When used outside a vapor will not remain in place because even a small wind will dilute it and carry it away. However, when dispersed inside there would be no wind, and the agent vapor would remain and the concentration would build, at least until the ventilation system removed it. On the other hand, the ventilation system could well be the means of dissemination. The site of agent use would depend on the objectives of the user. To make a statement and as a warning of attacks to come, a site in a smaller community might be the target.
Terrorist attacks are unpredictable.
We do not know where an attack might be. They are similar in structure and biological activity to some commonly used insecticides, such as Malathion7, and are similar in biological activity to carbamates used as insecticides, such as Sevin7, and used in medicine, such as Mestinon7, Neostigmine7, and Antilirium7. When the enzyme acetylcholinesterase is inhibited the neurotransmitter acetylcholine accumulates to cause overstimulation of those structures innervated by the cholinergic nervous system, namely skeletal muscles, smooth muscles, exocrine glands, and certain nerves both in the central nervous system and at ganglia. Most exposures to sarin have been from vapor. After exposure to a small amount of vapor, the triad of miosis, rhinorrhea, and airway effects sometimes occurs, but often only one or two of these effects occur.