Small quantities can have a tremendous effect. An often quoted example is that, "one small vial holds enough to kill every person on the earth."
In practice, however, dissemination methods are not nearly that efficient. For comparison purposes, a fragmentation hand grenade has an effective casualty radius of 15 meters. The same quantity of chemical agent (about 1.7 pounds), disseminated in a practical way, could fill a 600 foot long subway platform with a concentration which would injure or kill every person who remained on the platform for two minutes. It would take much less than an ounce of biological material to result in the same hazard. A radiological agent, spread in the same location, while it would likely not cause immediate injury, could have the potential to shut down the facility until it could be thoroughly removed.