Hydrogen is the cleanest burning fuel. When it combusts, the only byproduct is water, with zero carbon dioxide, carbon monoxide, or soot leaving the exhaust. NASA uses hydrogen to launch rockets, and the water produced is pure enough for astronauts to drink. Among conventional fuels, natural gas is the cleanest, producing about 53 kg of CO2 per million BTU, roughly half that of coal.
How Fuels Rank by Emissions
The U.S. Energy Information Administration tracks how much carbon dioxide each fuel releases per unit of energy. Measured in kilograms of CO2 per million BTU, the ranking from dirtiest to cleanest looks like this:
- Coal: 95.99 kg CO2
- Gasoline: 70.66 kg CO2
- Propane: 62.88 kg CO2
- Natural gas: 52.91 kg CO2
- Hydrogen: 0 kg CO2
These numbers capture only what comes out of the flame itself. Coal sits at the bottom because it’s a complex solid with high carbon content and significant sulfur, mercury, and particulate emissions on top of the CO2. Gasoline produces about 34% more CO2 per unit of energy than natural gas. Propane falls in between, which is why it’s sometimes marketed as a “clean” alternative for grills, generators, and fleet vehicles, even though it still releases meaningful carbon when burned.
Why Hydrogen Isn’t a Simple Winner
Hydrogen produces no carbon emissions at the point of use, but the full picture is more complicated. Most hydrogen today is made from natural gas in a process that releases significant greenhouse gases. Lifecycle analyses assign different labels based on how the hydrogen is produced:
- Gray hydrogen (from natural gas, no carbon capture): 12.3 to 13.9 kg CO2 equivalent per kg of hydrogen
- Blue hydrogen (from natural gas with carbon capture): 7.6 to 9.3 kg CO2 equivalent per kg
- Green hydrogen (from water using wind power): 0.6 kg CO2 equivalent per kg
- Green hydrogen (from water using solar power): 2.5 kg CO2 equivalent per kg
So hydrogen made with wind energy is genuinely near-zero emission across its entire lifecycle. Hydrogen made from natural gas without carbon capture is cleaner at the tailpipe but carries a heavy upstream carbon cost. The method of production matters as much as the combustion chemistry.
There’s also a less obvious pollution issue. When hydrogen burns in air at very high temperatures (above 1,500°C), nitrogen and oxygen in the atmosphere react to form nitrogen oxides, the same smog-forming pollutants produced by diesel engines. The U.S. Department of Energy notes that because hydrogen burns hotter than natural gas, it can actually produce comparably higher nitrogen oxide emissions in certain industrial applications like turbines and furnaces. Fuel cells sidestep this problem entirely because they convert hydrogen to electricity through a chemical reaction rather than combustion, producing only water.
Natural Gas: Cleanest of the Fossil Fuels
If you’re comparing conventional fuels you can actually buy today, natural gas wins on CO2. Its simple chemical structure (one carbon atom bonded to four hydrogen atoms) means more of the energy comes from burning hydrogen and less from burning carbon. It also produces virtually no particulate matter or sulfur dioxide when burned, which is why coal-to-gas switching in power plants has been a major driver of air quality improvements in cities.
The catch is methane leaks. Natural gas is mostly methane, and methane that escapes unburned during drilling, processing, and pipeline transport is a potent greenhouse gas. Over a 100-year window, methane traps 27 to 30 times more heat than CO2. Over 20 years, that figure jumps to 81 to 83 times, according to the latest IPCC assessment. A natural gas system with significant leaks can erase much of its combustion advantage over coal, depending on where and how the gas is extracted.
Indoors, natural gas cooking produces nitrogen dioxide and fine particulate matter. Measurements in homes with gas stoves show average indoor nitrogen dioxide levels around 21.8 parts per billion, with spikes as high as 148 ppb during heavy use. For context, the World Health Organization recommends annual average exposure stay below about 5 ppb. Proper ventilation with a range hood that vents outdoors makes a significant difference.
Solid Fuels and Particulate Matter
When people ask about “clean burning,” they often care about more than just CO2. Wood, coal, and other solid fuels produce heavy loads of fine particulate matter (PM2.5), the tiny particles that penetrate deep into your lungs and contribute to heart disease, asthma, and premature death. This is the main reason wood smoke in valleys and urban areas is such a persistent health concern.
Modern EPA-certified wood stoves and pellet stoves are capped at 2.0 grams of PM2.5 per hour when tested with standardized fuel, and 2.5 grams per hour with cord wood. That’s a dramatic improvement over older uncertified stoves, which could emit 10 times as much. Still, even a certified wood stove produces orders of magnitude more particulate pollution than a natural gas furnace, which emits essentially none. If indoor air quality and respiratory health are your priority, gas and electric heat are far cleaner choices than any wood-burning appliance.
Renewable Diesel and Liquid Alternatives
For vehicles and equipment that need liquid fuel, renewable diesel (also called HVO, for hydrotreated vegetable oil) is one of the cleanest drop-in options. It’s chemically similar to petroleum diesel but made from fats and plant oils. In real-world driving tests, renewable diesel reduced nitrogen oxide emissions by about 13% and cut particle emissions by 25% to 41% compared to standard diesel. It also slightly improved fuel efficiency by 6% to 10%. Unlike biodiesel blends, which can increase carbon monoxide in cold starts, pure renewable diesel performs consistently across conditions.
Propane is another practical option for fleets and off-grid applications. At 62.88 kg CO2 per million BTU, it produces about 11% less carbon than gasoline and burns with very low particulate and sulfur emissions. It’s not as clean as natural gas per unit of energy, but it’s easier to store and transport to remote locations.
The Bottom Line on “Clean”
The answer depends on what kind of “clean” you care about. If you mean zero carbon at the point of combustion, hydrogen is unmatched. If you mean the lowest lifecycle greenhouse gas emissions with current technology, green hydrogen produced from wind power comes closest at just 0.6 kg CO2 equivalent per kilogram. If you mean the cleanest fossil fuel you can realistically use in your home or vehicle today, natural gas produces the least CO2 per unit of energy at about 53 kg per million BTU, roughly 45% less than coal and 25% less than gasoline. And if your concern is air quality and particulate pollution, any gas or liquid fuel burns far cleaner than wood or coal.