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Crying Wolf: The Terrorist Crop-Duster

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The warning could hardly have been more frightening, the source seemingly more credible. Eight years after the horror of 9-11 and the ensuing lethal anthrax-letter mailings, the nation remains so vulnerable to bioterrorists that an attack on a major city by one single crop-duster spraying two to four pounds of anthrax spores could "kill more Americans than died in World War II."

That's the message the congressional Commission on the Prevention of Weapons of Mass Destruction Proliferation and Terrorism first delivered on Oct. 21 and which now has become carved in stone as background for news and opinion pieces on our vulnerability to a massively lethal anthrax attack.

Luckily nothing of the kind is plausible, because had this been a terrorist slam-dunk, more than 400,000 of us would be dead, victims of that lone aircraft flying out of the sun. We know from published reports that 9-11 ring-leader Mohammed Atta considered a crop-duster attack before alighting on a plan that would seem impossibly harder: Not crop-dusters but jetliners, not one but four. And we know that al Qaeda deputy Ayman al Zawahiri has made a decade-long effort to develop anthrax into such a weapon.

So why didn't they do that on 9-11 or on any of the thousands of days since? Because possibilities bred under ideal laboratory conditions become engulfed in real-world details, and taking our eyes off those details is extremely dangerous. Danger 1: We will focus attention and limited resources in a struggling economy on threats with virtually no chance of being carried out, so real threats will not get attention -- as many biological security experts have argued for years. Danger 2: The years-long mania for sharply increasing countermeasure development in high bio-security labs is putting us at higher and higher risk of the theft of weapons-grade anthrax that could make the commission report a self-fulfilling prophecy -- but even then not plausibly anywhere near such a death toll.

Soon after 9-11, the U.S. began throwing billions of dollars into "growing" hundreds of high biosecurity labs throughout the country to study deadly pathogens in case we might need to defend against them in bio-attack scenarios as improbable as one featuring a lone crop-duster. That, instead of massing resources into research that would protect us against biological threats of all kinds, natural or intentional. That, instead of leading international efforts to strengthen existing treaties banning biological and chemical weapons.

But the best way to show how wrong-headed this approach has been is to give a terrorist everything the commission says he needs, then follow along in a crop-duster headed for Washington, often cited as the target of such an attack, calculating fatalities based on available data and scientific studies. Follow? "Off-the-shelf" agricultural aircraft are single-seaters and can't be hijacked. Buying, leasing or stealing one also is much tougher since Atta's interest emerged after 9-11. Never mind. The weapon will be a state-of-the-art agricultural sprayer launched from the countryside. It will fly at 229 m.p.h. and spray 660 gallons of liquid that in a single pass might cover as much as three square miles of cityscape. The aim is to spread inhalational anthrax through the population below.

But first, the terrorist needs to overcome many huge obstacles just to launch the attack. The amount of anthrax required to cause even the radically fewer fatalities we arrive at below would dwarf two to four pounds - that small an amount would result in few fatalities. Thanks to well designed experiments based on worldwide, decades-long agricultural spraying using a harmless relative of anthrax we have data to calculate what he will need and how he must use it.

Modern crop "dusters" deliver only liquids, so the killing power must come from sprayed droplets that by chance remain suspended as an extremely fine mist and are breathed in by those on the ground. Most drops will fall without much effect. To achieve the required dosage of deadly Ames strain anthrax the spores must be suspended in liquid so finely that their clumps are no larger than five microns -- smaller than a red blood cell - and it will take 115 pounds of those spores, not four. Where does he get a deadly anthrax strain? The only likely place: Fort Detrick, Maryland, source of the spores that killed five people and left two dozen permanently injured in the "Amerithrax" letter attacks of October, 2001. The anthrax in those letters reportedly was enough to kill many thousands of people -- under ideal conditions -- and those spores were almost certainly stolen by a lab insider with access. If a few grams were stolen, how could anyone handle such deadly pathogens and culture them from a few stolen grams into 115 pounds -- then weaponize them to the sophisticated degree that even the Amerithrax powder was not. How will the payload spores be kept from clumping in the tanks as they naturally do? Beyond the five-micron size limit they will not penetrate deeply into the victims' lungs; few infections will follow. No matter. Given: the terrorist is airborne with this most improbable cargo.

Now the spores are released. At 229 m.p.h, it would take only four to five minutes for the "duster" to unload over three square miles. If the terrorist can get from takeoff to target without detection by air defense - what are the odds? -- he may be able to cover that area before being shot down. Everyone in the drop zone, of course, must be out in the open and fail to notice an aircraft only 60 yards overhead spraying mist, and they must fail to seek medical treatment. Inhalational anthrax is treatable if caught early. Again, no matter. Given: they all have been so exposed. Under these ideal conditions, which stretch even the most fearful imagination, we calculate the maximum death toll at 3,100.

However improbable an idea and however difficult to execute, couldn't a terrorist attempt it, somewhere, sometime, with some success? Of course. A few or a dozen people might even die with the two to four pounds of anthrax the commission deems sufficient. But that is not the kind of event on which a nation beset by real pandemics and other major public health issues focuses its biological security strategy, and that is precisely what the commission is urging.

Ours is not merely a civilian view of the bioterror threat. Col. Ted Cieslak, the Department of Defense's liaison to the Centers for Disease Control and Prevention, told an Oklahoma audience a few months ago that "over the past decade, too much emphasis has been put on bioterrorism, and too many billions of taxpayer dollars have been wasted on somewhat dubious efforts for bioterrorism defense." The primary value of bio weapons lies in the publicity they might generate for terrorists, he noted.

There is the final danger. The awful image of a single crop duster killing off most of a major American city primes the public to be so terrified that any attack would arouse terror.

Bottom line: Biological weapons for the foreseeable future are far more easily stolen than made. The best way for the United States to attack the problem is to lead international initiatives to strengthen the Biological Weapons Convention, passed in 1975, and the sister Chemical Weapons Convention that became law twenty years later. The best way to keep biological weapons out of the hands of terrorists is to make sure they are in no one's hands, and that their feedstocks are studied in only a small number of laboratories under the most secure conditions.

Klotz and Sylvester are the authors of Breeding Bio Insecurity: How U.S. Biodefense Is Exporting Fear, Globalizing Risk, and Making Us All Less Secure, University of Chicago Press, Oct. 15, 2009.