British airports are now doubling their number of security body scanners to defend against hard-to-detect explosive that has been coming from Yemen.

The plastic-based explosive, known as PETN, is a low-vapour substance that has been built into the printer bombs intercepted in the recent years. In these devices, PETN is hidden in ink cartridges and typically fitted with a mobile phone circuit board for a timed detonation of the bomb.

Hiding PETN in the cartridge prevents revealing spillage and evades detection by airport security scans.

Dropping or setting PETN on fire will not typically cause it to detonate. A shock wave from a blasting cap or an exploding wire detonator is needed to set off the substance.

PETN, however, is relatively sensitive and widely used by insurgent groups because it is "very easy to set off," Roland Alford, managing director of Alford Technologies, which develops bomb disposal systems, told the BBC.

PETN residues can be picked up by swabs from clothing and belongings and are readily identified with a chemical detector.

In 2010 two printer bombs en route to the U.S. were intercepted; both were sent from al-Qaeda in Yemen. A similar device was found in Yemen in May 2010.

Ibrahim al-Asiri of the al-Qaeda in the Arabian Peninsula is believed to be the mastermind behind the design and implementation of all three bombs that managed to slip past airport security.

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