When the impossible became reality and the World Trade Center came tumbling down, America braced itself for a second wave of terror.

So when Attorney General John Ashcroft said Monday that crop-dusting planes across the country had been grounded, fear of another kind of terror from the sky surfaced — a fear of chemical or biological attacks.

Experts have long known that biological and chemical weapons would become part of the arsenal of terrorists; the question was just when. But experts say the very nature of much-feared compounds like the anthrax-causing bacteria, along with the mechanics of crop dusters, make concerns about an imminent biological attack a little bit of a stretch.

"A biological or chemical attack is conceivable,” says Barbara Rosenberg, director of the Federation of American Scientists’ chemical and biological weapons program, “But I don’t think it’s very likely at the moment."

Rosenberg says a crop duster would be very useful for a biological or chemical attack — if you wanted to attack crops.

The problem, she said, is that only a very narrow range of particle sizes — smaller than 10 microns — can lodge deeply enough into human lungs to infect or damage the body. Crop dusters are fitted with much larger dispensers meant for insects and plants. And though a terrorist could modify the plane or dispenser, Rosenberg says, it wouldn’t be easy.

"It’s not extremely difficult, but you don’t go out to the store and buy one off the shelf," Rosenberg said. "They might go to a scientific supplier, but it would have to be constructed to order."

Attacking humans would also require an unwieldy amount of pathogens or poison, according to Milton Leitenberg, a senior fellow at the Center for International and Security Studies and the University of Maryland at College Park.

"With chemical agents you need a very, very large amount — thousands of pounds of agent — in order to kill substantial numbers of people," he said. "If they made an aerosol, they would have to have hundreds and hundreds of gallons of the highest quality nerve agent."

Crop dusters can hold from 200 to 800 gallons of insecticide, but before they can be loaded, the agent must be created. In the case of a biological agent, which most experts think is more likely to be used as a weapon of mass destruction, creating enough to do damage is a major project.

In 1995, the Japanese Aum Shinri Kyo killed 12 people and injured thousands of others in a sarin gas attack on the Tokyo subway. But they "had massive resources and buildings that looked like small factories, scientists and money and four years to work in peace and quiet on biological weapons," Leitenberg said.

“There’s no indication the people involved in (the World Trade Center) attacks have any of that kind of facility anywhere in this country,” he said. “This is not something you’d do in a garage."

There are three kinds of organisms scientists say might be suited as biological weapons, but each has its drawbacks.

The one most often brought up causes anthrax, Bacillus anthracis. It’s a bacterium normally seen in livestock that can be transmitted in spore form and whose fatal form causes lesions in the lungs and brain. It was an anthrax bacterial strain mistakenly released into the air that killed 66 people outside a military lab in the Soviet Union in 1979.

"It’s extremely stable and long lived, always the first choice," Rosenberg said. "But it is not spread from person to person. Each person had to come into contact with it directly."

There’s also smallpox, or variola, a virus that is less stable than anthrax but is highly contagious and causes bleeding and crusty lesions over the body. It was officially declared eradicated by the World Health Organization in 1979.

"It readily spreads from person to person," Rosenberg said. "Infect a few people and they go out and cough and spread it through aerial contact. It spreads like the common cold, but the agent itself is fragile."

But the only two places to have supplies of live smallpox are the Centers for Disease Control in Atlanta and a high-security Russian installation in Novosibirsk.

The third agent often mentioned is Pasteurella pestis, the bacterium that causes the bubonic plague, or pneumonic plague if spread by aerosol. A victim of the plague gets black sores all over the body and his lymph nodes swell up painfully. With the pneumonic plague, death usually occurs in 24 hours.

But working with the plague is even more problematic.

"It’s one those agents that has been weaponized in the past, but I think it’s much less likely to be used than anthrax because it’s not as stable," Rosenberg said. "It requires more care, it’s touchy."