than gasoline engines, though both contribute.
Sulfur Dioxide
Sulfur naturally present in fuel converts to sulfur dioxide (SO2) and sulfate particles during combustion. The amount released depends almost entirely on how much sulfur is in the fuel. Modern fuel standards have dramatically reduced this. Older diesel fuel averaged around 300 to 500 parts per million of sulfur, but regulations requiring low-sulfur fuel cut SO2 and sulfate emissions by about 95%. Sulfur control matters for another reason too: SO2 damages catalytic converters, reducing their ability to clean up the other pollutants in exhaust.
How Catalytic Converters Reduce Harmful Gases
Modern cars use a three-way catalytic converter to transform the most harmful exhaust components before they leave the tailpipe. It performs three chemical reactions simultaneously. First, it breaks nitrogen oxides apart into harmless nitrogen and oxygen. Second, it converts carbon monoxide into carbon dioxide. Third, it burns off unburned hydrocarbons, turning them into carbon dioxide and water vapor.
These reactions would happen on their own eventually, but far too slowly to matter. The catalyst, typically platinum, palladium, and rhodium coated onto a honeycomb structure, lowers the energy needed to trigger the reactions so they happen almost instantly as exhaust flows through. The converter doesn’t eliminate CO2, though. In fact, by completing the combustion of CO and hydrocarbons, it actually increases CO2 output slightly while dramatically reducing the toxic gases.
The Bigger Picture: Cars and Climate
Transportation accounts for 29% of all greenhouse gas emissions in the United States. Within that, passenger cars are responsible for 20% and light-duty trucks (SUVs, pickups, and minivans) for 37%. Together, personal vehicles represent the single largest chunk of transportation emissions. The primary greenhouse gas from cars is CO2, though vehicles also release small amounts of methane and nitrous oxide, both of which trap heat more effectively than CO2 molecule for molecule but are emitted in much smaller quantities.
The CO2 that cars produce is not something a catalytic converter can fix. It’s the unavoidable end product of burning any carbon-based fuel. Reducing it requires either burning less fuel (through better fuel economy or driving less) or switching to fuels that don’t contain carbon, which is the basic logic behind electric vehicles and hydrogen fuel cells.