The risks of nuclear terrorism are commonly misunderstood. It’s unlikely, given technological difficulties and security measures, that an extremist group could build a large-scale atomic weapon. But a group could easily obtain material for a smaller radiological weapon: a so-called dirty bomb.

Thousands of businesses and hospitals around the world, for example, keep potentially deadly stockpiles of the radioactive isotope cesium-137. While a cesium-137 blast would kill no more people than a conventional explosion would, millions of people in the fallout area could be exposed to cancer-causing radiation. Cleanup could cost tens of billions of dollars.

Yet supplies of cesium-137 and three other substances – cobalt-60, iridium-192 and americium-241 – are ill-protected from theft. What’s needed is an international effort to secure these radiological materials – just as the International Atomic Energy Agency has helped member countries protect nuclear energy waste.

Cesium-137 is used in about 500 U.S. hospitals to purify blood for transfusions. These irradiators aren’t even necessary anymore. High-powered X-ray machines can now do the same job. Cobalt-60, used in so-called Gamma Knife cancer treatment, and iridium-192 and americium-241, which have industrial applications, are not so easily replaced. But the growing terrorist threat should give industry and government agencies new incentive to ramp up research to find alternatives.

All 168 member states of the IAEA should make mandatory the agency’s recommendations for increasing security of stockpiles, notifying neighbors of potential thefts and phasing out the use of dangerous isotopes.

Small, deadly and capable of spreading terror: Extremist groups such as Islamic State and deadly isotopes have a lot in common. Keeping the world safe means keeping them apart.