What Gets Released From Cars

Cars release a mix of exhaust gases—primarily carbon dioxide (CO2), water vapor, nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (VOCs), and particulate matter—along with non-exhaust pollutants such as tire and brake particles, evaporated fuel vapors, leaked refrigerants, heat, and noise. Even electric vehicles eliminate tailpipe emissions but still produce tire wear particles and some brake dust. Below is a detailed breakdown of what comes off vehicles, why it matters, and how it’s changing under new technology and regulations.

Tailpipe emissions: what combustion produces

When gasoline or diesel burns in an internal combustion engine, it creates a range of gases and particles. The items below summarize the most common compounds and their effects.

• Carbon dioxide (CO2): The principal greenhouse gas from cars; about 8.9 kg of CO2 is released per gallon of gasoline burned (roughly 2.3 kg per liter).
• Water vapor (H2O): A benign byproduct of combustion that condenses as visible exhaust on cold days.
• Carbon monoxide (CO): A colorless, odorless gas that reduces oxygen delivery in the body; modern catalysts greatly reduce it but cold starts remain a hotspot.
• Nitrogen oxides (NO and NO2, collectively NOx): Key contributors to smog and respiratory irritation; also form secondary particulates and ozone downwind.
• Unburned hydrocarbons/volatile organic compounds (VOCs): Include reactive species that form ozone; some are toxic (e.g., benzene, formaldehyde, 1,3-butadiene).
• Particulate matter (PM2.5 and PM10): Soot and ultrafine particles harmful to lungs and heart; diesel engines are major sources without filters, and gasoline direct injection can emit fine particles without GPFs.
• Black carbon: The light-absorbing fraction of soot; a short-lived climate forcer with strong warming potential.
• Sulfur dioxide (SO2): Now minimal in places with ultra-low-sulfur fuels but still present where fuel sulfur remains higher.
• Nitrous oxide (N2O) and methane (CH4): Potent greenhouse gases formed in small amounts during combustion and aftertreatment.
• Ammonia (NH3): Can slip from selective catalytic reduction (SCR) systems on some vehicles, contributing to secondary particulate formation.

Together, these pollutants shape air quality and climate impacts. Modern engines and aftertreatment have cut many emissions substantially, but conditions like cold starts, aggressive driving, and poor maintenance can increase them.

How modern controls change the mix

Three-way catalytic converters, oxygen sensors, and precise fuel injection sharply reduce CO, VOCs, and NOx when hot and functioning properly. Diesel particulate filters (DPFs) remove most soot, while SCR with urea tackles NOx on modern diesels; gasoline particulate filters (GPFs) curb fine particles from direct-injection gasoline engines. However, emissions spike during warm-up, high-load operation, or when systems are degraded. Plug-in hybrids can avoid tailpipe emissions on short electric trips, and battery-electric vehicles have none at the tailpipe—though non-exhaust releases still occur.

Non-exhaust emissions: particles and vapors beyond the tailpipe

Not all vehicle emissions come from burning fuel. Wear, evaporation, and ancillary systems produce pollutants that now dominate urban roadway PM in many regions. The list below highlights the main non-exhaust sources.

• Tire wear particles: Microscopic rubber fragments and fillers (a major source of microplastics) shed during driving; chemicals such as 6PPD transform into 6PPD-quinone, implicated in aquatic toxicity.
• Brake wear particles: Metal-rich dust (iron, copper, antimony) released during braking; regenerative braking in hybrids/EVs can cut these emissions significantly.
• Road dust resuspension: Vehicles stir up previously deposited particles, including legacy pollutants and de-icing salts, adding to measured PM.
• Evaporative fuel vapors: VOCs escape from tanks and fuel systems via diurnal heating, permeation, and refueling; modern canisters and onboard diagnostics reduce but do not eliminate these losses.
• Refrigerant leaks: Air-conditioning systems can release small amounts of refrigerant; older R‑134a has high global warming potential (~1,430), while newer HFO‑1234yf is around ~1 GWP.
• Fluid leaks and drips: Engine oil, coolant, transmission and brake fluid can reach roads and waterways, affecting soil and aquatic life.
• Heat and noise: Thermal plumes and traffic noise degrade urban comfort and health, even though they are not chemical pollutants.

As tailpipe controls tightened, these non-exhaust sources became proportionally more important. Policies and technologies are beginning to address them, including limits on brake and tire particle emissions in new European standards.

Electric and hybrid vehicles: what still gets released

Battery-electric vehicles eliminate tailpipe emissions but still generate tire wear particles, some brake dust (often 50–90% lower thanks to regen), resuspended road dust, refrigerant leaks, heat, and noise at higher speeds. Hybrids vary: under engine-on operation they emit like efficient conventional cars; under electric drive they behave like EVs. Upstream emissions from electricity or fuel production depend on the local energy mix and are separate from what the vehicle itself releases on the road.

Health and environmental impacts

The emissions above have distinct consequences for people and ecosystems. The following points summarize key effects.

• Human health: PM2.5 and ultrafines penetrate deep into lungs and bloodstream, increasing risks of heart disease, stroke, asthma, and adverse pregnancy outcomes; NO2 exacerbates respiratory illness; certain VOCs like benzene are carcinogenic.
• Smog and ozone: NOx and VOCs react in sunlight to form ground-level ozone, worsening urban smog and reducing crop yields.
• Climate change: CO2 dominates long-lived warming; black carbon and N2O add near- and long-term climate forcing; refrigerants can be potent if leaked.
• Water and soil: Tire and brake particles, metals, and leaked fluids wash into waterways, affecting aquatic organisms; microplastics accumulate in sediments and food webs.
• Noise and heat: Chronic traffic noise links to stress and cardiovascular effects; heat from traffic contributes to local urban heat islands.

