The state-owned New Millennium Experience Company has selected a system using HFC which, molecule for molecule, is more than 1,000 times more effective than carbon dioxide at trapping heat in the Earth's atmosphere.
That is why HFC is one of six gases which developed nations, including Britain, agreed to curb at the Kyoto global warming summit in Japan just three months ago.
Now, Greenpeace is asking why the British government is specifying the gas for one of the biggest air-conditioning contracts it will ever award. ''This is a very bad decision and we'll do everything we can to stop it being implemented,'' said Peter Melchett, Greenpeace UK's executive director.
The Dome is expected to need year-round air conditioning as thousands of people flock through its vast enclosed space. When the New Millennium Experience Company (NMEC) advertised for a contractor last summer, it specified that HFC should be used. Now a major US company, Carrier, has won the pounds 1.5m contract which will use nearly seven tonnes of HFC.
This was regarded as much greener than the now banned CFCs and soon-to- be-banned HCFCs used in many conventional air-conditioning systems. CFCs and HCFCs are frowned on because they are destroying the Earth's protective ozone layer, which keeps out harmful ultra-violet radiation.
Yet HFC could make a major contribution to an even bigger environmental problem, global warming, if its use as a CFC-replacement continues to grow rapidly.
Yesterday the NMEC said only one per cent of the gas would be allowed to leak out in a year. Chief executive Jennifer Page said that being able to remove and sell the air conditioning system once the year-long millennium exhibition was over was a key consideration. Whoever takes over the gigantic structure may well not need such a large system.
The Dome organisers also wanted air conditioning which was tried and tested and could be installed quickly, rather than opting for something less orthodox which set new standards in protecting the environment.
But the decision has come as a bitter disappointment to some UK air conditioning contractors and Greenpeace. It had hoped that John Prescott, the Deputy Prime Minister and Secretary of State for the Environment - who played a leading role in brokering the Kyoto climate agreement - would intervene to stop HFCs being used.
In the end, he decided not to, confining himself to writing a letter to the minister in charge of the Dome, Peter Mandelson, which merely expressed disappointment at the choice of air conditioning system.
There are alternatives to HFCs, CFCs and HCFCs, which have been used in large office blocks in Britain. London Transport has two offices with air conditioners which use hydrocarbons.
Nick Cox, sales director with air conditioning firm Earth Care, said: ''Using HFCs is like switching from high tar to low tar cigarettes. The Dome [company has] to America for the quickest, easiest option when they should have been promoting green, British technology. It's a disappointment for UK industry and the environment.''
Greenpeace had earlier threatened to disrupt the Dome's construction because it would have been roofed with PVC plastic, but the Government changed its mind and chose Teflon instead.Reuse content