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A screening system to automatically check airline luggage for explosives has been installed at Heathrow Airport in London.

The system is the first of its kind and can screen up to 20 bags a minute, according to the BAA, the operator of Heathrow and six airports in Britain.It uses three techniques to search for explosives. Bags are sent through a computerized X-ray machine that analyzes the atomic weight and density of their contents. About 80 percent of the bags are typically cleared using this technique.

Any luggage with materials that match the characteristics of explosives are then diverted to a second screening process in which the materials are highlighted by color coding and studied more closely.

If the contents are still suspect, they are sent on to a third stage in which sensors check for vapors given off by certain explosives. Finally, the bag is then checked by hand with its owner present, or moved to a safe area if its owner can not be located.

The system - which will operate at a cost of about $1 per passenger - was created in response to a directive after the bombing of Pan Am flight 103 over Lockerbie, Scotland, in 1988, that all luggage must be screened before it is put into an aircraft's hold. The 183 countries that are members of the International Civil Aviation Organization signed the agreement, but it is not legally binding. The system is currently being used to screen bags transferred between international flights, but will soon be used for baggage that is checked in at the airport.

By 1996 the system will be in place at the seven airports in Great Britain operated by BAA, the company that runs Heathrow, Gatwick, Stansted and others.