Taken together, these impacts explain why transport remains a priority sector for air quality, climate policy, and urban health planning worldwide.

Policy and technology trends (2024–2025)

Regulators are tightening standards on both greenhouse gases and air pollutants. In the United States, the EPA finalized stronger greenhouse gas standards for model years 2027–2032 light-duty vehicles, steering manufacturers toward more efficient and electric models. In Europe, the forthcoming Euro 7 framework includes, for the first time, provisions to limit brake and tire particle emissions and to maintain low real-world exhaust emissions over a vehicle’s life. Ultra-low-sulfur fuels are now standard in many markets, enabling advanced aftertreatment. Several jurisdictions are phasing down copper in brake pads, expanding low- and zero-emission zones, and exploring controls on tire-wear chemicals. Meanwhile, automakers are deploying gasoline particulate filters, improved evaporative controls, low-GWP refrigerants (HFO‑1234yf), and onboard monitoring to keep systems effective over time.

How to reduce what gets released

Drivers, fleets, and cities can meaningfully cut vehicle emissions with practical steps. The list below outlines effective actions.

• Maintain vehicles: Keep engines, catalysts/filters, and evaporative systems in good condition; fix check-engine lights and fluid leaks promptly.
• Drive smoothly: Gentle acceleration, lower cruising speeds, and avoiding idling cut fuel use, NOx, and particle spikes.
• Choose cleaner vehicles: Opt for EVs, plug-in hybrids, or the most efficient models available; use low-sulfur fuels and follow correct oil specs.
• Optimize tires and brakes: Maintain tire pressure and alignment; select low-wear, low-emission tires where available; enable maximum regenerative braking.
• Refuel and cool smartly: Tighten gas caps, avoid topping off, and service A/C systems to prevent refrigerant leaks.
• Adopt capture tech: Emerging brake dust filters and tire-wear mitigation devices can reduce non-exhaust particles, especially for fleets.
• Urban measures: Street cleaning, porous pavements, traffic calming, and green corridors help reduce and disperse road dust and pollution exposure.

Combining vehicle technology with thoughtful driving and urban design yields the largest and fastest reductions in harmful releases from road traffic.

Summary

Cars release more than just exhaust: along with CO2, NOx, CO, hydrocarbons, and fine particles from combustion, they shed tire and brake dust, evaporate fuel, leak small amounts of refrigerant and fluids, and add heat and noise. Modern controls have lowered many tailpipe pollutants, but non-exhaust sources now make up a growing share of on-road emissions. As policies expand to cover brake and tire particles and automakers shift toward electrification, the mix is changing—but maintenance, smooth driving, cleaner models, and urban strategies remain essential to reduce what cars put into our air, water, and neighborhoods.

What is the biggest pollutant from cars?

carbon monoxides
Across the U.S., vehicle emissions are the largest source of carbon monoxides (56% nationwide and up to 95% in cities) and nitrogen oxides (45% is attributed to the transportation sector). California’s transportation sector accounts for nearly 80% of nitrogen oxide pollution and 80% of the pollutants that cause smog.

What is released when driving a car?

Burning gasoline and diesel fuel creates harmful byproducts like nitrogen dioxide, carbon monoxide, hydrocarbons, benzene, and formaldehyde. In addition, vehicles emit carbon dioxide, the most common human-caused greenhouse gas.

What is released from a car?

Cars primarily emit nitrogen gas, carbon dioxide (a greenhouse gas), and water vapor from the tailpipe, along with smaller amounts of pollutants like carbon monoxide, nitrogen oxides, unburned hydrocarbons, and particulate matter. These harmful substances, produced during the incomplete combustion of fuel, are partially reduced by a catalytic converter, but a certain amount still enters the atmosphere, impacting air quality and contributing to health problems and climate change. 
Major Components of Car Exhaust

• Nitrogen gas (N2): The largest component, largely unaffected by the combustion process. 
• Carbon dioxide (CO2): A greenhouse gas produced from the burning of fuel, contributing to climate change. 
• Water vapor (H2O): Also a product of fuel combustion, it can condense to form liquid droplets that drip from the exhaust. 

Harmful Byproducts

• Carbon monoxide (CO): A toxic gas from incomplete combustion that can be deadly when inhaled. 
• Nitrogen oxides (NOx): Formed at high combustion temperatures, they contribute to smog and are reactive in the atmosphere. 
• Unburned hydrocarbons (HC): Also known as volatile organic compounds (VOCs), these are unburnt fuel particles. 
• Particulate matter (PM): Tiny particles, like soot from diesel engines, that reduce air quality and can harm respiratory health. 
• Sulfur dioxide (SO2): Found in gasoline and diesel, it forms acids and smog when burned. 

The Role of the Catalytic Converter 

• Modern cars are equipped with catalytic converters that convert many of the harmful pollutants, such as carbon monoxide, nitrogen oxides, and hydrocarbons, into less dangerous substances.
• However, even with advanced technology, some pollutants are still emitted, and a faulty converter or engine can result in excessive harmful emissions.

What are the emissions released by cars?

A typical passenger vehicle emits about 4.6 metric tons of CO2 per year. This assumes the average gasoline vehicle on the road today has a fuel economy of about 22.2 miles per gallon and drives around 11,500 miles per year. Every gallon of gasoline burned creates about 8,887 grams of CO2.