Blame the millions of vehicles that pour into the city daily, clogging the city's arteries as surely as soot sticks to a chimney. Three-quarters of all air pollutants in London come from the roads; so the promise of a new generation of vehicle that leaves no pollution should prove as welcome as an open window on a sweltering day.

London is investing heavily in hydrogen, and spearheading this transport revolution is one of the city's most recognisable symbols. The first stop on the road to a hydrogen-based economy is being made by the red London bus.

Capital-dwellers might have seen one of the three hydrogen buses patrolling the RV1 route between Covent Garden and the Tower of London. Save for stickers displaying its green credentials, a hydro-bus looks exactly like a regular single-decker. But listen carefully: it is barely louder than a milk float. Look closely and you'll notice its only emission is a plume of steam.

Soon, up to a dozen buses will be added as London spends tens of millions on its experimental fleet of "hydro-vehicles". If the test proves successful, London's bus fleet could switch to hydrogen sometime in the next decade. By 2010, the hydro-fleet will have swollen to 70: we could also expect hydrogen-powered emergency vehicles, police cars and road sweepers.

The London mayor, Ken Livingstone, has become one of the strongest advocates of hydrogen power. Along with unlikely fellow travellers George W. Bush and Arnold Schwarzenegger, the mayor holds high hopes that hydrogen could help create cleaner air and dilute the city's dependence on fossil fuels. If hydrogen can be made and distributed cheaply enough - and in volume - it could, he argues, help make fossil fuels history.

Supply lines

London is not alone. Hydrogen-powered bus projects are being prepared all over the world. From Cambridge to California, Norway to Nagoya, Perth to Porto, pilot schemes are being readied and supply lines put down. Last week, Shell announced a partnership with the Dutch bus manufacturer Man that will see 20 hydro-buses on the streets of Rotterdam by 2009.

The appeal of hydrogen is easy to understand. It is the most abundant element, and abundantly available on Earth (though locked up in water), and its oxidation produces huge amounts of energy per unit mass. The hydro-buses - which experts say are as safe as conventional ones - use a process invented in 1839 by William Grove, a British barrister and amateur physicist, in which hydrogen is combined with oxygen within a fuel cell to generate a powerful electric current. The hydro-buses are quiet because the fuel cell, which is held in the roof, removes the need for an engine. The only exhaust is steam, because the hydrogen removes the need for the diesel engine.

But it's not quite so simple as filling a bus with hydrogen and driving away. The hydrogen molecules must first be extracted from another source, usually either water or a fossil fuel such as coal or gas. The cleanest way to make hydrogen is to electrolyse water using electricity generated from renewable sources; solar power has been used to power fuel-cell buses in Perth in Western Australia. But in London's case, "this means obtaining hydrogen from natural gas", says Mark Watts, the London mayor's adviser on energy, transport and air quality. The trouble is, he explains, that this process still produces carbon dioxide, "although it is about 30% less than the amount from the equivalent diesel engine". This also means the carbon dioxide can be released into the atmosphere miles from London.

The use of hydrogen is being held down by price. It costs up to 10 times more to take a bus passenger one mile using hydrogen rather than diesel.

Paul Medlicott, of the London Hydrogen Partnership, an organisation developed by the mayor's office "to drive London towards a hydrogen economy" says London can help drive down the cost of hydrogen and the cost of the fuel cells. Medlicott hopes that by ordering more buses and sharing the cost with "partner cities", London can kick-start its own hydrogen economy. He also stresses that hydrogen is at least as safe as petrol.

Liquid energy

But will procuring a few buses kick-start a full-scale hydrogen economy? Some think London's preferred choice of liquid hydrogen is not the best way.

Powerfuel's Richard Budge is talking with companies about using gaseous hydrogen to fuel a new generation of buses that would operate in the conurbations of Yorkshire. When Hatfield Colliery reopens next year it will fuel a nearby power station. The coal will be burned to turn the turbines that produce electricity; one byproduct will be almost pure hydrogen gas, unsuitable for fuel cells, but ideal for hydrogen-powered internal combustion engines.

Budge believes that a cheap supply of the gas could be funnelled into tankers. "It would be suitable given the short distances they travel to use gaseous hydrogen," he argues. "From our plant [near Doncaster] you could use buses with standard transmission, standard engines with just slight modifications. It would be a lot cheaper than the millions paid for fuel cell buses [in London]. I'm not saying fuel cell buses shouldn't go ahead... but to kick-start a hydrogen economy you want to use the simplest thing, which is gaseous hydrogen."

But even as a gas, hydrogen has disadvantages. Although it can be produced much more cheaply, it remains expensive to transport over long distances.

However, the cost of hydrogen fuel cells is also falling. According to Mike Stannard of Bac2, a UK manufacturer of fuel cell components, sales of commercial fuel cell products are increasing in niche markets. "Various mainstream electronics companies are saying they will have fuel cell powered laptops in the shops next year."

And perhaps there lies the truth: we are closer to a hydrogen economy of household goods than of mass transit. But eco-futurists should take hope. As noted by the former editor of the Harvard Business Review, Nicholas Carr, new technologies often take years to take hold. "The future arrives in fits and starts," he wrote on his website, Digital Renderings. "Early versions of new technologies are often prohibitively expensive ... and that can restrict their use to a small slice of the market for many years."

The technologies he talks about - fax machines, PCs and even railways - took years to reach the masses. Today, each is as common as a red London bus.