Diesel vs. Gasoline: Which Pollutes More?

Diesel engines generally pollute more for local air quality because they emit higher nitrogen oxides (NOx) and particulate matter (soot), while gasoline engines typically contribute more to climate warming per comparable vehicle class because they tend to emit more carbon dioxide (CO2) per mile—unless the gasoline car is a high-efficiency hybrid. In other words: diesel is usually worse for health-harming air pollution; gasoline is often worse for greenhouse gases.

What “gas” means here

In this article, “gas” refers to gasoline (petrol), not natural gas. The comparison focuses on road vehicles using diesel or gasoline in typical passenger and light-duty contexts, with notes on heavy-duty trucks where diesel dominates.

How diesel and gasoline differ by pollutant

Different fuels and engines excel—or falter—on different pollutants. Below is a breakdown of key emissions and how diesel and gasoline compare in real-world use under modern regulations in the U.S., EU, and similar markets.

• NOx (nitrogen oxides): Diesel engines tend to emit more NOx, a major driver of smog and respiratory irritation. Even with advanced aftertreatment (SCR) and tougher real-world testing, controlling NOx in all conditions (cold starts, urban stop‑and‑go, towing) remains challenging.
• Particulate matter (PM/soot): Historically far higher from diesel; modern diesels with diesel particulate filters (DPFs) slash soot dramatically. Gasoline direct-injection (GDI) engines can emit notable ultrafine particles unless equipped with gasoline particulate filters (GPFs), increasingly common in Europe.
• CO2 (climate warming): Per gallon, diesel contains more carbon and emits more CO2 than gasoline. But diesels are usually more fuel-efficient (often 15–25% better miles per gallon in similar vehicles), so CO2 per mile is typically lower for diesel than for a comparable conventional gasoline car. High-efficiency gasoline hybrids can outperform both.
• VOCs and CO (smog precursors): Gasoline vehicles typically emit more volatile organic compounds (including evaporative emissions from tanks and lines) and carbon monoxide than diesels, especially in older fleets.
• Sulfur and toxics: Ultra-low-sulfur fuels are now standard in the U.S. and EU for both diesel and gasoline, greatly reducing sulfate particles. Diesel exhaust is classified as carcinogenic to humans (IARC Group 1). Evidence for gasoline exhaust carcinogenicity is less conclusive but still concerning.

Taken together, diesel’s strengths (efficiency) and weaknesses (NOx, soot) contrast with gasoline’s relative strengths (lower NOx/PM) and weaknesses (higher CO2 per mile in non-hybrids, VOCs/CO). The public-health burden from diesel’s NOx and particles is especially acute in dense urban corridors and near freight routes.

Greenhouse gases vs. local air quality

The “which pollutes more” answer hinges on what you prioritize: climate or local air quality. Below are the main considerations for each frame.

For climate impacts, consider these points.

• CO2 per mile: Comparable diesel cars often emit roughly 5–15% less CO2 per mile than conventional gasoline cars because of higher efficiency, even though diesel has higher CO2 per gallon.
• Hybrids change the math: Modern gasoline hybrids commonly beat diesel cars on CO2 per mile; plug-in hybrids and battery electric vehicles cut tailpipe CO2 dramatically or entirely.
• Life-cycle (“well-to-wheel”) factors: Differences in refining, distribution, and aftertreatment are real but usually smaller than the efficiency gap between diesel and gasoline engines; the vehicle type and driving pattern dominate outcomes.

For local air quality, health impacts weigh heavily.

• NOx and PM dominate health risks: Diesel’s higher NOx and historical soot burdens are strongly linked with asthma, heart disease, and premature mortality. Modern controls cut these sharply, but real-world exceedances have been common in the past, and heavy-duty diesel still drives much of urban NOx.
• Real-world testing improved but not perfect: Europe’s Real Driving Emissions (RDE) and U.S. on-road portable emissions measurements have narrowed the lab-to-road gap, yet extremes of temperature, load, and duty cycle can still spike emissions.
• Exposure matters: Diesel-heavy corridors (ports, warehouses, freight routes) concentrate pollution in nearby communities, amplifying health inequities.

Bottom line: If your primary concern is climate, a diesel car can outperform a comparable non-hybrid gasoline car on CO2 per mile. If your concern is breathable air and smog, gasoline typically has the edge—and electrified options outperform both.

