Could terrorists or other criminals use off-the-shelf drones to launch chemical, biological or radiological attacks? That was the question on many readers' minds after last week's look at how the Islamic State has used drones in Iraq and Syria. At the time, I wrote that the hype surrounding the group's drone program would inspire jihadist sympathizers (and perhaps other criminals and terrorists) to use drones to try to conduct attacks in the West. I concluded, however, that the payload limits of commercially available drones, combined with a lack of access to military munitions, would limit the damage any drone attacks could wreak.
The public's interest in chemical, biological and radiological weapons is not surprising given the high profile (and somewhat exaggerated capabilities) ascribed to them by the media and Hollywood. Though the threat of an attack using such weapons could be grave in theory, there are practical constraints that would blunt its impact. By and large, these are the same constraints that would hamper any attempt to use biological, chemical or radiological weapons, regardless of how they are delivered.
The Difficulties of Biological Weapons
Of the three unconventional weapons, biological agents are the most capable of causing a true mass-casualty event. Though commercial drones are limited in the amount of weight they can carry — several kilograms at most — they could, at least in theory, convey enough of a biological agent to kill millions of people.
The nature of biological agents themselves curb their effectiveness as a drone-delivered weapon.
A single gram of weaponized anthrax, the amount included in letters mailed to U.S. Sens. Tom Daschle and Patrick Leahy in October 2001, can contain up to 1 trillion spores — enough to cause somewhere between 20 million and 100 million deaths if the disease were allowed to run its course. The volume of anthrax in the two letters, plus the five or so sent to major media outlets around the same time, was more than enough, if administered efficiently, to wipe out the entire U.S. population. Nevertheless, the attack infected only 27 people. Five died, and the rest recovered after receiving treatment.
The incident dominated the headlines, but it's a prime example of how tough it can actually be to cause mass casualties with even a highly potent, weaponized biological agent like anthrax. After all, if anthrax spores could be spread widely by a drone, infections could easily be treated with antibiotics. People exposed to the spores distributed in a highly obvious drone attack could start courses of antibiotics early, well before the spores have time to incubate, mitigating the impact of the attack.
Non-state actors have tried to develop biological weapons for decades but have struggled to concoct virulent agents. Consider the Japanese cult Aum Shinrikyo, which in the 1990s employed a team of trained scientists and spent tens of millions of dollars to develop sophisticated biological weapons research, production laboratories and other facilities. The group experimented with a variety of agents, including anthrax, botulism, cholera and Q fever. In the end, it couldn't produce a deadly biological agent. The cult shifted its laboratory's efforts toward making chemical weapons such as sarin and sodium cyanide, which it was able to use with some success. But its biological weapons ambitions led to one dead end after another, despite the money and years of effort it expended.
Techniques such as gene editing might someday make it possible for a person to develop and produce an extremely effective and virulent biological agent in a makeshift laboratory. But right now, the only actors capable of creating the types and quantities of weaponized biological agents required for a widespread attack are nation-states.
The Chemical Route
Aum Shinrikyo used nerve agents and cyanide gas in its attack on the Tokyo subway system in 1995. Al Qaeda, moreover, has demonstrated that it possesses the rudimentary knowledge to make a device that generates hydrogen cyanide gas. At his 2001 trial in the "millennium bomb plot," Ahmed Ressam described training he had received at al Qaeda's Deronta facility in Afghanistan in how to build a cyanide-emitting device. Videos found by U.S. troops after the invasion of Afghanistan supported Ressam's testimony, as did confiscated al Qaeda training manuals that held recipes for biological toxins and chemical agents.
There have been other examples as well. In February 2002, Italian authorities arrested several Moroccan men who allegedly were planning to attack the U.S. Embassy in Rome. They were found with about 4 kilograms (9 pounds) of potassium ferrocyanide. Five years later, al Qaeda in Iraq used chlorine gas in vehicle bomb attacks, albeit to little effect. More recently, the Islamic State has used mustard gas and chlorine in Iraq and Syria. But these attacks' psychological impact has far outweighed their tactical significance on the battlefield.
Small quantities of cyanide or sarin could certainly kill many people, but it would not be easy for terrorists to deploy these chemicals in a way that would do so.
There is a reason that military plans for chemical attacks include extensive barrages of artillery shells or rockets carrying large quantities of chemical agents to generate a thick, choking cloud. Smaller releases of chemical agents are far less effective, and as Aum Shinrikyo learned, it is difficult to administer a lethal dose of something like sarin, a volatile substance that decomposes and dissipates quickly.
Because a drone could deliver only a small amount of a chemical agent, whether dropped in a container that would break on the ground or in some sort of airburst, it probably couldn't create the type of heavy cloud needed to drive up the number of fatalities. By comparison, a crop-dusting plane — which could transport and spray hundreds of gallons of a chemical agent — would be a far more effective means of aerial delivery.
The Radiological Danger
Considering the relative ease with which radiological material can be gotten — and the spotlight the media has shined on dirty bombs — it's a little surprising that a dirty bomb or other type of radiological dispersal device hasn't been used in a terrorist attack in the West. Dispersing a radiological isotope with a drone would require aerosolizing, or finely powdering, the material. But unless large amounts of a highly radioactive material are used, its effects would be minimal. To be harmful, radiation exposure must occur either in a high dose over a short period of time or in smaller doses over a longer period of time. Though radiation can be dangerous, of course, limited exposure wouldn't necessarily cause any measurable harm. In fact, people who fly in airplanes or climb mountains are often exposed to more radiation than their peers on the ground, but those levels are manageable.
By their very nature, dirty bomb or radiological drone attacks are unlikely to be very effective in killing people, even if they draw the public's attention.
To maximize the harmful effects of radiation, victims must be exposed to the highest possible concentration of a radioisotope. But with a conspicuous delivery method, the targets are given warning and can be evacuated from the affected area to be decontaminated and treated. This makes it difficult for perpetrators to administer a deadly dose of radiation with a drone attack, which would cause widespread panic but very little death or damage. Like dirty bombs, then, a radiological drone attack would be more a weapon of mass disruption than destruction.
Even in war zones, where military munitions are widely available and where groups such as the Islamic State have access to chemical agents and civilian sources of radioisotopes, conventional attacks are significantly more effective and less complicated than those using drones. Beyond war zones, where terrorists' capabilities are even more limited, jihadist operatives will not be able to inflict the kind of carnage with drones that they have been able to achieve with simple armed assaults or vehicular attacks. All in all, there are few drone attack scenarios — whether using conventional explosives or biological, chemical or radiological weapons — that couldn't be undertaken far more effectively with a much larger general aviation aircraft.