Car emissions are the gases and particles your vehicle releases into the air, both from the tailpipe when the engine burns fuel and from the fuel system itself as gasoline evaporates. A typical gasoline-powered passenger vehicle produces about 400 grams of carbon dioxide per mile driven, and that’s just one of several pollutants coming out of the exhaust. Understanding what these emissions are, where they come from, and how your car controls them helps make sense of everything from smog check requirements to that check engine light on your dashboard.
Two Types of Car Emissions
Most people picture exhaust coming out of the tailpipe, but cars actually produce emissions in two distinct ways.
Tailpipe (exhaust) emissions are the byproducts of burning fuel inside the engine. When gasoline or diesel combusts, it produces a mix of gases that exit through the exhaust system. These are the emissions most regulations target, and they include everything from carbon dioxide to soot particles.
Evaporative emissions come from gasoline vapors escaping the fuel system. These happen whether the car is running or just sitting in a parking lot on a hot day. Fuel in the tank naturally produces vapors, and without a containment system, those vapors drift straight into the atmosphere. Research from the University of Tennessee’s Tickle College of Engineering notes that evaporative emissions contribute substantial amounts of hydrocarbon pollution, yet they’re frequently underestimated or left out of pollution inventories entirely.
What Comes Out of the Tailpipe
The exhaust from a gasoline or diesel engine contains a mix of harmful substances. They fall into two broad categories: pollutants that directly damage air quality and health, and greenhouse gases that trap heat in the atmosphere.
The main pollutants are:
- Carbon monoxide (CO): A colorless, odorless gas produced when fuel doesn’t burn completely. It’s dangerous in enclosed spaces and reduces the blood’s ability to carry oxygen.
- Nitrogen oxides (NOx): Formed when nitrogen and oxygen in the air react under the extreme heat inside the engine. These gases contribute to smog, acid rain, and ground-level ozone, which damages crops, trees, and lungs alike.
- Hydrocarbons (HC): Unburned or partially burned fuel molecules that escape through the exhaust. They react with nitrogen oxides in sunlight to form ground-level ozone.
- Particulate matter (PM): Tiny soot particles, especially common in diesel engines. The smallest particles (PM2.5) are fine enough to pass deep into lung tissue and even enter the bloodstream.
- Carbon dioxide (CO2): The primary greenhouse gas from vehicle exhaust. It doesn’t harm you directly at roadside concentrations, but it’s the largest contributor to climate change from driving.
Diesel engines tend to produce more nitrogen oxides and particulate matter than gasoline engines, while gasoline engines produce more carbon monoxide. Both emit carbon dioxide in proportion to how much fuel they burn.
Why These Pollutants Matter for Health
Living or spending time near heavy traffic carries real health consequences. A 2024 systematic review in the medical literature linked traffic-related air pollution to cardiovascular disease, respiratory illness, cancer, cognitive decline, preterm birth, diabetes, and dementia. That’s not a theoretical list. Studies across multiple countries found measurable increases in disease risk tied to specific pollutants from vehicles.
Children are particularly vulnerable. Kids in high-traffic areas were three times more likely to experience chest tightness compared to those in low-traffic neighborhoods, according to a Malaysian study included in the review. Research in the United States found that children exposed to higher levels of fine particulate matter showed lung function deficits of 2% to 3%. In France, increased nitrogen oxide exposure was associated with persistent wheezing in four-year-olds.
For adults, the risks shift toward the heart. A large English study of over 211,000 people found that higher traffic-related nitrogen oxide levels were associated with a 10% increased risk of heart failure. Long-term exposure to fine particles triggered elevated inflammatory markers in the blood, a precursor to cardiovascular disease.
How Your Car Controls Emissions
Modern vehicles come with several systems specifically designed to reduce what gets released into the air. The two most important are the catalytic converter and the evaporative emission control (EVAP) system.
The Catalytic Converter
This device sits in the exhaust pipe between the engine and the muffler. A three-way catalytic converter performs three chemical reactions simultaneously: it breaks nitrogen oxides apart into harmless nitrogen and oxygen, converts carbon monoxide into carbon dioxide, and turns unburned hydrocarbons into carbon dioxide and water. The converter doesn’t eliminate all pollutants, but it dramatically reduces the most toxic ones before exhaust leaves the tailpipe.
The EVAP System
The evaporative emission control system captures fuel vapors before they can escape into the air. Its key component is a charcoal canister filled with activated charcoal that traps gasoline vapors from the fuel tank. When conditions are right, the car’s computer opens a purge valve that pulls those stored vapors into the engine’s intake, where they get burned during normal combustion instead of being wasted.
Other parts of the system include vapor lines connecting the tank to the canister, a vent valve that lets fresh air in to prevent vacuum buildup, and a fuel tank pressure sensor that reports conditions to the car’s computer. Even the gas cap plays a role by sealing the system. A loose or cracked gas cap is one of the most common reasons for an EVAP-related check engine light. Leaks from failed gaskets or O-rings can also let vapors escape, sometimes producing a noticeable fuel smell near the car.
What Happens During an Emissions Test
Many states require periodic emissions inspections, commonly called smog checks. The method depends on your vehicle’s age.
Cars from 1996 and newer have an on-board diagnostic (OBD-II) system, which is essentially a built-in emissions monitor. During an OBD-based inspection, a technician plugs a scanner into a port under your dashboard and reads data from the car’s own computer. The system runs self-tests called “readiness monitors” that verify whether emission control components are working correctly. If the check engine light (formally called the Malfunction Indicator Lamp) is on while the engine is running, the vehicle fails. It also fails if the light doesn’t illuminate during the initial key-on check, since that suggests the bulb has been removed or disabled.
Older vehicles may still undergo a traditional tailpipe test, where a probe inserted into the exhaust pipe measures the actual concentrations of pollutants coming out. This method directly samples what the car is emitting rather than relying on the car’s self-reported data.
Common reasons for failing include a faulty catalytic converter, oxygen sensor problems, misfiring spark plugs, or EVAP system leaks. If your car’s check engine light comes on, getting it diagnosed sooner rather than later usually means a simpler, cheaper fix and a smoother time at your next inspection.
How Fuel Type Affects Emissions
Gasoline and diesel produce different emission profiles. Diesel exhaust contains higher levels of nitrogen oxides and particulate matter, which is why diesel vehicles often need additional equipment like particulate filters and selective catalytic reduction systems to meet modern standards. Gasoline engines run cleaner on those two pollutants but produce more carbon monoxide.
Both fuel types generate carbon dioxide in rough proportion to gallons burned. The EPA’s figure of 400 grams of CO2 per mile is an average for gasoline passenger vehicles. A fuel-efficient compact car will produce less, while a large truck or SUV burning more fuel per mile will produce more. Electric vehicles produce zero tailpipe emissions, though they still have upstream emissions from electricity generation depending on the power grid’s energy sources.