There really aren't many clear-cut answers to your questions....
Not all biologicals can be aerosolized...at least not in a volume that makes them portable in a crop duster...
I think there numerous agencies working on this issue right now. Our company does research in the area of biological detectors using DNA.
There are other agencies and programs working on counterdefenses to aerosols and topical exposures through air and HVAC systems....
Our family has a philosophy that we live our life as safely as we can and try to stay as aware as we can. I live in a rather large city,,,a crop duster flying over spraying liquid would not be in the air for long, IMHO. However, anything is possible. I think the odds are other things may kill us first. I'm not so sure I'd want to stay around after such an event if everything in the landscape has been contaminated. What kind of life is that?
It's a difficult question and it's not wrong to think about it.
Thank you for being so helpful and understanding my concern.
Long enough. In most places it would take longer to scramble fighters and have them get there, or get the local guard unit to bring out some Stingers, than it would for the guy to empty his tanks. Besides, with Jihadies, the pilot would probably crash the aircraft into the juciest target he could find as soon as the spray tanks went dry. Or consider that the target might be a sporting event, say a college or pro football game. That way he could spray first and crash later, all in the same place. The airspace over such events is restricted, but like gun control laws, such restrictions aren't going to stop the bad guys, unless the authorities can react quicker than I suspect they can in most places.