Heavy-duty trucking vs. passenger cars

Most heavy-duty trucks and buses run on diesel and have outsized impacts on urban NOx and PM. Passenger cars, particularly in North America, skew gasoline. The sectoral split changes the story by vehicle class.

Key differences across segments include:

• Heavy-duty diesel: Modern systems (DPF + SCR) drastically reduce soot and NOx when functioning properly. New U.S. rules for 2027 heavy-duty engines will drive NOx down further; maintenance and durability enforcement are critical.
• Passenger cars: Gasoline dominates in the U.S.; diesel car sales remain higher in parts of Europe but have fallen post‑2015. Hybrids and EVs are rapidly reducing gasoline’s CO2 footprint.
• Freight alternatives: Battery-electric trucks are scaling for short-haul/urban duty; renewable diesel and compressed or liquefied natural gas are transitional options with mixed climate and air-quality benefits depending on source and controls.

Because heavy-duty vehicles contribute disproportionately to NOx and PM, cleaning up diesel in freight corridors can yield the largest immediate health benefits, while electrifying light-duty fleets cuts CO2 fastest at scale.

Regulations and technology—what’s changed recently

Policy and technology since the mid‑2010s have reset expectations for both fuels, narrowing some gaps while highlighting others.

Here are the most consequential developments.

• Aftertreatment maturity: Diesel particulate filters (DPFs) and selective catalytic reduction (SCR) are now standard on modern diesel, slashing PM and NOx. Gasoline particulate filters (GPFs) are spreading on GDI engines, especially in Europe.
• Sulfur cuts in fuel: Ultra-low sulfur diesel and low-sulfur gasoline enable advanced catalysts to work effectively, reducing sulfate particles and allowing tighter standards.
• Real-world compliance: Europe’s RDE and U.S. portable emissions measurement systems reduce lab-only optimization; U.S. heavy-duty standards tighten further from model year 2027.
• Electrification push: New U.S. light- and medium-duty greenhouse gas standards through 2032 and EU fleet CO2 targets accelerate market share for hybrids and EVs, reshaping the gasoline-vs-diesel calculus.

The net effect: When systems are maintained and driven as designed, modern diesel and gasoline vehicles can both be very clean at the tailpipe for certain pollutants, but diesel’s NOx control remains more complex, and electrification outperforms both on most metrics.

When each can be “cleaner”

Context matters—vehicle type, technology package, and driving pattern can flip the answer in specific cases.

• Diesel can be cleaner if: You compare a new, well-maintained diesel car with DPF+SCR to an older, non-hybrid gasoline car on long highway trips—CO2 per mile is likely lower, and PM may be minimal. For heavy towing or high-load duty cycles, diesel efficiency helps.
• Gasoline can be cleaner if: You compare a modern gasoline hybrid to a conventional diesel—CO2 per mile often favors the hybrid, while NOx/PM remain low. Short, cold-start city trips also tend to favor gasoline emissions control.
• Electric beats both: In urban stop‑and‑go, battery electric vehicles eliminate tailpipe NOx/PM and, in most regions, reduce life-cycle GHGs as grids decarbonize.

If you must choose between diesel and gasoline, match the powertrain to the duty cycle; if you can choose an electrified option, it will usually outperform both on health and climate grounds.

The takeaway

Asked broadly, “What pollutes more, diesel or gas?” the most accurate answer is: diesel for local air pollutants (NOx and soot), gasoline for climate (CO2)—with important exceptions based on technology, maintenance, and how the vehicle is used. Modern controls and regulations have reduced both fuels’ emissions substantially, but the cleanest path forward is rapid adoption of hybrids and zero-emission vehicles, alongside strict real-world standards and enforcement for remaining combustion engines.

Summary

Diesel engines typically emit more NOx and, historically, more particulate matter, making them worse for local air quality and health. Gasoline engines generally produce more CO2 per mile than comparable diesels, though modern gasoline hybrids can reverse that. New aftertreatment, low-sulfur fuels, and real-world testing have narrowed gaps, but diesel NOx control remains more complex in practice. For most use cases, electrified options beat both on air quality and climate metrics.

Are gas or diesel fumes worse?

