Terrorism works not as much by causing death as by causing fear. Bombings and shootings kill a few, or a few dozen or few hundred or even a few thousand, but frighten millions. The unpredictable random attacks in public places leave us all feeling vulnerable, afraid, just what terrorists hope to achieve. So wouldn’t you assume that among all the things that governments are doing to reduce the danger from terrorist attacks, that a big part of that effort would be to try and minimize the fear these attacks cause? You’d think so, but with one of the most fear-inducing weapon terrorists might use, you’d be wrong.

The weapon is a dirty bomb, a conventional explosive mixed with radioactive material that would be dispersed across a community (it's technically known as a radiological dispersal device). In the aftermath of the Paris attacks last November Belgian authorities discovered evidence that the terrorists involved in the Paris attacks were surveilling a high-level Belgian nuclear official who had access to radioactive material—not the kind that could be used to build a nuclear weapon, but perfect for a dirty bomb. And Reuters reported that radioactive material was stolen in Iraq last November from an oilfield company that was using the material to test the integrity of oil pipelines. No one knows who took it, or where it is.

The prospect of such a bomb seems terrifying, but anyone who knows the basic science of radiation biology knows that it wouldn't cause much health damage, because the dose of radioactivity to which most people might be exposed would be very low. And experts know, based on the 65 year Life Span Study of the survivors of atomic bomb explosions in Japan, that even at extraordinarily high doses, ionizing radiation only raises lifetime cancer mortality rates a little bit—just two thirds of one percent for survivors who were within three kilometers of ground zero. And despite popular belief, it causes no genetic damage that is passed on to future generations. At the low doses most people might get from a dirty bomb, the health risk is infinitesimal. Not zero, but tiny.

But most people don’t know that. They believe that any exposure to nuclear radiation is really dangerous. Radiophobia is deeply carved into public belief. So if a dirty bomb goes off, and the global media screams with dramatic alarms about the danger of radiation, fear will spread faster and further than the isotopes of iridium or cobalt or whatever nuclear material terrorists have used. And that fear will do immense harm.

Should such a weapon go off in a city, much of that city will be shut down, and major areas evacuated, for weeks or months. Tens of millions of people in the wider surrounding region, especially downwind, will be afraid. The economic costs will be vast. So will the health effects—not from radiation, but from the sweeping physical impacts of stress, including increased cardiovascular risk and weakened immune systems. A dirty bomb will likely produce a global cry for dramatic retaliation against known terrorist havens, and heads of state will find it hard to resist. Short of the disastrous physical harm of a nuclear weapon itself, it’s hard to imagine a terrorist attack that could do more damage.

So what are governments doing to protect us? They're doing a great deal to keep such a device from going off in the first place. And thank goodness those efforts have been successful, so far. But compared to the hundreds of millions of dollars being spent to prevent such an attack, practically nothing is being done to proactively defuse the fear a dirty bomb would produce. There are no attempts to put the actual danger of nuclear radiation in perspective for the public or the news media.

The US Nuclear Regulatory Commission (NRC) and Centers for Disease Control (CDC) have websites about radiological emergencies. (The EPA, which has significant authority over the public and environmental health effects of radiological emergencies, has practically nothing on its site about such events.) The CDC site is mostly about what to do and how to decontaminate yourself, with little about how low the risk is. The NRC states

Just because a person is near a radioactive source for a short time or gets a small amount of radioactive dust on himself or herself does not mean he or she will get cancer. Any additional risk will likely be extremely small.

But posting information on a bureaucracy’s web site is hardly proactive public outreach. Much more should be done. A coordinated, persistent, multi-faceted, multi-agency communication campaign should be conducted to reach the public with this information. A key part of this outreach should be the news media, so they understand in advance the actual threat of radiation should a dirty bomb be used. An information campaign could partner with a wide range of sources more trusted than the government; scientific, health, and medical authorities and organizations, local officials, well-known figures on social media and in popular culture, even faith leaders concerned about public well-being. The information could be embedded in the story lines of movies and TV shows that often feature terrorist attacks in their plots. And yes, this information could be presented simply, clearly, without the technical or bureaucratic language or scientific nuance that so often interferes with effective risk communication.

Would such a campaign totally dispel the excessive fear of nuclear radiation? Of course not. The roots of that fear run far too deep to dig up entirely. But a carefully researched and carefully designed risk communication campaign could help diminish that fear, and take at least some of the power of a dirty bomb to terrorize us out of the hands of terrorists. Governments that are working hard to protect us from such attacks must also take this important step. And soon.