Its been feared that terrorists might try to get a tanker truck of noxious chemicals and run a hose from the tank to an air inlet for a major office building. This is difficult, as the air intakes are usually high off the ground (so as not to suck in street level smells and odors.) Getting past security with portable tanks of poison, and up to the floors where access to the air circulation system is another option. But this sort of thing is difficult, as building security personnel are looking for just such an attack. Not exactly Mission Impossible, but definitely Mission Difficult.
Just opening up a tanker full of noxious gas in an urban area full of many people is another possibility. But the number of casualties would probably be low. Thats because this sort of thing happens fairly often, 100-200 times a year. These more common events are called accidents, although most of them take place in thinly populated areas. A deliberate noxious chemical spill in Manhattan or downtown Washington, DC, would be another matter. However, cops are alert to where tanker trucks should, and should not, be. Manhattan or downtown DC are not normal places for these trucks.
Another option is to run a tanker down a route that would not attract police attention, but intersects with a subway system (as is found in New York city and Washington, DC.) Pour the gas down one of the street level air vents and you could do some serious, but not catastrophic, damage.
The problem with noxious chemical gasses is that, to be really effective, you either have get your victims to stand still long enough to inhale a fatal, or at least debilitating, dose, or catch people when they are asleep, or trapped in a subway or building. The perfect situation is when your victims are asleep, as at Bhopal. Another possibility is to pump the gas into a sealed office building. The people running these buildings are determined to prevent an attack like this. The nightmare scenario would be someone with getting a tank of nerve gas into the air ducts of a major building. Difficult to do, but not impossible.
Terrorists have found that, while deadly chemicals can be used for a spectacular attack, they are much more difficult to handle than explosives. If you are driving a tanker truck downtown, its much more effective to have it rigged as a fertilizer bomb than for a gas attack. So while there is much talk about deadly chemical attacks, this is unlikely as long as its so much easier just to do your dirty work with a bomb.
What kind of industrial chemicals can terrorists use to cause massive death and destruction? There are several that are quite deadly and widely available, in particular those used to manufacture insecticides or weed killers. There are mainly organophosphate pesticides, like malathion and parathion, as well as other made from carbamates. Insecticides are related to nerve gas, but are formulated to attack the nervous systems of insects instead of mammals. But if humans inhale enough of this stuff, the effect can be fatal, or at least harmful. We know this from the disaster at Bhopal, India, in 1984. A worker at an insecticide plant mistakenly put water into a tank holding 40 tons of methyl isocyanate, one of the components of insecticide. The water caused a reaction that ruptured the tank and allowed the poisonous gas to pour down a hill onto a densely populated shanty town. It was the middle of the night, and several hundred thousand people got a wiff of the gas. Over two thousand died immediately, several thousand died later, and over 150,000 were injured to one degree or another. This was an ideal situation, as it was at night and several hundred thousand people were asleep in flimsy shanties downhill from the tank releasing the poisonous gas.