Cobalt-60 is a radioactive isotope of the metallic element cobalt and the gamma rays it emits destroy tumours.

However, touching, ingesting or just being near to it can cause cancer if it is not properly handled and sealed.

Besides radiotherapy, cobalt-60 can be used to irradiate food, sterilise health care products and measure thicknesses, densities and other properties in industrial processes.

It is just one of many radioactive substances used in hospitals, universities and industry worldwide. Others include iridium-192, americium-241 -- used in smoke detectors -- and caesium-137.

There have been numerous incidents of these substances falling into the wrong hands and causing serious illness and deaths, for example in Brazil in 1987, Turkey in 1999, Thailand in 2000 and India in 2010.

But a bigger worry is that extremists could get hold of the materials and use them in a "dirty bomb" -- a device whereby conventional explosives disperse radioactive materials. Cobalt-60 is particularly well suited.

Such a device would be considerably easier for extremists to make than a nuclear explosive device, which uses nuclear fission or a combination of fission and fusion with either highly-enriched uranium or plutonium.

Although the damage and loss of life caused by a "dirty bomb" -- also known as a "radiological dispersal device" or RDD -- would be a fraction of that unleashed by an atom bomb, it could still cause mass panic.

In recent years governments have made efforts to reduce stockpiles of highly-enriched uranium, not least in the former Soviet Union, and to reduce the risks posed by civilian uses of nuclear technology.

This includes tightening security measures at facilities containing nuclear materials, converting reactors producing medical isotopes and more stringent border checks.

US President Barack Obama hosted a summit in 2010 on the subject, followed by another in Seoul last year. A third is planned in The Hague next year, but much remains to be done, not least in improving international cooperation and introducing binding global rules, experts say.

"This incident (in Mexico) is reminder of the need to make nuclear and radioactive security a top international priority," said Michelle Cann, analyst at the Partnership for Global Security.

"Strengthening transport security for radioactive sources is one of the issues that will be discussed at the March 2014 Nuclear Security Summit in The Hague," Cann told AFP.