Terror gas exercises to be held

Harmless perfluorocarbons (PFCs) will be released later this month in Marylebone in an effort to see how vapours move around a built-up area.

A computer model will then be designed for use if there is ever an attack.

The environmental impact of the gas released - equivalent to a return car journey from London to Sheffield - is to be "offset", an official said.

Weather

Similar tests have been carried out recently on the London underground network.

The PFCs, which are invisible and odourless, will be released in 50 loads over a period of four to six weeks.

Scientists hope to see how they can deal with chemical attacks in all weather conditions, where to position response teams and how best to evacuate attack areas.

They will look at the effects of street layout and the size of buildings.

An official said: "The point we want to be at is that when a frontline team get to the incident, we are running the models and saying 'this is where we want you'."

No specific threat

He added that the information found could be used in the event of an attack at the 2012 London Olympics or other large-scale events.

The official said: "We continue to be concerned about the possibility of terrorists using chemical or biological weapons."

But the tests were part of a long-term programme rather than a response to a specific threat.

They are being carried out for the government by university experts in air pollution.