The future of cars might be electric, but some automakers are hedging their battery bets with hydrogen fuel cells. Despite having even less overall infrastructure than battery-powered EVs, hydrogen cars do have some inherent advantages. However, just like EV batteries, hydrogen tech is only as clean as what powers it. And according to a new study, that source isn’t as clean as many think it is.

How natural gas becomes hydrogen for powering cars

On the surface, a hydrogen fuel-cell car appears to be fairly environmentally friendly. After all, its only emission is water. However, the hydrogen in the fuel cell has to come from somewhere. But while hydrogen is the most abundant element in the universe, it’s usually chemically bound to other elements. And breaking those bonds takes a significant amount of effort and energy—which, depending on the process, means significant emissions.

In the US, the primary method of hydrogen production is natural gas reforming, which involves several different reactions, the Department of Energy says. Steam reforming, aka ‘methane reforming,’ has the methane react with high-temperature steam to produce carbon monoxide (CO) and hydrogen. Partial oxidation involves reacting the methane with a less-than-ideal amount of oxygen, again making CO and hydrogen. Finally, the water-gas-shift reaction converts the carbon monoxide to hydrogen using more steam, producing carbon dioxide (CO2) along the way.

Making hydrogen in this manner produces what’s known as ‘gray hydrogen.’ But if you capture the CO2, it’s called ‘blue hydrogen.’

It’s cleaner to burn natural gas than to turn it into blue hydrogen for cars, a new study says

On the surface, electric cars powered by blue hydrogen are perfectly clean; after all, they only emit water. But while using hydrogen is environmentally friendly, natural gas reforming isn’t by any means. And according to a peer-reviewed study recently published in Energy Science & Engineering, the process is more harmful than first thought.

Researchers performed a life-cycle analysis to determine all the emissions associated with making blue hydrogen. That means adding up not just the CO2 emitted during natural gas mining, but also the CO2 emitted during the reforming process, the New York Times explains. After all, running all those reactions requires a significant amount of power, not all of which is clean. Plus, a small amount of methane inevitably escapes during the mining process. And methane is up to 86 times more potent than CO2 in terms of warming potential, Autoblog notes.

In the end, the researchers concluded that the “greenhouse gas emissions from blue hydrogen are still greater than from simply burning natural gas.” And while blue hydrogen is cleaner than gray hydrogen, its total greenhouse gas emissions are only 18%-25% lower.

Are there more environmentally-friendly production methods?

All of that sounds like a significant blow for hydrogen cars, especially in the US. However, that’s not necessarily the case.

Firstly, it’s worth remembering that a hydrogen fuel cell EV produces fewer emissions during use than an internal combustion car. The problem here isn’t the hydrogen itself, but the method used to make it. And that varies significantly across the planet.

The hydrogen used in Toyota’s prototype Corolla race car, for example, isn’t ‘blue hydrogen.’ Instead, it’s ‘green hydrogen,’ produced solely by tapping renewable energy sources; in this case, solar power. And green hydrogen isn’t some low-scale pipe dream. China and Germany recently opened up green hydrogen production plants that use wind and solar power. Shell operates the latter, which makes hydrogen through water electrolysis, not natural gas reforming, Power explains.

Speaking of Shell, it’s also partnering with Mercedes-Benz to open up hydrogen filling stations for trucks and cars. These stations will likely use the green hydrogen made in the German plant. If so, that would make Mercedes’ semis significantly cleaner overall than any diesel- or gasoline-powered alternative.

So, while blue hydrogen cars aren’t a perfect environmental solution, that doesn’t mean hydrogen itself is a poor choice. It just means the production companies have to clean up their acts.

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