Car makers have tough safety problems to solve if hydrogen fuel cells are to provide petrol-free, pollution-free transportation for the world in the next decade, Automotive News Europe reported.

Hydrogen is tricky to handle and far more flammable and explosive than petrol. Other challenging hydrogen properties, according to several experts:

It burns with a clear flame, which means great harm can occur before people realise a fire needs to be fought.

It is odourless, which means leaks are difficult to detect.

It can penetrate some metals and make them brittle.

"These are the problems we are going to face and overcome," says Bill Reinert, national manager of the advanced technology group at Toyota Motor North America.

The company has taken all necessary precautions with two fuel cell-powered SUVs it has put into service at the University of California, he says.

Incidentally, the trucks were recalled in May when a hydrogen leak was detected.

Similarly, Honda cars leased to the city of Los Angeles, called the FCX, have extensive safety features, according to American Honda Motor.

Still, it can be unnerving for a novice to see an FCX refuelled for the first time. The vehicle has two fuel doors. The first opens to reveal a connection for a grounding wire. Then the second door can be opened for the high-pressure nozzle to be attached with a leak-proof seal to the fuel receptacle.

In other words, the two-door system is meant to prevent anyone from even trying to refuel the car without first grounding it. It is a graphic reminder of the concern among the experts that a static charge can ignite hydrogen.

Buford Lewis, manager for fuels development at ExxonMobil, says hydrogen "has far more safety issues than gasoline." His company — one of the world's biggest sellers of petrol — has a vested interest in the debate, but he notes that it also processes hydrogen and would expand that operation if markets develop.

Rhoads Stephenson, an engineer and big fan of hydrogen, says that car makers and others need to take extraordinary precautions.

"One spectacular [hydrogen] fire or explosion on the evening news will set them back 20 years," he says.

Stephenson is a consultant to the independent, non-profit Motor Vehicle Fire Research Institute, which studies fire dangers in cars. Kennerly Digges, president of the institute, says concerns about hydrogen go beyond the element's natural properties — the low-ignition point, clear flame and such.

Digges says most fuel cell vehicles use high-pressure fuel tanks to hold hydrogen. A lot of mechanical energy would be released if a tank ruptures, he says.

Potential problems with high-pressure tanks are a reason why some companies, such as DaimlerChrysler, are looking at storing on-board hydrogen in a solid material, such as a borax-like substance.

Stephenson says he's convinced that with good engineering, hydrogen-fuelled vehicles "can be made as safe or safer than what we are currently driving."

Most experts say mass-produced hydrogen-powered vehicles are at least a decade away.

Karen Miller, vice president of the National Hydrogen Association, says: "Hydrogen is no less safe than gasoline or other fuels and in some cases can be much safer. The key to any fuel is handling it safely."