r their aircraft is level, climbing or descending.

Depleted uranium (DU) is a heavy substance, 1.7 times as dense as lead, and used in armour-piercing munitions. Many Gulf war veterans believe it is implicated in a range of medical problems they are suffering from, known collectively as Gulf War Syndrome.

Malcolm Hooper, professor emeritus of medicinal chemistry at the University of Sunderland told BBC News Online: "If no precautions were taken at the crash scene, people will have been exposed to hazards that could prove fatal."

Debris from the Korean 747 scattered across fields

DU has also been used in aircraft to make counterweights in the tailplane. A Boeing spokesman told BBC News Online: "The company began using DU in the early 1960s.

"Boeing replaced it with tungsten in the early 1980s, on grounds of cost and availability.

"The Korean 747 was delivered to the airline in June 1980. We think it contained about 300 kg of DU.

"But it would need to have been exposed to a fire of 800 degrees Celsius for more than four hours before it emitted uranium oxide.

"And even then, if it was breathed in it would be only 40% of the amount deemed harmful."

Dutch crash

On 4 October 1992 an El Al cargo 747 crashed into a block of flats in an Amsterdam suburb. It had been carrying 282 kg of DU counterweights.

Only 130 kg were recovered in the clear-up after the crash, and the Dutch commission of inquiry concluded that some of the rest had been released as particles, which would have been inhaled by rescue workers and local people.

The plane was also carrying chemicals used to manufacture the nerve gas sarin, which local people blamed for ensuing health problems.

Over 150 kg of DU was lost in Amsterdam

More than 800 residents and rescue workers were reported after the crash to be complaining of a range of problems, including fatigue, skin complaints, joint and bone pains, kidney ailments and respiratory problems.

The commission of inquiry did not conclude that these problems had been caused by the DU lost in the crash.

DU is known to vaporise into a spray of burning dust on striking a hard object and some studies suggest that it can form uranium oxides at lower temperatures.

A report in 1985 from the Battelle Pacific Northwest Laboratory said that after two hours' exposure to a fire of 700 degC, 22% of a DU munition appeared to have burnt off.

The Korean Air jet crashed in flames and Professor Hooper said: "Those who were handling the wreckage should have been advised of the risk. They should have been taking all the precautions they didn't take in the Amsterdam crash.

"I can't see any way you could have a significant fire in a crash like this without producing the conditions that would allow a potentially hazardous release of DU."