No, diesel emissions are not inherently worse than gasoline, and modern diesels with emissions controls are often cleaner for certain pollutants, while still emitting more nitrogen oxides (NOx) and less carbonaceous aerosol than gasoline cars. While older diesels were notoriously dirty, modern systems with diesel particulate filters (DPFs) are highly effective at reducing particulate matter, which is a significant concern with gasoline vehicles. However, due to their combustion process, diesels still produce higher levels of NOx, which can cause respiratory problems. 
Modern Diesel Emissions vs. Gasoline

• Particulate Matter: Opens in new tabModern diesels with DPFs can emit less particulate matter (soot) than gasoline cars. Gasoline cars, especially older ones or those with specific additives, can produce significant amounts of carbonaceous aerosol and ultrafine particles. 
• Nitrogen Oxides (NOx): Opens in new tabDiesel engines inherently produce more NOx than gasoline engines, which can lead to respiratory issues like asthma and decreased lung function. 
• Carbon Dioxide (CO2): Opens in new tabDiesels have better fuel efficiency, meaning they emit less CO2 per mile than gasoline cars, even though the fuel itself contains more carbon. 
• Carbon Monoxide (CO): Opens in new tabGasoline cars are the primary source of carbon monoxide, a toxic pollutant that diesel cars generally do not produce. 

Why the Comparison is Complex

• Vehicle Age and Technology: Opens in new tabOlder diesel and gasoline vehicles often have higher emissions than their modern counterparts. Newer emissions control technology, such as DPFs and catalytic converters, significantly improves overall air quality. 
• Vehicle Type and Fuel Additives: Opens in new tabThe presence of fuel additives, like those in some high-octane gasolines, can contribute to ultrafine particulate matter, while some diesels do not require such polluting additives. 
• Real-World vs. Lab Testing: Opens in new tabEmissions testing methods and the implementation of emissions control systems impact a vehicle’s real-world performance, and a vehicle’s specific engine and driving conditions can significantly alter its emission profile. 

Which type of fuel is most polluting?

Coal is a fossil fuel, and is the dirtiest of them all, responsible for over 0.3C of the 1C increase in global average temperatures. This makes it the single largest source of global temperature rise. Oil releases a huge amount of carbon when burned – approximately a third of the world’s total carbon emissions.

Is diesel or gas better for the environment?

Neither gas nor diesel is definitively “better” for the environment; the choice depends on prioritizing climate change versus local air quality. Diesel engines are more fuel-efficient, producing less carbon dioxide (CO2) per mile, which benefits climate change. However, diesel vehicles historically emit more particulate matter and nitrogen oxides (NOx), harmful to human health and local air quality. Modern diesel engines with advanced emission controls have significantly reduced these pollutants. 
Diesel’s Environmental Pros:

• Lower CO2 Emissions: Opens in new tabDiesel fuel is more energy-dense, and diesel engines are more fuel-efficient, resulting in lower CO2 emissions per mile compared to gasoline engines. 
• Refinery Efficiency: Opens in new tabDiesel fuel requires less energy-intensive refining, which also contributes to lower overall greenhouse gas emissions. 

Diesel’s Environmental Cons:

• Particulate Matter: Diesel exhaust historically contains more harmful particulate matter, which contributes to smog and respiratory illnesses. 
• Nitrogen Oxides (NOx): Diesel combustion produces higher levels of NOx, another air pollutant. 
• Fuel Properties: Diesel is a heavier fuel with longer carbon chains and may contain more impurities, leading to higher emissions of sulfur dioxide (SO2) compared to gasoline. 

Gasoline’s Environmental Pros:

• Lower Local Pollutants: Opens in new tabGasoline engines typically produce lower levels of particulate matter and nitrogen oxides compared to diesel engines. 
• Less Sulfur Content: Opens in new tabModern gasoline contains less sulfur than older diesel fuels, although modern ultra-low-sulfur diesel has a similar sulfur content to gasoline. 

Gasoline’s Environmental Cons: 

• Higher CO2 Emissions: Due to lower fuel efficiency, gasoline engines emit more CO2 per mile than diesel engines.

Key Considerations:

• Modern Technology: Opens in new tabAdvanced emission control systems on both diesel and gasoline engines significantly improve their environmental performance compared to older models. 
• Real-World vs. Laboratory Conditions: Opens in new tabWhile some studies show diesel having lower CO2, real-world driving conditions, fuel-intensive NOx reduction technologies, and engine weight can reduce this benefit for diesel cars. 
• Overall Picture: Opens in new tabWhen considering the impact, it’s important to look beyond just CO2 and consider the combined effect of greenhouse gases and local air pollutants. 

What wastes more, gas or diesel?

Diesels can achieve 25-30% better mileage than a gas engine due to the higher efficiency of diesel fuel, while the direct fuel injection during the combustion process wastes little fuel.