Car exhaust releases a mix of gases and tiny particles, most of which are harmless but a small fraction of which are genuinely toxic. By volume, about 71% of what comes out of a tailpipe is nitrogen, 18% is carbon dioxide, and 9% is water vapor. The remaining 1-2% contains the pollutants that matter for your health and the environment.
The Major Gases
The bulk of exhaust is unremarkable. Nitrogen makes up the majority because it’s already the main component of the air your engine pulls in. Carbon dioxide and water vapor are the natural byproducts of burning any hydrocarbon fuel. A small amount of leftover oxygen and trace noble gases round out the mix.
Carbon dioxide is the greenhouse gas that gets the most attention. A typical passenger vehicle produces about 400 grams of CO2 per mile driven, according to the EPA. That adds up to roughly 4.6 metric tons per year for an average car. CO2 isn’t directly toxic to breathe at roadside concentrations, but it’s the primary driver of climate change from transportation.
Carbon Monoxide
Carbon monoxide forms when fuel doesn’t burn completely. It’s colorless and odorless, which makes it dangerous in enclosed spaces like garages. CO binds to red blood cells far more effectively than oxygen does, which means even moderate exposure can starve your tissues of oxygen. At roadside levels it rarely reaches dangerous concentrations outdoors, but heavy traffic in poorly ventilated areas (tunnels, parking garages, dense intersections) can push levels high enough to cause headaches and fatigue.
Nitrogen Oxides and Ground-Level Ozone
Nitrogen oxides form when the extreme heat inside an engine forces nitrogen and oxygen from the air to combine. These gases irritate airways on their own, but they also trigger a secondary problem. When nitrogen oxides and volatile organic compounds from exhaust react in sunlight, they produce ground-level ozone, the main ingredient in smog. This is not the protective ozone layer high in the atmosphere. Ground-level ozone inflames lung tissue, worsens asthma, and reduces lung function even in healthy people.
Diesel engines tend to produce more nitrogen oxides than gasoline engines because they run at higher temperatures and pressures. This is one reason diesel vehicles face stricter emissions regulations in many cities.
Volatile Organic Compounds
Exhaust contains dozens of volatile organic compounds, several of which are known carcinogens. Benzene is the most prominent, with tailpipe emission rates measured at roughly 11,900 micrograms per kilometer. 1,3-butadiene, another established carcinogen, is emitted at about 2,100 micrograms per kilometer. Formaldehyde, acrolein, and crotonaldehyde also appear in measurable quantities. These compounds contribute to cancer risk with long-term exposure and, along with nitrogen oxides, fuel the formation of ground-level ozone.
Particulate Matter
The tiny solid particles in exhaust are among the most harmful components. Fine particulate matter (PM2.5, particles smaller than 2.5 micrometers) is a complex mixture of elemental carbon (soot), organic carbon compounds, and metals including iron, zinc, calcium, aluminum, and sodium. Trace amounts of more toxic metals like cadmium, chromium, and lead also show up.
Diesel vehicles produce dramatically more particulate matter than gasoline vehicles. In one study, diesel engines emitted about 50 milligrams of total carbon per kilometer compared to just 1.2 milligrams from gasoline engines, roughly a 40-fold difference. The composition also differs: diesel soot is mostly elemental carbon, while gasoline particulates contain proportionally more organic carbon.
Modern diesel vehicles fitted with wall-flow particulate filters capture the vast majority of these particles before they leave the tailpipe, but older diesels without filters remain significant polluters.
Ultrafine Particles and Health Effects
The smallest particles in exhaust, ultrafine particles less than 0.1 micrometers in diameter, pose a unique threat. They’re so small they pass through lung tissue into the bloodstream and travel to organs throughout the body, including the brain (where they can reach neural tissue through the nasal passages). Their enormous surface area relative to their size lets them carry toxic compounds deep into tissues where larger particles can’t reach.
The health consequences are well documented. Short-term exposure to high concentrations raises blood pressure, disrupts heart rhythm, and increases markers of inflammation and blood clotting. Over months to years, chronic exposure promotes atherosclerosis (hardening of the arteries) and is linked to metabolic syndrome. Studies in Boston found that even in a general adult population, higher ultrafine particle counts in the air were associated with elevated blood pressure.
The WHO identifies particulate matter as the air pollutant with the strongest evidence of harm, noting that inhaled particles trigger inflammation, oxidative stress, and cellular damage across the lungs, heart, and brain.
Sulfur Dioxide
Sulfur dioxide comes from sulfur impurities in fuel. Modern gasoline and diesel contain far less sulfur than they did decades ago thanks to refining standards, so SO2 emissions from cars are relatively low today. It remains a concern primarily from older vehicles or in countries with less stringent fuel quality regulations. Even at low levels, sulfur dioxide irritates the airways and can trigger asthma attacks in sensitive individuals.
How Catalytic Converters Help
Nearly every gasoline car built since the 1970s uses a catalytic converter to clean exhaust before it exits the tailpipe. Modern three-way catalytic converters simultaneously reduce carbon monoxide, unburned hydrocarbons, and nitrogen oxides. Combined with diesel particulate filters and selective catalytic reduction systems on diesel vehicles, these technologies cut up to 90% of harmful emissions. Some systems achieve even higher efficiency: volatile organic compound emissions from engines with modern catalytic converters can be over 99% lower than from engines without them.
This is why a well-maintained modern car produces far less pollution per mile than vehicles from earlier decades, even though the raw combustion process hasn’t fundamentally changed. It also means a failing catalytic converter can dramatically increase the pollutants your car releases.